Plitvice Lakes 3031203771, 9783031203770

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Springer Water

Marko Miliša Marija Ivković   Editors

Plitvice Lakes

Springer Water Series Editor Andrey Kostianoy, Russian Academy of Sciences, P. P. Shirshov Institute of Oceanology, Moscow, Russia Editorial Board Angela Carpenter, School of Earth & Environment, University of Leeds, Leeds, West Yorkshire, UK Tamim Younos, Green Water-Infrastructure Academy, Blacksburg, VA, USA Andrea Scozzari, Institute of Information Science and Technologies (CNR-ISTI), National Research Council of Italy, Pisa, Italy Stefano Vignudelli, CNR—Istituto di Biofisica, Pisa, Italy Alexei Kouraev, LEGOS, Université de Toulouse, Toulouse Cedex 9, France

The book series Springer Water comprises a broad portfolio of multi- and interdisciplinary scientific books, aiming at researchers, students, and everyone interested in water-related science. The series includes peer-reviewed monographs, edited volumes, textbooks, and conference proceedings. Its volumes combine all kinds of water-related research areas, such as: the movement, distribution and quality of freshwater; water resources; the quality and pollution of water and its influence on health; the water industry including drinking water, wastewater, and desalination services and technologies; water history; as well as water management and the governmental, political, developmental, and ethical aspects of water.

Marko Miliša · Marija Ivkovi´c Editors

Plitvice Lakes

Editors Marko Miliša Department of Biology Faculty of Science University of Zagreb Zagreb, Croatia

Marija Ivkovi´c Department of Biology Faculty of Science University of Zagreb Zagreb, Croatia

ISSN 2364-6934 ISSN 2364-8198 (electronic) Springer Water ISBN 978-3-031-20377-0 ISBN 978-3-031-20378-7 (eBook) https://doi.org/10.1007/978-3-031-20378-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Plitvice Lakes (Croatian: Plitviˇcka jezera) are a remarkable ecosystem on a global scale, exceptional in a diversity of almost all environmental aspects—climate, hydrology, geology, geography, and biology—from frozen to warm, from flooding to drying channels, from springs to lakes, from dolomite to limestone, from valley to canyon. This edition aims to compile the research done so far in all these aspects, and all focusing toward understanding the exciting interplay of these factors. The interplay of all these factors results in some of the most exciting and beautiful landscapes that have developed through the phenomenon of calcite precipitation— tufa deposition. Very soluble carbonate rocks readily dissolve in precipitates and in the underground. Since precipitation is (was) abundant, plenty of springs developed that formed the stream system. When the underground water, supersaturated with respect to calcium, bicarbonate, and carbonate ions and slightly acidic due to excess dissolved carbon dioxide, reaches the surface, the carbon dioxide outgasses, shifting the equilibrium of carbonate and calcium toward crystallization at appropriate places. These are the tufa barriers. They are the main phenomenon of this ecosystem. The barriers form seemingly randomly but from current knowledge, they are the result of changes in water flow dynamics, and biota that develops at these sites—mainly autotrophs. Both help evacuate carbon dioxide even further and facilitate the deposition of crystals. This process formed lakes and waterfalls over some 300 000 years. Tufa is a calcium carbonate deposit that developed in supersaturated waters of karstic origin. The mineral crystals are deposited on (as opposed to within) organic tissue. Mosses, algae, and even invertebrate cases or exuviae comprise a significant part of the tufa depositional frameworks. Even though this process is known in other places, at Plitvice lakes it is dramatically fast with a rate of growth of approximately 1 cm per year at places. This exciting ecosystem has attracted scientific attention since mid-late nineteenth century. The peak of interest that resulted in the proclamation of this area a National Park was through the work of a Croatian botanist Ivo Pevalek. Plitvice Lakes received international recognition in 1979, by being included in the UNESCO World Heritage List. v

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Among numerous fans of the area and scientific researchers, Pevalek was the first that wrote about his discoveries that water mosses and algae are crucial for the unique geomorphology of Plitvice Lakes. Subsequently, many of freshwater ecologists have studied and worked in this exceptional ecosystem. We all had a great time and are continuing to having it. It is our profound pleasure to present the works of the great researchers past and present that made this book possible. Zagreb, Croatia

Marko Miliša Marija Ivkovi´c

Contents

Geomorphological and Geological Properties of Plitvice Lakes Area . . . . Neven Boˇci´c, Uroš Barudžija, and Mladen Pahernik Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ˇ Ivan Canjevac, Ivan Martini´c, Maja Radiši´c, Josip Rubini´c, and Hrvoje Meaški

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Water Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Andrijana Brozinˇcevi´c, Maja Vurnek, and Tea Frketi´c

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Environmental Isotope Studies at the Plitvice Lakes . . . . . . . . . . . . . . . . . . Andreja Sironi´c, Ines Krajcar Broni´c, and Jadranka Bareši´c

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Recent Tufa Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Renata Matoniˇckin Kepˇcija and Marko Miliša Energy and Matter Dynamics Through the Barrage Lakes Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Marko Miliša, Maria Špoljar, Mirela Serti´c Peri´c, and Tvrtko Dražina Springs of the Plitvice Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Pozojevi´c Ivana, Ivkovi´c Marija, and Peši´c Vladimir Benthic Algae on Tufa Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Igor Stankovi´c, Beáta Szabó, Tomáš Hauer, and Marija Gligora Udoviˇc The Plitvice Lakes—An Interplay of Moss, Stonewort and Marshland Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Antun Alegro, Anja Rimac, Vedran Šegota, and Nikola Koleti´c Plankton Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Ivanˇcica Ternjej, Maria Špoljar, Igor Stankovi´c, Marija Gligora Udoviˇc, and Petar Žutini´c

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Aquatic Insects of Plitvice Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Marija Ivkovi´c, Viktor Baranov, Valentina Dori´c, Vlatka Miˇceti´c Stankovi´c, Ana Previši´c, and Marina Vilenica The Fish of the Plitvice Lakes—A Wealth of Simplicity . . . . . . . . . . . . . . . . 317 ´ Ivana Buj, Marko Caleta, Zoran Marˇci´c, Davor Zanella, and Perica Mustafi´c Caves in Plitvice Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 Kazimir Miculini´c, Tvrtko Dražina, Nikola Marki´c, and Neven Boˇci´c

Geomorphological and Geological Properties of Plitvice Lakes Area Neven Boˇci´c, Uroš Barudžija, and Mladen Pahernik

Abstract Plitvice Lakes are a world-famous karst phenomenon. Their origin and development is a consequence of a specific geological structure and geomorphological processes. The aim of this chapter is to provide an overview of geoscientific knowledge about this area with additional data from recent research. This area belongs to the outer Dinarides, the largest karst area in Europe. The majority of geological material consists of Triassic dolomite, Jurassic dolomites and limestones, and Cretaceous limestones. Differences in lithology also cause differences in hydrogeological characteristics and in exogenous geomorphological processes. Areas built of dolomite are mostly impermeable, so a drainage network and erosion processes have been developed on them. Karstification processes predominates on the limestones and there are lack of the surface streams. The most famous phenomena are tufa barriers and a series of cascading barrage lakes with waterfalls that make this area world famous. Keywords Karst · Fluvio-karst · Karst geology · Karst geomorphology · Karst lakes · Karst rivers · Plitvice lakes · Croatia · Dinaric karst

1 Introduction The Plitvice Lakes National Park was declared on April 8, 1949, covering an area of 192 km2 . Since 1979, the Plitvice Lakes National Park has been on the UNESCO N. Boˇci´c (B) Divison of Phisical Geography, Department of Geography, Faculty of Science, University of Zagreb, Maruli´cev trg 19, 10000 Zagreb, Croatia e-mail: [email protected] U. Barudžija Department of Mineralogy, Petrology and Mineral Resources, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia e-mail: [email protected] M. Pahernik Croatian Geomorphological Society, Maruli´cev trg 19, 10000 Zagreb, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_1

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World Heritage List. This confirms the uniqueness and global significance of this phenomenon. In 1997, the area of the National Park was expanded and now covers an area of 295 km2 . Due to their exceptional value, the Plitvice Lakes have been the subject of research by numerous researchers. As this is a world-famous karst phenomenon, numerous geomorphological and geological researches have been carried out. One of the oldest known descriptions of the area of Plitvice Lakes with a hypothesis about their origin is given by the geographer Milan Šenoa (Šenoa 1895). The first known geomorphological surveys were carried out by Hranilovi´c (1901) and Gavazzi (1903). Also, detailed descriptions of this area with an explanation of origin of the lakes are given by Hirc (1900) and Frani´c (1910). Poljak (1914) gives a detailed overview of the previously known caves and pits of this area. Pevalek (1925/26) writes about the geological significance of tufa deposits. First detailed geological investigations in the Plitvice Lakes area were performed by Koch (1916, 1926, 1932), who made the first geological map in the scale of 1:75.000 and defined Cretaceous carbonate rocks as predominant in the area. Later, Kochansky-Devide (1958) and Polšak (1959, 1960, 1963) performed geological and biostratigraphic research in the area and distinguished Triassic, Jurassic and Cretaceous rocks. Rogli´c (1951, 1958, 1974) analyzes the geomorphological features and the formation of the Plitvice Lakes. A detailed overview of caves and pits is given by Rendešek (1958), and Polšak (1974) writes about the geological aspects of the protection of the Plitvice Lakes. By basic geological mapping in the scale of 1:100.000 performed in the second half of the twentieth century, Biha´c (Polšak et al. 1976) and Otoˇcac (Veli´c et al. 1970) sheets of the Basic Geological Map were produced. These sheets were the basics for Geological Map of Croatia in 1:300.000 scale (HGI-CGS 2009), on which basic lithostratigraphy (Veli´c and Vlahovi´c 2009) in the Plitvice Lakes area can be observed. Srdoˇc et al. (1985) publish a detailed study on calcite deposition, and Horvatinˇci´c et al. (2000, 2003) provide data on the age of tufa. Explaining the conceptual hydrogeological model Biondi´c et al. (2010) also give a hypothesis of the origin of the Plitvice Lakes. Overview of geomorphology was also given by Boˇci´c (2009) and lithostratigraphy by Barudžija (2014). More recent contributions on tectonics and structural relations in the Plitvice Lakes area were given by Krnjak et al. (2019). Plitvice Lakes are located (Fig. 1) between Mala Kapela (1279 m) in the west and northwest, Liˇcka Plješivica (1646 m) in the southeast, Lika highlands with karst fields in the south and Una-Korana karst plateau in the northeast. A series of 16 lakes that generally stretch from south to north are an integral part of the Korana River valley. The southernmost Lake Proš´ce is the highest (636 m) while the northernmost lake, Novakovi´ca Brod, is the lowest (503 m) in the series of the Plitvice Lakes. The area of the National Park is built of carbonate rocks of Mesozoic age, and according to the lithological characteristics, several units can be distinguished that have different hydrogeological and geomorphological features. The aim of this chapter is to give an overview of previous knowledge about the geological structure and geomorphological features and their connection in the area of the Plitvice Lakes National Park. The review has been supplemented by more recent, primarily morphometric data.

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Fig. 1 Location and coverage of the Plitvice Lakes National Park in relation to the most important relief units

2 Bacis Geomorphometric Properties Morphometric analyzes were made based on a high-resolution digital elevation model (0.5 × 0.5) obtained by airborne lidar scanning. The highest point of the entire area of the National Park is Seliški vrh (1279 m), the lowest is in the Korana canyon at 367 m and average height is 803.7 m. The hypsometric map (Fig. 2a) indicates the basic orographic features of the terrain and suggests that the whole area consists of a north-eastern lower and a south-western upper part. The north-eastern part is a relatively flattened fragment of a karst plateau with a height range of 500–800 m. The lowest height is in the central part where the Korana canyon was cut, up to 150 m in relation to the level of the plateau. On the plateau itself there are some elevations, the highest of which are 867 and 889 m. Along the very northern edge of the park there is another series of relatively higher elevations. The south-western, higher part of the Park is much more heterogeneous. In its northern part is the highest elevation of the Park (Seliški vrh 1279 m) with the parallel ridge of Razdolje (1113 m). In the southern part, three main ridges extend along the dinaric orientation. Between the southeastern and midle series of ridges are two large karst depressions.

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Fig. 2 Geomorphometric maps of the Plitvica Lakes National Park: a hipsometry, b slope inclination, c slope aspect, d relative relief per 1 km2

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The average slope of the whole terrain is 12.5° and the spatial distribution of the slopes (Fig. 2b) shows their spatial heterogeneity. As a rule, lower slopes are primarily related to the bottoms of larger karst depressions (uvalas and poljes) and to the karst plateau. Large slopes are associated with slopes along tectonic zones. These zones are elongated and straight, mostly with dinaric orientation (NW–SE). Along with them, large slopes are related to the zones of deep incision of the drainage network. The most pronounced examples are in the Korana canyon where the canyon cliffs are often subvertical and vertical. The slope aspects show the orientation of the slope in relation to the sides of the world. This method is important because it can indicate morpholinaments resulting from active tectonics, but it also indicates the exposure of individual slopes to various exogenous influences. The map of slope aspects (Fig. 2c) indicates the predominant north-eastern and eastern orientations on the one hand, and south-western and western orientations on the other. Such slope orientations reflect the prevailing dinaric structures. It can also be noticed that the fragmentation of the slope orientation fields is significantly higher in the north-eastern part than in the south-western part, which is probably a consequence of the stronger karstification of the north-eastern part of the Park. The relative relief was calculated as the difference between the highest and lowest point per unit area (1 km2 ). The minimum recorded values are 11.3 m/km2 , the maximum is 440.1 m/km2 , and the average is 162.5 m/km2 . These values indicate a significant erosion potential, and indirectly tectonic activity, especially vertical movements. The map of relative relief (Fig. 2d) clearly outlines tectonic zones, mainly of Dinaric orientation. Similar to the slope, the smallest relative relief is related to the bottoms of larger karst depressions and to the plateau karst. The central, dinaric-oriented belt of increased values of relative relief is also noticeable, in which the Plitvice Lakes themselves developed.

3 Geological Properties Basic geological properties are shown on geological map (Fig. 3) made on the basis of data of the section of the Geological Map of Croatia in 1:300.000 scale (HGI-CGS 2009).

3.1 Lithostratigraphy The underground of the Plitvice Lakes is built of Triassic, Jurassic and Upper Cretaceous sedimentary rocks. All these rocks are almost exclusively carbonates (various types of limestones and dolomites), and the other sedimentary rocks are only subordinately present in the area. Upper Triassic deposits are almost exclusively well

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Fig. 3 Geological map of the Plitvice Lakes National Park area (partly modified after HGI-CGS 2009). Legend: 1-dip and strike of beds; 2-normal (continuous) contact of beds; 3-fault: determined; 4-fault: covered; 5-lowered block; 6-reverse fault: determined; 7-reverse fault: covered; 8-reverse fault: partly covered; 9-overthtrust: covered; 10-Holocene: deluvium and proluvium deposits; 11Upper Cretaceous: limestones; 12-Lower Cretaceous: limestones and dolomites; 13-Upper Jurassic: limestones and dolomites; 14-Middle Jurassic: limestones and dolomites (thick bedded); 15-Lower Jurassic: limestones and dolomites; 16-Upper Triassic: dolomites

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Fig. 4 a The Upper Triassic well bedded grey dolomites at banks of the Upper Lakes, b The Upper Cretaceous limestones in the canyon walls at the Lower Lakes. Photos by U. Barudžija

bedded grey dolomites (Fig. 4a). Lower Jurassic deposits are predominantly limestones, partly intercalated with dolomites. Middle Jurassic rocks are fossiliferous limestones (i.e. limestones with lithiotid bivalves), occasionally interlayered with thick beds of dolomites. Upper Jurassic deposits are well bedded limestones, occasionally with thin layers of dolomites and cherts. Lower Cretaceous deposits are represented by monotonous sequences of grey to brown coloured limestones. Upper Cretaceous well bedded to massive fossiliferous limestones often contain abundant fauna of rudist shells or its debris (Fig. 4b). Quaternary lake tuffa, which make the barriers and cascades of the Plitvice Lakes, are the youngest deposits in the area. These are highly porous grey or yellow rocks; whose total thickness occasionally exceeds 20 m.

3.2 Hydrogeology The Plitvice Lakes are situated in the Outer Dinarides, within the karst hydrological regime having strong water circulation, complex subterranean interconnections, strong karst springs and confined lakes development at the surface. By combination of these factors complex hydrogeological mechanisms were formed, depending mainly on the permeability of the rocks and on their tectonic and structural relationships as well. Some of the rocks represent the barriers for water percolation and subsequently leads to the accumulation of water, while the other rocks allow water to circulate throughout. Depending to their permeability and ability to accumulate water, rocks are considered as permeable, low permeable or impermeable.

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The impermeable barriers are the Upper Triassic dolomites as well as conformable Lower Jurassic limestones and dolomites. The Upper Triassic dolomites have low primary and slightly enhanced (due to dolomitization) secondary porosity, and they are generally considered as low permeable rocks. However, dolomites are considered also as impermeable barriers due to impermeable Upper Triassic (Carnian) clay layers below, which additionally prevent vertical water circulation. Cracks and fissures in the Upper Triassic dolomites are therefore well saturated and the water is accumulated and drained in numerous springs. Due to these impermeable barriers, surficial watercourses started to develop and flow, and the present-day Upper Lakes were eventually formed, almost without loss of water in the karst underground. Lower Jurassic impermeable carbonates with marls and chert interlayers additionally consolidate this barrier, preventing vertical water circulation. Another barrier for water circulation is the horizon of impermeable Upper Turonian platy to laminated clayey limestones, situated below heavily karstified rudist limestones. These impermeable rocks are responsible for water accumulation in the Lower Lakes area. Thick sequences of the Middle to Upper Jurassic and Lower Cretaceous limestones and dolomites, as well as the Upper Cretaceous rudist limestones, are mainly permeable rocks. The areas with these rocks are the main waterways due to various karst features developed within, such as dolines and caves. Water percolates mainly through the cavities, and it is drained from the underground at the springs located near the impermeable barriers.

3.3 Tectonics and Structural Elements The folded and overthrusted Dinaric Mountains chain, part of which are also the Plitvice Lakes, originated by convergence of the Adriatic Microplate and the EuroAsian Plate, since the Upper Jurassic until the present-day (Pami´c et al. 1998; Vlahovi´c et al. 2005; Schmid et al. 2008, with accompanying references). The oldest deposits in the External (Karst) Dinarides are of Carboniferous age, and succession of Palaeozoic, Mesozoic and Cenozoic predominantly carbonate deposits follows. These carbonates are genetically linked with the tectonic evolution of the Adriatic Microplate, and clastic deposits subordinately appear. In the Plitvice Lakes area Triassic, Jurassic and Cretaceous deposits are present. Among the structural elements, faults predominate, and the older folds can be seen as well. Main tectonic units stretching along the NW–SE strike. The Upper Triassic dolomites in the middle part of the terrain are delineated on the NE from folded Upper Cretaceous rudist limestones by almost vertical reverse fault. The Lower and Upper Jurassic rocks continues upon the Upper Triassic dolomites towards the SW. Faults with predominant NW– SE (so-called “Dinaric”) strike prevail in the area (Polšak 1974; Polšak et al. 1976, 1978), and they originated in the Middle-Upper Eocene and Oligocene (Vlahovi´c et al. 2005). These faults are later intersected by the faults with E-W strike, which enabled intense karstification and disintegration of the Mesozoic deposits (Herak 1955; Polšak 1959, 1960; Veli´c et al. 1970; Polšak et al. 1976, 1978, with accompanying references). After Polšak et al. (1978) and Veli´c et al. (1970), two regional

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tectonic units, Mala Kapela-Liˇcka Plješivica and Lipovaˇca-Cazin, are outlined in the Plitvice Lakes area. These regional units consist of small-scale structural units: Veliki Javornik, Brezovica-Krbavica, Plitvice Lakes, Trovrh-Gola Plješivica and ˇ Catrnja-Lipovaˇ ca.

4 Morphogenesis 4.1 Processes and Forms The area of the Park is exogenously shaped by karst and erosion processes. The spatial distribution of the predominant influence of these processes primarily depends on the lithological structure of the terrain (Figs. 3 and 5). However, all exogenous processes are driven by a high energy of relief that significantly exceeds the local erosion base— the Korana River, which is within the Park at 367 m above sea level, but on the nearby Una-Koran Plateau is even lower, at about 320 m (Fig. 1). Such orographic relations are also a consequence of tectonic movements, mostly the relative uplift of this area in relation to the erosion base in the northeast. The most characteristic karst form of this area are dolines. These circular and subcircular depressions can be considered as a diagnostic form of karst (Ford and Williams 2007). Based on the topographic map at a scale of 1: 25,000, 7238 dolines were singled out in the Park area. As previous experiences show that due to generalization, these maps show about 50% of all existing dolines, we can assume that there are about 15,000 of them in the Park area. The spatial distribution (Fig. 5a) and dolines density (Fig. 5b) are very uneven. There are areas without them while the maximum density is 113.7 dolines/km2 , which falls into the category of very high density of dolines (Pahernik 2012). The average density for the entire area of the Park is 24.1 dolines/km2 . The main factor of this spatial distribution is the difference in lithology (Pahernik 2012). Most of them are in the area built of Upper Cretaceous limestones. This is confirmed by the data obtained from the Lidar high resolution digital elevation model. In the area made of Upper Cretaceous limestones, the average density of all dolines was measured as high as 156.15 dolines/km2 . In addition to lithology, the slope inclination also plays an important role in the spatial distribution of dolines. This is particularly pronounced in the highest parts of the park which, although lithologically quite homogeneous (Lower Cretaceous limestones), have a heterogeneous distribution of dolines. They mainly appear on slopes of small and medium inclination, while on steep slopes they are absent. In addition to dolines, large karst depressions also occur in this area (Fig. 5c). The largest are located in the southern part of the Park. These are the shallow karst uvala Maˇcje doline (2.7 km2 ), the Homoljaˇcko karst polje (4.7 km2 ) and the largest karst depression in the Park-Brezovac (4.7 km2 ). In the northern part of the Park, the karst uvala Štropovi (1.8 km2 ) stands out, which is the deepest karst depression of the Park with a depth of 90.1 m. There are no major depressions in the northeastern part of

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Fig. 5 a Spatial distribution of the dolines, b dolines density, c spatial distribution and depth of karst depressions, d spatial distribution of ridges

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the Park because it is a karst plateau. It is assumed that this plateau is an elevated fragment of a large Una-Korana plateau (Boˇci´c et al. 2010, 2015). The area of the National Park is rich in numerous underground karst phenomena— caves. About it see in a separate chapter of this book (Miculini´c et al. 2022). In areas built of dolomite, mechanical erosion predominates, and thus the development of a surface drainage network (Fig. 5a). Such relief is most developed on the impermeable Upper Triassic dolomites as well as Lower Jurassic limestones and dolomites. The most significant relief forms of this area are gullies and valleys. The valleys are relatively short. The longest valleys are Sartuk and Plitvice (9.2 km), Rijeˇcica (6.3 km), Bijela rijeka and Matica (5.5 km), Draga (4.8 km) and Crna rijeka (3 km). They have steep sides, mostly have a V cross-section and mostly deeply cut into the surrounding terrain (often over 100 m). The flat valley bottom with accumulated material appears only sporadically (e.g. the valley of Bijela rijeka and Matica). Numerous gullies, ravines and smaller valleys with rather strong erosion appear in the valley heads and valley sides. Although morphologically this relief is most similar to the valley type (fluviodenudational) due to the carbonate structure and the karst solution present in the total denudation, it is actually a fluvio-karst on the dolomites. In addition to dolomite fluvio-karst, some forms of contact fluvio-karst (Gams 1986; Mihevc 1991) are present here, such as pocket valleys, dry and blind valleys. The most pronounced pocket valley is the amphitheatre depression around the source of the Plitvice stream with steep slopes higher than 150 m. Also, parts of the hydrographic network that have been cut in the limestone terrains have canyon-type valleys and gorges. The most pronounced example is the canyon in which the Lower Lakes are located, then the canyon of the river Korana and parts of the valley of Sartuk and Plitvice. There are also positive forms that are primarily associated with ridges (Fig. 5d). The highest and most emphasized are the ridges that were formed as a result of structural relations, and they are here related to higher elevations, namely the dinaric orientation. The denudation ridges are much more common, representing slower denuded parts of the terrain between depressions. In the terrain with predominant karst denudation, the ridges are of higher density and close the polygons around dolines and other karst depressions. In the part of the terrain with pronounced surface erosion, the ridges are somewhat rarer and represent local watersheds between valleys and deep gullies. In addition to denudation forms, the processes of material accumulation also play an important role in the morphogenesis of this area. Along with the alluvial end proluvial fans, sporadic alluvial deposits and colluvial forms on the slopes, the most significant accumulation forms of this area are tufa deposits. This is the most recognizable phenomenon of this area, and at the same time the main cause of the fundamental phenomenon of the Park—the lakes and waterfalls themselves.

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Tufa is formed by crystallization of calcium carbonate, i.e. calcite from water (for details see Sironi´c et al. 2022; Matoniˇckin Kepˇcija and Miliša 2022). It consists of hollow, extremely porous limestones formed on the waterfalls of karst rivers and lakes and on springs. Calcite crystals are excreted from water in certain physicochemical conditions and are caught on rocks, moss, aquatic plants and submerged trees by means of algae and bacteria. Such sedimentation creates tufa barriers over which water flows from a higher lake to a lower one. In this way, one of the most important landscape phenomena in the Park is created—numerous waterfalls. As the barriers rise, the water level in each lake also rises. The tufa barriers of the Plitvice Lakes are very complex morphologies, and 15 large complexes of tufa barriers are grouped. A study of the age of the Plitvice Lakes tufa showed that the current active barrier began to form before about 6–7 ky. Tufa was also found in the area of the Park in paleobarriers aged 90–130 and 250–300 ky (Horvatinˇci´c 1999; Horvatinˇci´c et al. 2000). It has also been found that the growth rate of tufa barriers is different and when the downstream barrier grows faster, the lake can submerge the upstream barrier. Such submerged barriers have been found, for example, in Lake Kozjak. In periods favorable for their growth, tufa barriers created barrage lakes, which become depression favourably for sedimentation. Slowing down the water flow and sedimentation processes reduces the erosion impact of the stream on the deepening of the riverbed. When climatic conditions are unfavorable for the growth of tufa, conditions are created for increased erosion of tufa formations, lake sediments and the bedrock itself. Tufa barriers and barrage lakes also have an impact on the water level in the riverbed, i.e. on the local erosion base that defines the incision of the surrounding valleys and gullies. It also has an impact on the groundwater level, i.e. the local karstification base.

4.2 Relief Development The development of the relief, and especially the origin of the Plitvice Lakes, has not been sufficiently researched to date. Only a few works try to give an explanation of the origins of this area while giving different interpretations. Nevertheless, all the proposed hypotheses can be classified into two groups. The first is that the Upper Lakes depressions were primarily formed in which lakes were created later by the accumulation of water from endoreic watershed. Only later, there was a cascading overflow of water into lower depressions and the final creation of the Korana river valley as an effluent (outflow) river. Šenoa (1895) and partly Hranilovi´c (1901) supported this hypothesis, believing that depressions were primarily of karst, i.e. exogenous origin. Koch (1926), on the other hand, believed that lake depressions were formed as a result of folding, so the larger Upper Lakes developed in syncline depressions. Biondi´c et al. (2010) also belong to this group of hypotheses, they believe that Lake Proš´ce and Lake Kozjak basins were formed as tectonic depressions. Initially, the higher lake was created in such depression—Proš´ce, and then there was an overflow of water into the lower Lake Kozjak.

Geomorphological and Geological Properties of Plitvice Lakes Area

13

The second group of hypotheses (e.g. Gavazzi 1903, Rogli´c 1951, 1958, 1974) starts from the fact that a surface drainage network was originally created whose backbone is the valley of the former course of the Korana. With the formation of tufa barriers, the valley was partitioned and cascading barrage lakes were created. The difference in the morphology of the Upper and Lower Lakes is explained by the difference in the lithological background. The part of the valley in which the Upper Lakes were formed was developed in dolomite, in which, due to more intensive weathering, the valley sides became flatter, and the valley itself became wider. In contrast, the Lower Lakes were formed in limestone, so the valley itself has a canyon morphology. Based on the geomorphological, especially morphometric features of the relief; we are currently more inclined to adhere to the second hypothesis of partitioning a previously developed valley, although there are still a number of open questions. According to currently known data, we believe that the development scenario was approximately the following. Due to the described hydrogeological conditions, surface watercourses with the direction of runoff towards the north or northeast were primarily formed in this area. Due to the neotectonic uplift of the area of Mala Kapela and Liˇcka Plješivica, there was an increased incision of the main watercourse—Korana. The incision was made possible by the large gradient and consequently the erosion force of the river, but also because the bedrock was tectonically fractured. In the zone built of dolomite, a wide and mild valley with numerous surface tributaries was formed, while the downstream part of the valley in the area built of limestone took the form of a steep and deeply incised canyon without surface tributaries. The surrounding area was exposed to intensive karstification and the development of underground karst hydrography. The strong incision of the Korana canyon is also evidenced by the 76 m high waterfall with which the Plitvica Stream flows into the Korana valley. Due to favorable conditions for tufa formation, tufa barriers began to block the Korana Valley and thus create series of lakes in the valley. The difference in the shapes of the lakes thus formed is defined by morphological differences along the submerged valley. The Upper Lakes were formed by flooding of the part of the valley incised into the dolomite base, so they are wider and more spacious and with slightly inclinated shores. The Lower Lakes were formed by the flooding of a canyon valley incised into limestone, so they are therefore narrow and elongated, with very steep rocky sides (Fig. 6). In less favorable glacial periods, there was a cessation of tufa deposition and a renewed increased erosion and incision. In more favorable, interglacial periods, tufa and lakes formation in the valley occurred again. One such period continues today. Today, however, this process is increasingly threatened by either direct or even more indirect human impact (e.g. climate change) and there is an increasing danger to the survival of this very fragile natural balance.

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Fig. 6 a The shores of Prošˇcansko Lake formed in dolomite (photo: N. Boˇci´c), b The Lower lakes were formed by immersing a deeply incised canyon in a limestone base due to rising of tufa barriers (photo: K. Boˇci´c), c Sastavci—by merging water from the Lower Lakes and water of the Plitvice stream, which falls down a 76 m high waterfall, begins today’s course of the river Korana (photo: N. Boˇci´c), d the canyon of the river Korana incised into a limestone plateau (photo: H. Cvitanovi´c)

References Bareši´c J, Sironi´c A, Krajcar Broni´c I (2022) Environmental isotope studies at The Plitvice Lakes. In: Miliša M, Ivkovi´c M (eds) Plitvice Lakes environment. The handbook of environmental chemistry. Springer (this volume) Barudžija U (2014) Plitviˇcka jezera—geološke okolnosti postanka i zaštita (Plitvice Lakes–geological setting, origin and environmental protection). In: Veli´c J, Malvi´c T, Cvetkovi´c M (eds) Croatian geological summer school lectures book 2014. Croatian geological summer school. Zagreb, pp 193–210. https://geoloskaljetnaskola.hr/ Biondi´c B, Biondi´c R, Meaški H (2010) The conceptual hydrogeological model of the Plitvice Lakes. Geol Croat 63:195–206 Boˇci´c N (2009) Plitvice Lakes–the world known karst geomorphological phenomena. Internation Interdisciplinary Scientific Conference “Sustainability of the Karst Environment–Dinaric karst and Other Karst Regions”, Plitvice Lakes, Croatia, 2009. Excursion Guidebook, Center for karst, Gospi´c, pp 3–7 Boˇci´c N, Pahernik M, Bognar A (2010) Geomorfološke znaˇcajke Slunjske zaravni. Hrv Geograf Gl 72:5–26. https://doi.org/10.21861/hgg.2010.72.02.01

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Boˇci´c N, Pahernik M, Mihevc A (2015) Geomorphological significance of the palaeodrainage network on a karst plateau: the Una-Korana plateau, Dinaric karst, Croatia. Geomorphology 247:55–65. https://doi.org/10.1016/j.geomorph.2015.01.028 Croatian Geological Institute-Croatian Geological Survey HGI-CGS (2009) Basic geological map of the Republic of Croatia 1:300.000. HGI-CGS, Department of Geology, Zagreb Ford D, Williams P (2007) Karst hydrogeology and geomorphology. Wiley, Chichester Frani´c D (1910) Plitviˇcka jezera i njihova okolica. Tisak Kr. Zemaljske tiskare, Zagreb Gams I (1986) Kontaktni Fluviokras. Acta Carsol 14(15):71–88 Gavazzi A (1903) Geneza Plitviˇckih jezera. Glasnik Hrvatskog Naravoslovnog Društva 15:1–9 Herak M (1955) O nekim hidrogeološkim problemima Male Kapele (On some hydrogeological problems of Mala Kapela Mt.). Geološki Vjesnik 8–9:19–37 Hirc D (1900) Lika i Plitviˇcka jezera. Lav. Hartman (Kugli i Deutsch), Zagreb Horvatinˇci´c N (1999) Starost sedre Plitviˇckih jezera. Priroda 861(4):20–22 ˇ c R, Geyh A-M (2000) Interglacial growth of Tufa in Croatia. Quat Res 53:185– Horvatinˇci´c N, Cali´ 195. https://doi.org/10.1006/qres.1999.2094 Horvatinˇci´c N, Krajcar Broni´c I, Obeli´c B (2003) Diferences in the 14 C age, δ13 C and δ18 O of Holocene tufa and speleothem in the Dinaric Karst. Palaeogeogr Palaeoclimatol Palaeoecol 193:139–157. https://doi.org/10.1016/S0031-0182(03)00224-4 Hranilovi´c H (1901) Geomorfološki problemi iz hrvatskog Krasa. Glasnik Hrvatskog Naravoslovnoga Društva 13(1–3):93–133 Koch F (1916) Izvještaj o geološkim odnošajima u opsegu lista Plitvice (Report on geological relationships for Plitvice Sheet). Vijesti geološkog povjerenstva 5–6 Koch F (1926) Plitviˇcka jezera. Prilog poznavanju tektonike i hidrografije krša. (Plitvice Lakes. Contributions to tectonics and karst hydrography.) Vijesti geološkog zavoda 1:155–179 Koch F (1932) Geological map of Yugoslavia 1:75 000—Plitvice Sheet. Kochansky-Devide V (1958) Izmjena generacija vrste Orbitopsella Praecoursor u lijasu Plitvica (Exchange in the generations of Orbitopsella Praecoursor within the Lower Triassic (Lias) deposits of Plitvice). Geološki Vjesnik 11:77–86 Krnjak H, Matoš B, Paviˇci´c I et al (2019) An overview of tectonic evolution of the Plitvice Lakes National Park based on structural data. In: Horvat M, Matoš B, Wacha L (eds) 6th Croatian geological congress with international participation: Abstracts Book, Zagreb, pp 110–111 Matoniˇckin Kepˇcija R, Miliša M (2022) Tufa formation–potential environmental indicator. In: Miliša M, Ivkovi´c M (eds) Plitvice Lakes environment. The handbook of environmental chemistry. Springer (this volume) Miculini´c K, Dražina T, Marki´c N, Boˇci´c N (2022) Plitvice Lakes underground environment. In: Miliša M, Ivkovi´c M (eds) Plitvice Lakes environment. The handbook of environmental chemistry. Springer (this volume) Mihevc A (1991) Morfološke znaˇcilnosti ponornega kontaktnega krasa v Sloveniji. Geografski Vestnik 63:41–50 Pahernik M (2012) Prostorna gusto´ca ponikava na podruˇcju Republike Hrvatske. Hrvatski Geografski Glasnik 74(2):5–26. https://doi.org/10.21861/HGG.2012.74.02.01 Pami´c J, Guši´c I, Jelaska V (1998) Geodynamic evolution of the Central Dinarides. Tectonophysics 297:251–268. https://doi.org/10.1016/S0040-1951(98)00171-1 Pevalek I (1925/26) Oblici fitogenih inkrustacija i sedre na Plitviˇckim Jezerima i njihovo geološko znamenovanje. Spomenica u poˇcast Dr. Dragutinu Gorjanovi´c–Krambergeru, Hrvatsko prirodoslovno društvo, Zagreb, pp 101–110 Polšak A (1959) Geološko istraživanje okolice Plitviˇckih jezera (Geological investigations at the Plitvice Lakes area). Ljetopis JAZU 63, Zagreb Polšak A (1960) Prilog poznavanju hidrogeoloških odnosa okolice Plitviˇckih jezera (Contribution to hydrogeological relationships in the Plitvice Lakes area). Ljetopis JAZU, 64, Zagreb Polšak A (1963) Rudisti senona Plitviˇckih jezera i Liˇcke Plješivice (Senonian rudists in the Plitvice Lakes and Liˇcka Plješivica Mt.). Geološki vjesnik 15/2

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Polšak A (1974) Geološki aspekti zaštite Plitviˇckih jezera (Geological aspects of the Plitvice Lakes protection). In: Plitviˇcka jezera—ˇcovjek i priroda (The Plitvice Lakes—Man and the nature). NP Plitvice Lakes, Zagreb, pp 226–235 Polšak A, Juriša M, Šparica M et al (1976) Basic geological map of SFRY 1:100.000, Biha´c Sheet (L33–116). Geological Institute Zagreb, Federal Geological Institute Belgrade Polšak A, Crnko J, Šimuni´c A et al (1978) Basic geological map of SFRY 1:100.000, Geology of Biha´c Sheet (L33–116). Geological Institute Zagreb, Federal Geological Institute Belgrade Poljak J (1914) Pe´cine hrvatskog krša II dio. Pe´cine okoliša Plitviˇckih jezera, Drežnika i Rakovice. Prirodoslovna istraživanja JAZU, 1–25 Rendešek V (1958) Topografski opis pe´cina Nacionalnom parku Plitviˇcka jezera. In: Šafar J (ed) Nacionalni park Plitviˇcka jezera. NP “Plitviˇcka jezera”, pp 295–328 Rogli´c J (1951) Unsko-koranska zaravan i Plitviˇcka jezera–geomorfološka promatranja. Geografski Glasnik 13:49–66 Rogli´c J (1958) Karakteristika pejsaža i mogu´cnosti Plitviˇckih jezera. In: Šafar J (ed) Nacionalni park Plitviˇcka jezera. NP “Plitviˇcka jezera”, pp 409–434 Rogli´c J (1974) Morfološke posebnosti Nacionalnog parka Plitviˇcka jezera. Plitviˇcka jezera – cˇ ovjek i priroda, Zagreb, pp 5–22 Schmid S-M, Bernoulli D, Fügenschuh B et al (2008) The Alpine-Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss J Geosci 101:139–183. https://doi.org/ 10.1007/s00015-008-1247-3 Srdoˇc D, Horvatinˇci´c N, Obeli´c B et al (1985) Procesi taloženja kalcita u krškim vodama s posebnim osvrtom na Plitviˇcka jezera. Krš Jugoslavije 11(4–6):101–204 Šenoa M (1895) Rijeka Kupa i njezino porjeˇcje. Rad JAZU, Knjiga CXXII, Zagreb Veli´c I, Bahun S, Sokaˇc B et al (1970) Basic geological map of SFRY 1:100.000, Otoˇcac Sheet, L33–115, Geological Institute Zagreb, Federal Geological Institute Belgrade Veli´c I, Vlahovi´c I (2009) Geology for basic geological map of the Republic of Croatia 1:300.000. Croatian Geological Institute-Croatian Geological Survey (HGI-CGS), Zagreb Vlahovi´c I, Tišljar J, Veli´c I et al (2005) Evolution of the adriatic carbonate platform: paleogeography, main events and depositional dynamics. Palaeogeogr Palaeoclimatol Palaeoecol 220:333–360. https://doi.org/10.1016/j.palaeo.2005.01.011

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes Area ˇ Ivan Canjevac, Ivan Martini´c, Maja Radiši´c, Josip Rubini´c, and Hrvoje Meaški

Abstract The Plitvice Lakes where the first place in Europe to be included in the UNESCO World Natural Heritage List as a phenomenon that is created primarily by water. The main phenomenon of the National Park is a series of barrage lakes created by the growth of tufa barriers on the old riverbed of the Korana River. In addition, the highest waterfall in Croatia, 78 m high Veliki slap Waterfall is one of the main attractions. In this chapter, main hydrological, hydrogeological and hydromorphological characteristics of the Plitvice Lakes National Park are presented. Basic climatological components are analysed following data on groundwater storage and circulation. Final part of the chapter is dedicated to surface water, i.e. streams and lakes of the Plitvice Lakes area, their basic hydrological characteristics and hydromorphological status according to the Water Framework Directive. Keywords Hydrology · Hydrogeology · Hydromorphology · Karst · Barrage lake · Water balance · Discharge regime · Climate change

ˇ I. Canjevac (B) · I. Martini´c Divison of Physical Geography, Department of Geography, Faculty of Science, University of Zagreb, Maruli´cev trg 19, 10000 Zagreb, Croatia e-mail: [email protected] I. Martini´c e-mail: [email protected] M. Radiši´c · J. Rubini´c Department for Hydraulic and Geotechnical Engineering, Faculty of Civil Engineering, University of Rijeka, Radmile Matejˇci´c 3, 51000 Rijeka, Croatia e-mail: [email protected] J. Rubini´c e-mail: [email protected] H. Meaški Faculty of Geotechnical Engineering, University of Zagreb, Hallerova Aleja 7, 42000 Varaždin, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_2

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1 Introduction Because of its exceptional natural values and beauty, the Plitvice Lakes area was legally protected already in 1928 and declared a national park in 1949 as the first national park in Croatia. In 1979 the lakes were included in the UNESCO World Natural Heritage List (Boˇci´c 2009). The lakes were the first place in Europe to be included in the list as a phenomenon that is created primarily by water (Meaški 2011). The main phenomenon of National Park is a series of barrage lakes created by the growth of tufa barriers on the old riverbed of the Korana River (Boˇci´c 2009). In addition, the highest waterfall in Croatia, 78 m high Veliki slap Waterfall is one of the main attractions. Plitvice Lakes National Park (PLNP) is located in the karst area of the Croatian highlands, on the border of the Lika and Kordun regions, and the Ogulin-Plaški valley. The area is predominantly hilly-mountainous. Mala Kapela Massif with its highest peak Seliški vrh (1279 m a.s.l.) dominates the southwestern half of the park. The northeastern part of the park is of lower altitude, down to about 360 m a.s.l. in the canyon of the Korana River (Fig. 1). The Plitvice Lakes belong to the Danube catchment area, i.e., to the Black Sea basin, and they are situated in the boundary area towards the Adriatic Sea basin, with the watershed divide crossing the Park in the area of Babin potok. According to current findings, the entire Plitvice Lakes catchment area is within the boundaries of the Park, except for small surfaces in the mountain area of Mala Kapela and area between Kuselj and Saborsko (PLNP Management Plan 2019). Dolostones and limestones of Mesozoic age form the greater part of the geological base of the park. The southwestern part of the park consists of limestones and dolostones of Jurassic age. The area is filled with dolines and caves, indicators of a well-developed karst. The central part of the park is built on Upper Triassic dolomite rocks with low permeability. From here flow the Bijela rijeka Stream and the Crna rijeka Stream whose waters form the Matica River and the Plitvice Lakes system. The northeastern part of the park consists of Upper Cretaceous limestones, which are strongly karstified. Such a lithological composition of the area indicates the presence of a developed network of groundwater flows. Comparing the topographic and hydrogeological watersheds of the Plitvice Lakes (Fig. 2), we can see that they are largely different. The hydrogeological watershed of the Plitvice Lakes system (including the Plitvica Stream) occupies about 152 km2 (Meaški 2011), while the topographic watershed covers only 72 km2 . It is important to note that water not only enters the lake system through underground routes but is also lost through them. Data from the National habitat map (2016) where used to analyze land cover. 83.5% of the park area is covered with forest. Only 1.12% of the area is covered with water, 0.93% cover the lakes and perennial streams, while intermittent streams cover 0.19% of the park. The rest of the area is covered with scattered patches of meadows, pastures, transitional areas, abandoned agricultural land and settlements. Administratively, the park is divided between 2 counties (Karlovac county and Lika-Senj county) and within them 4 municipalities and 20 settlements. According

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

Fig. 1 Position of the Plitvice Lakes National Park

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Fig. 2 Comparison of hydrogeological and topographical watershed of the Plitvice Lakes

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

21

to the 2011 census, 1411 inhabitants live in the National Park, whose total area is 296 km2 . The largest settlements, Plitviˇcka Jezera and Jezerce, have about 300 inhabitants (PLNP Management Plan 2019). Most of the settlements are located along the two main roads D1 and D52. According to the work of Rubini´c et al. (2008), and Benceti´c Klai´c et al. (2018), the first cartographic records with the toponym Plitvice Lakes appear in the seventeenth century, while the first textual records date from the eighteenth century. Greater attention was paid to the lakes in 1850, when a more detailed map was produced, a geological survey was conducted, first depths of lakes were measured, which is considered the first limnological work on the lakes (Frani´c 1910). The first comprehensive limnological survey was carried out by Gavazzi (1919). A significant turning point occurred in 1951, when for the first time a multidisciplinary scientific research project was organized at the Plitvice Lakes by a team led by Petrik (1958), and continuous hydrological monitoring of water level fluctuations in the lakes and flow at the sites of individual tributaries in the lake system as well as on the outflow of lakes began. In 2003, research began in the Plitvice Lakes and in the wider region within the framework of the international project “Investigation of post-war anthropogenic pollution and the definition of protective measures for the Plitvice Lakes National Park and the Biha´c region in the border area between Croatia and Bosnia and Herzegovina” (Anthroprol.prot 2006), within which the analysis of water exchange dynamics was carried out (Babinka 2007). The hydrogeological project “Sustainable water use in the Plitvice Lakes pilot area” analyzed surface and groundwater phenomena in Plitvice Lakes, as well as hydrological and hydrogeological relationships between them (Biondi´c et al. 2009, 2010; Meaški 2011). The hydrology of Plitvice Lakes is also addressed by Ridanovi´ c (1989) who brought first analyzes of hydrological data from gauging stations operating since the beginning of 1980s. Bonacci (2000, 2013a, b), was first to point out the problem of pronounced decreasing runoff trends, and Berakovi´c (2005) analyzed the water balance of the Plitvice Lakes. In the work of Zwicker and Rubini´c (2005), the trends of the course of water level and flow fluctuations were analyzed, and the average growth of tufa barriers was estimated, and in the work of Rubini´c et al. (2008) and Rubini´c and Zwicker Kompar (2009), the hydrological relationships between the hydrological elements of individual lakes were analyzed. In the 2016–2021 period an interdisciplinary project Hydrodynamics of Plitvice Lakes is organized by the Faculty of Science in Zagreb, the Faculty of Civil Engineering in Rijeka and the Faculty of Geotechnical Engineering in Varaždin. Following the acceptance and gradual implementation of the Water Framework Directive in Croatia contemporary hydromorphological analyzes where carried out in the PLNP. The research on hydromorphogical pressures and state of Lakes Proš´ce and Kozjak were done by interdisciplinary team from the University of Zagreb, Faculty of Science, Department of Geography and private company Elektroprojekt Consulting

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Engineers (Vuˇckovi´c et al. 2019a, b). In addition, systematic hydromorphological research was carried out in the 2019–2021 period in the Plitvica Stream catchment ˇ area (Canjevac et al. 2021). In this chapter, main hydrological, hydrogeological and hydromorphological characteristics of the PLNP are presented. Basic climatological components are analyzed following main data on groundwater storage and circulation. The final part of the chapter is dedicated to surface waters, i.e. streams and lakes of the PLNP.

2 Basic Climatological Characteristics of the Plitvice Lakes Area Despite the relatively early scientific-professional, as well as excursion-tourist interest in the Plitvice Lakes, continuous observations of climatic characteristics of the area began relatively late, in the 1950s, and are characterized by frequent changes of measuring stations and occasionally long interruptions. According to the available data (Makjani´c 1958), the first continuous monitoring of precipitation at the Plitvice Lakes refers to the 1903–1910 period, and it is estimated that the data after 1908 are unreliable. Full climatological monitoring at the Plitviˇcki Ljeskovac station was carried out during 1932–34, 1938–1941 and 1949–1950, but with occasional interruptions. At the beginning of the 1950s, a large number of climatological stations ˇ (Plitvice Ljeskovac, Plitvice Lakes, Stipanov Griˇc, Corkova uvala) as well as precipitation stations (Prijeboj, Liˇcko Petrovo selo, Gornji Babin Potok, Vrhovine, Titova Korenica) were established, but all these stations continued the practice of occasional interruptions of observations, and even their complete cessation of observations. Therefore, the climatological characteristics of the Plitvice Lakes basin are considered at the level of the basic climatological characteristics of the recorded time series—monthly and annual data on air temperatures and the amount of precipitation. Data from the climatological station Plitvice Lakes (1956–2019), located at an altitude of 579 m (44° 53' N, 15° 37' E) (https://meteo.hr), were used for the analyzes. The results of basic statistical data processing (Avg—mean of the series, Stdev—standard deviation, Cv—coefficient of variation, Max and Min—extreme registered values within the analyzed series of mean monthly or annual values) on precipitation and air temperatures at the mentioned station are given in Table 1. In some months, data on precipitation and air temperatures are missing, and due to the very good correlations, the missing data were supplemented by the climatological station Slunj.

0.58

0.48

3.7

Min

3.2

6.1

6.9

−9.3

−0.7

2.6

−3.8

4.6

−6.6

Avg

Cv

Max

Min

0.5

24.4

333.1

SD

Air temperature (°C)

293.3

Max

Cv

67.9

55.0

116.7

114.2

Avg

SD

II

I

Month

Precipitation (mm)

−1.5

8.9

0.55

2.3

4.2

27.6

254.5

0.42

49.2

117.3

III

4.5

13.4

0.19

1.6

8.7

8.9

311.3

0.38

54.2

142.1

IV

9.3

16.6

0.11

1.5

13.0

16.8

421.2

0.49

70.7

145.3

V

14.0

20.3

0.08

1.4

16.4

25.2

283.9

0.45

61.1

137.0

VII

15.8

21.2

0.07

1.3

18.2

13.8

323.5

0.59

63.2

107.9

VIII

13.7

21.7

0.09

1.6

17.6

3.0

346.7

0.61

73.2

120.5

IX

10.2

16.9

0.10

1.3

13.5

28.1

418.3

0.54

83.6

156.0

X

4.9

12.9

0.18

1.7

9.3

9.2

447.2

0.57

84.2

147.9

XI

0.0

10.6

0.49

2.3

4.7

27.5

362.6

0.46

78.1

169.5

XII

−4.4

4.5

4.2

2.2

0.5

4.0

345.1

0.53

75.5

141.1

Year

Table 1 Characteristic monthly and annual values of precipitation (mm) and air temperature (°C) at the Plitvice Lakes station (1956–2019)

7.4

10.9

0.10

0.8

8.8

1023.6

2213.9

0.17

277.2

1615.5

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes … 23

24

ˇ I. Canjevac et al.

2.1 Precipitation and Air Temperature At the Plitvice Lakes station, the average total annual precipitation in the 1956– 2019 period was 1615.5 mm and the average annual air temperature was 8.8 °C. There is a very large variability, both by individual months and at the level of annual precipitation. The annual variability ranges from 1023.6 mm (2011) to 2213.9 mm (2014). The maximum monthly precipitation during the entire monitoring period was 447.2 mm (October 1974) and the minimum was 3.0 mm (August 2000). The maximum mean monthly air temperature is 21.7 °C (August 2003) and the minimum −9.3 °C (February 1956). The trends of annual precipitation and air temperature at the analyzed station were also considered (Fig. 3). The display shows a slight trend of decreasing annual precipitation of about 49 mm/100 years and a very significant trend of increasing air temperature of 2.6 °C/100 years. A plot of the distribution of mean, maximum and minimum monthly precipitation and air temperatures within the year is also given (Figs. 4 and 5). Figure 4 shows that the mean monthly precipitation is very evenly distributed throughout the year. The lowest precipitation amount occurs in June (107.9 mm) and the highest in November (169.5 mm). However, when all monthly precipitation amounts are considered, including the recorded extremes, it can be seen that in certain months there is almost no precipitation at all (minimum monthly precipitation amounts of less than 10 mm were recorded in as many as 5 calendar months), while in some months (May, September and October) even more than 400 mm can fall. Figure 5 shows the usual distribution of mean monthly air temperatures. The highest air temperatures occur in the summer months of July and August, and the

Fig. 3 The course of the characteristic annual values of the recorded precipitation and air temperature at the station Plitvice Lakes (1956–2019)

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes … Avg

Precipitation (mm) 500

Max

25

Min

450 400 350 300 250 200 150 100 50 0 1

2

3

4

5

6

7

8

9

10

11

12 Month

Fig. 4 The distribution of mean, maximum and minimum monthly precipitation within the year on the station Plitvice lakes (1956–2019) Avg

Air temperatures (°C) 25

Max

Min

20

15

10

5

0

-5

-10 1

2

3

4

5

6

7

8

9

10

11

12 Month

Fig. 5 The distribution of mean, maximum and minimum monthly air temperatures within the year on the station Plitvice lakes (1956–2019)

ˇ I. Canjevac et al.

26 SUB-HUMID

HUMID

PERHUMID HOT

22 VII

20

VI

VII VIII

VIII

1961 - 1990

12

1991 - 2019

VI IX

10

IX

V V X

IV

8

X

6

MILD

14

IV

FRESHLY

16

XI

III

4

III XI

2

II XII

0 I II

-2

COLDLY

Mean monthly air temperature (°C)

18

XII

I

-4 40

60

80

100

120

140

160

180

200

Mean monthly precipitation (mm)

Fig. 6 Foster diagram for the station Plitvice Lakes (1961–1990 and 1991–2019)

lowest in January, where they take on an average negative value of −0.7 °C. Mean monthly temperatures of 0 °C and below were recorded in all months in the period November–March. The general climatic characteristics can be clearly represented by the so-called Foster diagram (Fig. 6), which gives average monthly data on precipitation and air temperatures for the station Plitvice Lakes for the standard 30-year climatic period 1961–1990 and for the recent 29-year period (1991–2019). From the given plot, the perhumid climate prevails in the Plitvice Lakes, which characterized all calendar months during the period 1961–1990. During the recent period (1991–2019), there is an increase in air temperatures in all calendar months, as well as a significant decrease in precipitation in the warmer season (June–August) and its increase in the otherwise wetter months (September–February). Under these circumstances, the two warmest and simultaneously driest months of the year, July and August, shift from the perhumid to the humid zone. Compared to the 1961–1990 period, the highest air temperature increase is in August, by an average of 1.9 °C, and in July and January, by 1.6 °C. This shows the current manifestations of climatic variations/changes in the analyzed area of the Plitvice Lakes.

2.2 Evapotranspiration The term evapotranspiration includes total evaporation from water surfaces, land surfaces, snow, ice, vegetation and other surfaces as well as biological transpiration and plays a very important role in the water balance. Evaporation from free water

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

27

surfaces (lakes) is a significant component of total evapotranspiration in PLNP. The process of evaporation depends on air temperature, water vapor pressure, wind speed, humidity, but also water quality (Žugaj 2000). Meaški (2011) calculated the average annual evaporation from the lakes using Meyer’s (1915) formula: E = 11.25 × e0 × (1 − R) × (1 + 0.225 × v) where E is evaporation from free water surface (mm/month); e0 is monthly mean saturation vapour pressure (mbar); R is monthly mean relative humidity (R (%)/100); and v is mean wind speed (m/s). For the calculation the data from the meteorological station Plitvice Lakes in the period 1995–2007 were used. The result is 422 mm (±19 mm) per year. The highest evaporation from the lakes occurs in the summer months, 58 mm in June, 63 mm in July and 62 mm in August. Evaporation in the Plitvice Lakes area is lowest in winter, with evaporation of 13 mm in December, 14 mm in January and 17 mm in February. Evapotranspiration cannot be measured directly from a larger surface or area, but direct measurements of other meteorological parameters can be used to calculate it, as can the water balance method and empirical approaches based on graphical determination. The water balance method calculates evapotranspiration as the difference between total and effective precipitation. The methods of Turc and Coutagne were used to calculate total evapotranspiration from land area. These methods were chosen because of their good results on Dinaric karst areas in previous studies. Moreover, these methods express the value of evapotranspiration in one place, which allows objective estimation of evapotranspiration in the studied area with different interpolation methods (Meaški 2011). For the values of total evaporation, the Turc’s simplified formula (1953, 1954) can be used. Mean air temperature and mean precipitation are required as input data. The formula can also be used, with certain modifications, for the calculation of monthly evapotranspiration. √ E T T = P/ 0.9 + (

P 2 ) L

ET T —Turc’s evapotranspiration (mm); P—mean annual precipitation∑(mm); T ∑ — mean annual air temperature (°C); L = 300 + 25 × T + 0.05 × T 3 ; Tp = (Pi/Ti)/ Pi—used instead of T in the equation for L if there are dana for Pi (monthly mean precipitation (mm)) and Ti (monthly mean air temperature (°C)). Coutagne (1954) presented several equations using the mean air temperature and mean precipitation of the studied area as input parameters. The choice of equation depends on the particular conditions determined by the Courtagne parameter λ: E T Ct = P − λP 2 , when 0.125/λ ≤ P ≤ 0.5/λ

(E1)

E T Ct = P, when P ≤ 0.125/λ

(E2)

ˇ I. Canjevac et al.

28

E T Ct = 0.2 + 0.035 × T , when P ≥ 0.5/λ

(E3)

where ET Ct is Coutagne’s evapotranspiration; P is mean annual precipitation (mm); T is mean annual air temperature (°C); λ = 1 / (0.8 + 0.14 × T ). Courtagne’s parameter λ for PLNP was calculated. The results have shown that the condition P ≥ 0.5/λ is valid for the entire study area. Consequently, the third Eq. (E3) was applied. The results of both Turc’s and Courtagne’s methods were spatially displayed at a 25 m grid (Fig. 7). The mean annual evapotranspiration obtained by Turc’s method in the study area varies between 420 and 520 mm. The lower values (less than 440 mm) are characteristic of relatively colder areas, i.e. areas with higher altitude. The higher annual evapotranspiration values (more than 500 mm) are shown in the northwestern parts of the study area, where air temperatures are slightly higher than in the rest of the area. Coutagne’s method gave slightly smaller values of ET, from 400 to 520 mm. The spatial distribution of the values depends mainly on the air temperature, which gives us a very similar distribution to Turc’s method (Meaški 2011). Seasonal changes were analyzed by the modified Turc equation using monthly temperature and precipitation values. The results have shown a very good correlation between the evapotranspiration values and the monthly mean temperature distribution. Winter months have the lowest values of evapotranspiration. In February, the evapotranspiration value is minimum, about 22 mm. The period from May to September has the highest evapotranspiration values, about 53 mm. The differences

Fig. 7 Mean annual evaporation (mm) according to Turc (left) and Coutagne (right) in the area of Plitvice Lakes National Park for the 1961–1990 period (Meaški 2011)

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

29

in evapotranspiration values between meteorological stations are highest in summer and lowest in winter.

3 Groundwater Storage and Circulation The most important surface water occurrences within the PLNP are barrage lakes of various sizes. However, there are also numerous karst springs, of which the most important are the permanent ones: Spring of the Crna rijeka Stream, Spring of the Bijela rijeka Stream and Spring of the Plitvica Stream. Within the PLNP several hydrogeological catchment areas have been identified. The Plitvice Lakes catchment and the beginning of the Korana River catchment are almost entirely within the PLNP, but there are also parts of the following catchments within the PLNP: the Liˇcka Jesenica Stream, the Gacka River, the Koreniˇcka rijeka Stream, the Prijeboj area and Spring Klokot (B&H). Hydrogeological catchments are mostly of a zonal character. However, due to simplified views, they are usually represented as lines. The Plitvice Lakes hydrogeological catchment area (PLC) is the largest catchment within the PLNP, with the surface area of approx. 152 km2 (Meaški 2011). It is a part of the Korana groundwater body (Biondi´c et al. 2013). The PLC is composed of a series of interconnected smaller subcatchment areas where all waters are directed towards the source area of the Korana River (Fig. 8). Those areas are: Matica, Plitvica and Jezera (Lakes) (Meaški 2011). Delineation of the PLC was carried out through the synthesis of research results that included: hydrogeological characteristics needed to define the conceptual position of subcatchments; hydrological and meteorological analyses for quantification of presumed hydrogeological catchments; hydrochemical data and isotopic analysis of groundwater for verification of obtained results. The PLC is a part of the Dinaric karst area in which the dynamic of groundwater is related to the process of karstification of carbonate rocks of Mesozoic age. Geological evolution of the Dinarides is well-researched (Dimitrijevi´c 1982; Herak 1986, 1991; Pami´c et al. 1998; Korbar 2009). Its evolution has had a significant impact on hydrogeological characteristics and water permeability assessments of this area. Five groups of rocks of different water permeability were identified: well-permeable carbonate rocks, medium permeable carbonate rocks, poorly permeable carbonate rocks, deposits of variable water-permeable properties of relatively small thickness and water-impermeable clastic rock (Fig. 9). Data collected in the field, locations of sources and sinks, karst geomorphological features, results of tracing tests, existing geological maps (Polšak et al. 1967; Polšak 1969; Veli´c et al. 1970) and other relevant and available data relating to the hydrogeology of this area (Bahun 1978; Biondi´c and Goatti 1976; Biondi´c 1982; Herak 1962) were taken into consideration during the hydrogeological analyses. Research of the direction of groundwater flows has been carried out on several occasions (Table 2). Until now, 17 tracer tests were carried out in this area (Meaški 2011).

30

ˇ I. Canjevac et al.

Fig. 8 Plitvice Lakes hydrogeological catchment area (PLC) with subcatchments (Meaški 2011)

Spatial and temporal distribution of effective rainfall, runoff and evapotranspiration was carried out to calculate the water balance of the presumed hydrogeological catchment area. Time series of the flow as well as identification of possible changes in the hydrological regime using the Rescaled Adjusted Partial Sums (RAPS) method (Garbrecht and Fernandez 1994) were also considered. It was determined that in the period 1959–2008, exchange of three wet and three dry periods were detected (Meaški 2011; Meaški et al. 2016). The final analysis of the water balance shown that the Matica River flow represents a major contribution of water to lakes system (Table 3); about 74% of water is from the Crna rijeka Stream and about 24% is from the Bijela rijeka Stream. The chemistry of groundwater is often dependent on the source rocks through which the water flows. Due to that, water sampling and laboratory analysis of chemical composition were carried out within the international research project during two hydrological years 2005–2007 (Biondi´c et al. 2008). The comparison of the HCO3 − concentration and the Ca2+ /Mg2+ ratio in source waters revealed that the area is built of predominantly carbonate sediments. Alkalinity values also showed that concentration of HCO3 − ions predominated in comparison with all other ions that constitute the total water alkalinity. The higher concentration of HCO3 − at springs and streams is a consequence of the lack of precipitation of calcite. In downstream parts of rivers and lakes, HCO3 − concentration gradually

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

31

Fig. 9 Hydrogeological characteristics and water permeability pattern in the Plitvice Lakes area (Meaški 2011)

decrease due to the deposition of tufa sediments along the lake system. Due to some differences in the hydrochemistry, it was possible to identified different parts of catchment areas (Meaški 2011). Sampling and analysis of stable isotopes of water were carried out to complete the picture of the natural water system: the identification/verification of the mean recharge altitude of springs and determination of the mean residence time of groundwater (Biondi´c et al. 2008). The content of stable isotopes δ18 O and δD in precipitation revealed strict seasonal changes with enriched content in summer and depleted content in winter months, but also the fact that both winter and summer rainfall proportionally contribute to the supply of karst aquifers—indicating good mixing of water in the aquifer. Previous investigation shown that clouds carried with winds from inland bring approximately 80% of precipitation to the Plitvice Lakes area, while winds from Mediterranean area bring approximately 20% of precipitation (Anthropol.prot 2006). This was visible in the slightly higher dexcess , due to the influence of evaporation occurring in the Mediterranean area (Gat and Carmi 1970; Fontes 1980). Based on the altitude effect on the isotopic composition of δ18 O and δD in precipitation in the region (isotope gradient), the mean altitude of the supply of the main

ˇ I. Canjevac et al.

32

Table 2 Overview of tracing groundwater flows in the wider area of the Plitvice Lakes Area

Date and place of the tracer injection

Altitude (m a.s.l.)

Place of the First tracer detection of appearance the tracer

Apparent velocity (cm/s)

Belac (ponor)

768

Sartuk (left) 12 days

0.12

Pepelarnica (ponor)

767

Kuselj

48 h

0.09

Kuselj (ponor)

758

Mlinište

17 days

0.1

22.09.1978

Kuselj (ponor)

758

Mlinište

17 days

0.1

09.09.1999

Kuselj (borehole)

n/a

Kuselj

15 min

–

ˇ Corkova uvala

15.05.1984

Crno jezero (ponor)

829

Spring of the Plitvica River

48 h

1.37

Brezovac

17.11.1979

Jadova (ponor)

758

Spring of the Crna rijeka Stream

14–17 h

10.79–8.88

Pe´cina

28 h

6.42

Spring of the Crna rijeka Stream

30–36 h

4.99–4.16

Kuselj area 19.05.1977 14.09.1977

Prijeboj

Turjansko & Trnavac

21.03.1980

Brezovac (ponor)

758

Pe´cina

46 h

3.72

28.03.1981

Vrtlovi (vrtaˇca)

n/a

–

–

–

07.05.1982

Uzelaˇcki zavoj (ponor)

780

Spring of the Crna rijeka Stream

68 h

2.08

Pe´cina

Yes (?)

1.95

13.05.1985

Kod bunara (sinkhole)

n/a

–

No

–

16.09.1982

Prijeboj (ponor)

690

n/a

n/a

n/a

07.11.1986

Prijeboj (ponor)

690

Klokot I, Klokot II

12 days

1

Žegar

17 days

1.09

Boreholes near estavelle

No

–

26.04.1988

Vranjkovac (estavelle)

810

(continued)

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

33

Table 2 (continued) Area

Upper part of the Korana River

Vrhovine

Date and place of the tracer injection

Altitude (m a.s.l.)

23.04.2010

724

Trnavac

Place of the First tracer detection of appearance the tracer

Apparent velocity (cm/s)

Klanac

164 h

2.71

Majerovo vrelo

164 h

3

21.04.2005

Rastovaˇca (sinkhole)

515

Klokot I, Klokot II

428 h

1.14

25.09.2007

PB-1 (borehole)

375

Klokot I

443 h

1.12

Gavrani´ca vrelo

571 h

0.31

Baraˇcevo vrelo

679 h

0.26

Zalužnica

306 h

0.88

13.01.1975

Ponor area

Table 3 Calculated hydrometeorological values for the Plitvice Lakes catchment (PLC) compared to measured flow rates (period from 1981 to 2008)

750

Sinac

66 h

5.01

Majerovo vrelo

30 h

9.17

Klanac

42 h

6.68

Tonkovi´ca vrelo

42 h

6.81

Description

% of PLC

Area (km2 )

Subcatchment MATICA

55

84

2.09

Subcatchment JEZERA

18

28

0.784 −0.125

Loses (evaporation, intake…) Subcatchment PLITVICA

27

Qcalc (m3 /s)

41

1.21

springs within the PLC were determined (Table 4). Depletion of heavier isotopes of deuterium and oxygen-18 in spring waters of the Bijela rijeka and Crna rijeka Streams and the Plitvica Stream, indicates a higher altitude of the supply area of these springs. The mean water retention time was estimated by a simplified equation of the exponential flow model, with the introduction of seasonal amplitude (Stichler and Hermann 1983; Clark and Fritz 1997; Geyh 2000). According to the maximum amplitude of δD in precipitation it is between 2 and 3 years, and according to the minimum amplitude of δD is between 1.1 and 1.6 years. Due to that, the average retention time is from 1.5 to 2.3 years.

ˇ I. Canjevac et al.

34 Table 4 Mean altitudes of the main springs within the PLC Description

m a.s.l

Mean δ18 O

h18 O

δD

hδD

Mean altitude

Spring of the Bijela rijeka Stream

710

−10.59

1098

−70.86

946

1022

Spring of the Crna rijeka Stream

670

−10.68

1122

−70.98

950

1036

The Rjeˇcica Stream

750

−10.48

1066

−70.45

933

999

Spring of the Plitvica Stream

606

−10.58

1094

−70.83

945

1020

The obtained water retention time is in accordance with the obtained values of previous research based on the analysis of tritium (Horvatinˇci´c et al. 1986; Krajcar-Broni´c et al. 1986), which determined 1–4 years as a time of water retention for karst springs of the Crna rijeka, the Bijela rijeka and Plitvica streams. The obtained mean water retention time should be taken as a rough estimate, which can serve to better understand the dynamics of water in the Plitvice Lakes basin, but also to help determine the protection of karst springs. More details about results regarding hydrogeological research, water balance determination and hydrochemical analyses within the PLC can be found in Biondi´c et al. (2008), Meaški (2011) and Meaški et al. (2016).

3.1 The Matica River Subcatchment Area The subcatchment of the Matica River includes all surface water and groundwater that drain towards the Matica River (Fig. 8), which flows into Lake Proš´ce. Waters are collected from the wide catchment areas of the Crna rijeka Stream, the Bijela rijeka Stream and the Ljeskovac Stream. Therefore, those are the main hydrogeological parts of the Matica River subcatchment area. The reason for directing groundwater towards the Matica River area is primarily hydrogeological. Namely, the water in its underground flows encounters less permeable dolomite rocks, which have been determined to be of Triassic age. The drainage zone of the Crna rijeka Stream (Fig. 8) in the hydrogeological sense comprises of high and medium water permeable carbonates (Fig. 9). These are mainly limestones and dolomites of Jurassic age, primarily formed during the Lias and Dogger age. Most of the water flows out at the Spring of the Crna rijeka Stream at approx. 680 m a.s.l. (Fig. 10). This is the largest spring (vrelo) in the Plitvice Lakes area that never dries out. The drainage zone of the Bijela rijeka Stream (Fig. 8) in the hydrogeological sense comprises of predominantly low water permeable carbonates (Fig. 9), mainly dolomites of Jurassic age that are primarily formed during the Upper Malm age. Most of the water does not flow out concentrated at one point, but in the wide zone at

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

35

Fig. 10 The Spring of the Crna rijeka Stream. Photo by I. Martini´c

approx. 710 m a.s.l. along the edge of the valley. The largest part of this drainage zone extends northwest, along the divide between the Adriatic and Black Sea catchment areas (Biondi´c and Goatti 1976; Deškovi´c et al. 1981, 1984; Kuhta 2010). Due to a lack of reliable hydrogeological data, there are still some open issues related to the recharge zones of this area. The drainage zone of the Ljeskovac Stream is located between Crna rijeka and Bijela rijeka streams (Fig. 8). In the hydrogeological sense one part of the drainage area is comprised of low water permeable carbonates formed during the Upper Malm age and the one part mainly consist of medium to high permeable limestones formed during the Dogger age (Fig. 9). The outflow zone consists of a series of smaller springs that eventually form a small stream that inflows into the Bijela rijeka Stream (Fig. 11), shortly before the junction of the Bijela rijeka Stream with the Crna rijeka Stream.

3.2 The Plitvica Stream Subcatchment Area The subcatchment of the Plitvica Stream in a simplified hydrogeological sense includes all surface water and groundwater that drain towards the Plitvica watercourse. All waters are collected from the catchment areas of the Sartuk Stream, the

36

ˇ I. Canjevac et al.

Fig. 11 The confluence of the Ljeskovac Stream (left) and the Bijela rijeka Stream (right). Photo by H. Meaški

Plitvica Spring and the Plitvica Stream, which are the main hydrogeological parts of the Plitvica Stream subcatchment area (Fig. 8). The specificity of the Plitvica Stream drainage area is in fact that the part of water outflow from this area via surface watercourse (called the Plitvica Stream) that ends at the Veliki slap Waterfall. After that, all these waters inflow into the area called Sastavci and merges with all waters from the lakes system. Other part of water from the Plitvica Stream sinks into several ponor (swallow-hole) zones along the watercourse and flows underground. This is a problem that has been observed ever since hydrological research in this area began in the late 1970s, particularly during summer dry periods. These loses are most noticeable at the Veliki slap Waterfall (Biondi´c et al. 2010; Meaški 2011). The drainage zone of the Sartuk Stream (Fig. 8) in the hydrogeological sense represents the low water permeable dolomites of Triassic age (Fig. 9). The formation of aquifers was not detected. The source area consists of a series of small springs that seep surface or subsurface towards several points of outflow. Eventually, they form a small watercourse that inflows into the Plitvica Stream, approx. 900 m downstream of the Plitvica Spring. The drainage zone of the Plitvica Spring (Fig. 8) in the hydrogeological sense comprises of the high to medium water permeable limestones formed during the Lower Cretaceous and Dogger age (Fig. 9). The formation of well karstified aquifers in this area is undoubtedly established with tracing test that showed that the main outflow zone is the Spring of the Plitvica Stream (Fig. 12) at approx. 610 m a.s.l.

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

37

Fig. 12 The Spring of the Plitvica Stream. Photo by H. Meaški

3.3 Jezera (Lakes) Subcatchment Area The subcatchment of the Jezera (Lakes) includes all water that flows directly into the lake system. The development of karst aquifers or groundwater flows towards the lake system have not been confirmed by hydrogeological research. The main surface inflows into the lake system are the Matica River and the Rjeˇcica Stream. The other important inflows that should be mentioned is the Sušanj Stream. The Matica River is by far the most significant inflow into the lake system. The area of the lakes in the hydrogeological sense is comprised of low water permeable dolomites of Triassic age (Fig. 13) that are well known base of the Upper Lakes system and the largest part of Lake Kozjak. The drainage zone of the Rjeˇcica Stream represents a low water permeable dolomites of Triassic age. In this area there is a well-developed surface network of streams, without any dominant karst spring and without formation of karst aquifers. All waters are drained towards permanent stream Rjeˇcica that inflows into Lake Kozjak at approx. 535 m a.s.l. and the highest part of the catchment is located at approx. 840 m a.s.l. The drainage zone of the Sušanj Stream mainly consists of lower permeable dolomite of Malm age that seep surface towards a small watercourse that inflows into Proš´cansko Lake. Therefore, this watercourse often dries up. However, shortly

38

ˇ I. Canjevac et al.

Fig. 13 Upper Triassic dolomites near Lake Kozjak. Photo by H. Meaški

before Proš´cansko Lake, there is a spring in Limanska Draga (Fig. 14), which drains a smaller lens of somewhat better water-permeable limestone and thus maintains a constant flow of water.

Fig. 14 Spring of the Sušanj Stream in Limanska Draga. Photo by H. Meaški

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

39

4 Streams Surface water represents approximately 1% of the Park surface, with the total volume of 22.95 million m3 of water (PLNP Management Plan 2019). The most important permanent (perennial) karst springs forming surface watercourses in the area of the National Park are the Spring of the Crna rijeka Stream, the Spring of the Bijela rijeka Stream, and the spring of the Plitvica Stream (Fig. 15). For the lake hydrological system, as mentioned before, main water sources are the Crna rijeka Stream and the Bijela rijeka Stream. The Spring of the Crna rijeka Stream is situated at the foothill of Kik, north of the settlement of Uvalica, and it represents the largest water source in the area of the Plitvice Lakes with average yearly discharge of around 1.5 m3 /s (yearly minimums of around 0.5 m3 /s; yearly maximums of around 7.5 m3 /s). The Crna rijeka Stream is 2.48 km long and does not receive significant surface tributaries before joining with the Bijela rijeka Stream and forming the Matica River in Plitviˇcki Ljeskovac. ˇ The source area of the Bijela rijeka Stream is situated in the area of Cudin klanac, where water originates at the edge of the valley on several locations. In extremely dry conditions, upper part of the watercourse runs dry (PLNP Management Plan 2019). The Bijela rijeka Stream is 4.35 km long (Fig. 16). Along its course it receives numerous small tributaries, e.g. water from the Vukmirovi´ca spring (left tributary) and just before the junction with the Crna rijeka Stream in Plitviˇcki Ljeskovac, right tributary the Ljeskovac Stream. The Bijela rijeka Stream average yearly discharge is around 0.5 m3 /s (yearly minimums around 100 L/s; yearly maximums 1.7 m3 /s). The Crna rijeka Stream as a main contributor, together with the waters of the Bijela rijeka Stream, the Kavga Stream (950 m long; Qav = 0.75 m3 /s) and the Pe´cina Stream (less than 100 m long; Qav = 150 L/s) join together in the settlement of Plitviˇcki Ljeskovac and form the Matica River. The Matica River is the main inflow into Lake Proš´ce, the highest lake of the Plitvice Lakes system. The Matica River is 1.43 km long with average yearly discharge of around 2.43 m3 /s, ranging from the yearly minimum of around 0.85 m3 /s to the yearly maximum of around 9.6 m3 /s. Other bigger streams of the Lake system include the Sušanj Stream and the Rjeˇcica Stream. They are both perennial streams without a strong spring. The Sušanj Stream source area is on the eastern slopes of the Mala Kapela (western part of PLNP). It is around 4.3 km long and empties into Lake Proš´ce in the Limanska draga bay. In the lower part of the course it has a permanent spring (Fig. 14). It has an average discharge of around 0.06 m3 /s. The Rjeˇcica Stream source area is in the eastern part of the PLNP and consists of several hilly brooks which joined form a 6.5 km long perennial stream which empties into southern bay of Lake Kozjak. The Plitvica Stream is not feeding the Lake system but joins the Korana River at the last large tufa barrier of Novakovi´ca brod (Sastavci). It has a strong permanent spring, i.e. vrelo (Qav of around 1 m3 /s) after which it receives the longest left tributary (7.43 km long) the Sartuk Stream (Qav of around 0.1 m3 /s). Due to occasional spilling and sinking of water into subterranean karst in the middle section, the Plitvica Stream

40

Fig. 15 Hydrological gauging stations in Plitvice Lakes National Park

ˇ I. Canjevac et al.

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

41

Fig. 16 The Bijela rijeka Stream. Photo by H. Meaški

has smaller discharge at the mouth, i.e. the Veliki slap waterfall (Qav of around 0.6 m3 /s). The Veliki slap Waterfall, highest in Croatia, is 78 m high and is one of the most attractive sites in the PLNP. The Korana River downstream of Sastavci loses water into subterranean karst and remains without water during dry summer periods already in the canyon part within the PLNP area (PLNP Management Plan 2019). The Korana River downstream of the Korana settlement (exit from the PLNP area) has Qav of around 2.67 m3 /s. Discharge regimes show timing and magnitude of annual maxima and minima of average monthly discharge as a result of dominant feeding of river, i.e. from rainfall, snow or ice melt (Ridanovi´ c 1993). Discharge regimes were calculated for larger streams of the Plitvice Lakes system (Fig. 15) for the period of measurements after the reestablishment of the hydrological stations network following the Homeland War at the end of the twentieth century. Module coefficients (Mc, Pardé 1933), calculated as the ratio between mean monthly and yearly discharge, where used in order to compare regimes of streams of different size (Table 5, Fig. 17). All streams of the ˇ Plitvice Lakes system have a variation of the Dinaric snow-rain regime (Canjevac 2013). Primary, spring maximum occurs in March or April with Mc values above 1.6. Secondary, late autumn maximum occurs in November or December with Mc values of around 1.2. Primary, summer minimum occurs in August with Mc values usually bellow 0.5. Secondary, winter minimum occurs in January with Mc values around 1. Some streams, like the Bijela rijeka Stream and the Rjeˇcica Stream are close to

42

ˇ I. Canjevac et al.

simple regime, without the secondary late autumn maximum and winter minimum. This occurs in some years on other streams as well. Discharge regimes of streams in PLNP are largely determined with the occurrence and duration of snow, i.e. ratio of snow in total precipitation within the year. Moreover, changes in the occurrence and duration of snow in the last twenty years because of temperature increase are the main triggers of change in the discharge regime of ˇ streams in the PLNP (Canjevac et al. 2021). The typical change in discharge regime of streams in the Plitvice Lakes area are shown on the example of the Plitvica Stream (Fig. 18). Discharge regime of the Plitvica Stream in the recent period (2001–2020) has smaller amplitude between yearly maxima and minima. In addition, primary spring maximum (April, May) is not so pronounced and secondary late autumn/early winter maximum (December) has moved to November, i.e. occurs earlier. Spring maximum is less pronounced due to lack of long-lasting snow cover and February and November module coefficients are higher again because of the rain-snow ration favoring rain in the recent period and resulting in higher discharge of previously colder months. General hydrological analysis of Plitvice Lakes has determined a general trend of decreasing quantity of water flowing through the Plitvice Lakes system in the period 1952–1991 (PLNP Management Plan 2019), continuing in the present period. This is caused by presented general trends of increasing mean annual air temperatures and decreasing precipitation (see Sect. 2). The hydromorphological state of several rivers in the PLNP has been assessed ˇ et al. 2021) which within several different projects (Vuˇckovi´c et al. 2019a; Canjevac gives a good impression of general hydromorphological pressures resulting from human activity on streams. The assessment followed an adapted methodology based on European Normative EN 15843: 2010 EU (EN 2010) and guidelines and methodologies for hydromorphological monitoring and measures in Croatia (Croatian waters 2013, 2016). The elements affecting the hydromorphological status of streams are defined by the EU Water Framework Directive (EU WFD 2000) and are divided into three categories: hydrological regime, longitudinal connectivity and morphological features. Grades ranging from 1 to 5 were assigned for each individual element within the three categories. A grade of 1 reflects a natural or near-natural state (i.e. the best condition), while a grade of 5 reflects a significantly altered condition, i.e. the maximum deviation from the natural/reference condition. The total hydromorphological grade is given for the whole water body as well as for each of the three categories. They are calculated as the arithmetic mean of all grades within the category. The hydrological state of the Crna rijeka Stream is very good/almost natural, without significant anthropogenic influences (Table 6). The longitudinal connectivity of the Crna rijeka Stream is not interrupted, which means that sediment flow and species migration within the stream are almost natural. The morphological state of the stream is good, only insignificantly altered. Almost all changes are in the lower part of the stream, just before the confluence with the Bijela rijeka Stream in Plitviˇcki Ljeskovac. In this part of the stream, the banks are moderately altered. The riparian vegetation is partially removed in this area. The erosion-sedimentation processes are

Rodi´c Poljana

Plitvice

Sartuk

Plitvica

2001–2020

Plitviˇcki Ljeskovac

Plitviˇcki Ljeskovac

Crna rijeka

Bijela rijeka

2015–2020

Plitviˇcki Ljeskovac

Kavga

1999–2020

2001–2020

2002–2020

2001–2020

1996–2020

Plitviˇcki Ljeskovac

Luketi´ci

Matica

Korana

Period

2003–2020

Hydrological station

Plitviˇcki Ljeskovac

Stream

Rjeˇcica

0.87

0.98

1.04

1.05

0.93

1.14

0.97

1.02

I

1.02

1.23

1.14

1.19

1.20

1.24

1.13

1.09

II

1.54

1.51

1.50

1.64

1.71

1.67

1.43

1.43

III

1.68

1.60

1.73

1.59

1.29

1.81

1.49

1.46

IV

1.17

1.30

1.48

1.19

1.40

1.35

1.16

1.24

V

0.95

0.75

1.02

0.75

0.84

0.73

0.82

0.91

VI

Table 5 Monthly module coefficients (Mc) for selected streams in the Plitvice Lakes National Park VII

0.44

0.43

0.68

0.51

0.56

0.37

0.53

0.68

VIII

0.39

0.41

0.50

0.45

0.44

0.27

0.52

0.59

IX

0.58

0.57

0.56

0.59

0.48

0.50

0.77

0.66

X

0.81

0.92

0.57

0.73

0.83

0.63

0.90

0.82

XI

1.30

1.09

0.73

1.08

1.07

0.96

1.05

0.98

XII

1.24

1.21

1.04

1.23

1.25

1.33

1.25

1.12

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes … 43

ˇ I. Canjevac et al.

44 module coefficients 2 Rječica Korana Crna rijeka Sartuk

1,8 1,6

Matica Kavga Bijela rijeka Plitvica

1,4 1,2 1 0,8 0,6 0,4 0,2 0 I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Fig. 17 Discharge regimes of selected streams in the Plitvice Lakes National Park

module coeficient 2,5 mc 1980-1991

mc 2001-2020

2

1,5

1

0,5

0 I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Fig. 18 Discharge regimes of the Plitvica Stream at station Plitvice in the 1980–1991 and 2001– 2020 period

moderately altered only in the mentioned part of the stream. The ability for lateral movement of the channel and connectivity to the floodplain are close to natural. The morphology of the upper and middle parts of the stream is largely natural and thus in very good state. All in all, the total average grade of the Crna rijeka Stream indicates a very good hydromorphological state.

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

45

Table 6 Hydromorphological state of the Crna rijeka Stream Stream name

Crna rijeka

Hydromorphological element

Average grade

State

1. Hidrology (hydrological regime)

1.00

Very good

2. Longitudinal connection

1.00

Very good

3. Morphology

1.50

Good

Total average grade

1.38

Very good

The hydrological state of the Plitvica Stream is very good/almost natural despite communal water abstraction in the spring area (Table 7). The longitudinal connectivity of the Plitvica Stream is moderately disrupted. There are few anthropogenic alterations within the stream that potentially negatively impact sediment flow and species migration. The morphological state of the stream is very good, but not without anthropogenic influence. There are several sections of the stream where the banks were moderately altered several decades ago, and the stream was channelized. Reasons for this were the reclamation of arable land in the floodplain and the operation of the local mills. Riparian vegetation is nearly natural. The erosion-sedimentation processes are only insignificantly altered. The ability for lateral movement of the channel and connectivity to the floodplain are nearly natural. Total average grade of the Plitvica Stream shows a very good hydromorphological state. The hydrological state of the Korana River within the National Park is very good, almost natural (Table 8). The longitudinal connectivity is not anthropogenically disturbed. The morphological state of the river is slightly altered. The banks are moderately altered with hard artificial materials. Riparian vegetation is moderately altered, in some sections even significantly. Erosion-sedimentation processes reflect a near-natural state. Lateral connectivity to the floodplain is moderately limited. The potential for lateral bed movement is severely limited in some sections due to bank alteration. Total average grade of the Korana River shows a good hydromorphological state. Table 7 Hydromorphological state of the Plitvica Stream Stream name

Plitvica

Hydromorphological element

Average grade

State

1. Hidrology (hydrological regime)

1.33

Very good

2. Longitudinal connection

3.00

Moderate

3. Morphology

1.17

Very good

Total average grade

1.41

Very good

ˇ I. Canjevac et al.

46 Table 8 Hydromorphological state of the Korana River Stream name

Korana

Hydromorphological element

Average grade

State

1. Hidrology (hydrological regime)

1.00

Very good

2. Longitudinal connection

1.00

Very good

3. Morphology

2.08

Good

Total average grade

1.81

Good

5 Lakes The Plitvice Lakes area consists of 16 named lakes and a number of smaller unnamed ones, formed in the uppermost part of the Korana River basin by the growth of tufa barriers. The lakes are mostly formed on Triassic dolomitic limestones, partly in Cretaceous and Jurassic limestones. The structural-tectonic structure of the Plitvice Lakes area is very complex (Polšak et al. 1976). Dolomitic limestones are less susceptible to the karstification process and therefore less permeable to water than pure limestones, in which underground karst forms such as abysses, pits and caves have developed. The Plitvice Lakes area present a very sensitive and complex karst hydrological system. The dynamic and direction of change in such an environment are defined not only by climate and hydrological conditions but also by biodynamic process of tufa creation which plays a key role in creation or dissaperance of lakes (Pevalek 1935, 1958). Even a seemingly small disturbance by natural or anthropogenic activities can have large and long-lasting consequences. The entire system of the Plitvice Lakes (Fig. 19) is supplied with water by surface streams and numerous springs, but water exchange through underground pathways is also very significant (Ridanovi´ c 1989; Biondi´c et al. 2010; Meaški 2011). Most of the water flows into the lake system of the Plitvice Lakes via the Matica River, which gravitates about 83.8 km2 of the basin, out of a total of 92.7 km2 of the entire basin of Lake Proš´ce, while the interflow of the downstream Lake Kozjak is about 20.8 km2 . These are the two largest lakes in terms of area and volume (together count approximately 89% of the total water volume in the Park), which also influence hydrological conditions in other lakes through their retention effect. According to the geographical position and thermals, the lakes in general belong to the dimictic lakes of the temperate zone. Lakes follow in cascade one after another by the overflow of lake water over tufa waterfalls from the first lake, Proš´ce, at an altitude of 636 m a.s.l. to the last lake, Novakovi´ca Brod (503 m a.s.l.), whose water joins that of the Plitvica Stream and further continues as the Korana River. The lakes are divided into the Upper and Lower Lakes. The Upper Lakes include 12 lakes (Table 9), located on Triassic-age dolomites along the Proš´canski vrh—Labudovac transverse fault, which extends from the northeast to the southwest. The lake system begins with Lake Proš´ce. Lake Proš´ce is the second largest lake after Lake Kozjak, with an area of 0.68 km2 (Fig. 20). The maximum depth of the lake is 38 m (PLNP Management Plan 2019)

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

47

Fig. 19 The 16 lakes of Plitvice Lakes National Park

and the length of the shore is almost 8 km. The area of the topographic catchment of Lake Proš´ce is 37 km2 . It generally belongs to the dimictic lakes of the temperate zone (Mihaljevi´c et al. 2013). Most of the water in Lake Proš´ce comes through the Matica River. In the western part of Lake Proš´ce, the Sušanj Stream flows into the lake in Liman draga bay. The water from Lake Proš´ce falls over tufa barriers into Lakes Ciginovac and Okrugljak. From Lake Okrugljak the water flows into Lake Batinovac, Lake Veliko jezero, Lake Malo jezero and Lake Vir. From these lakes, water flows into Lake

ˇ I. Canjevac et al.

48 Table 9 The 16 lakes of Plitvice Lakes—the Upper Lakes in gray, (PLNP Management Plan 2019)

Name of the lake

Area (ha)

Depth (m)

Lake Proš´ce

68

38

Lake Ciginovac

7

11

Lake Okrugljak

4

13

Lake Batinovac

1

5

Lake Veliko jezero

2

7

Lake Malo jezero

1

9.5

Lake Vir

0.6

4

Lake Galovac

12

24

Lake Milino jezero

0.1

1

Lake Gradinsko jezero

8

10

Lake Burgeti

1

5

Lake Kozjak

82

47

Lake Milanovac

3

18

Lake Gavanovac Lake Kaluderovac

0.9

10

2

14

Lake Novakovi´ca brod

0.3

4.5

Fig. 20 Lake Proš´ce in autumn. Photo from the PLNP archive

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

49

Galovac, the third largest lake in the system. Furthermore, the water flows through Lake Milino jezero, Lake Gradinsko jezero and Lake Burgeti, to final inflow into Lake Kozjak. Lake Kozjak is the largest and deepest of the 16 lakes of PLNP and the last one belonging to the Upper Lakes (Fig. 21). The area of the lake is 0.82 km2 , the maximum depth is 47 m (PLNP Management Plan 2019) and the total length of the lake shore is about 9 km (Martini´c et al. 2019). The area of the topographic catchment is 55 km2 . The lake is in a fault zone at the boundary between Triassic dolomites and Cretaceous limestones (Polšak et al. 1967). Lake Kozjak is largely fed by water flowing from the Upper Lakes through the large tufa barrier of the Burgeti Lake. The Rjeˇcica, Matijaševac and Jasenov potok Streams bring significant amounts of water and sediment from a surrounding terrain to Lake Kozjak (Mihaljevi´c et al. 2013). The water from Lake Kozjak further flows into a series of the Lower Lakes. The Lower Lakes include the last four lakes of the system, Lake Milanovac, Lake Gavanovac, Lake Kaluderovac and Lake Novakovi´ca brod. They were formed on a limestone bedrock of Upper Cretaceous age, in a deeply incised canyon. Coasts are therefore mostly steep and flat (Boˇci´c 2009). The Lower Lakes end with water overflowing from the Lake Novakovi´ca brod as a 25 m high waterfall at a place called Sastavci. At Sastavci, the water from the Plitvice Lakes joins the water from the Plitvica Stream, through an imposing 78 m high waterfall called Veliki slap Waterfall (Fig. 22).

Fig. 21 Lake Kozjak. Photo from the PLNP archive

50

ˇ I. Canjevac et al.

Fig. 22 Sastavci—on the left Lake Novakovi´ca brod, on the upper right the Veliki slap Waterfall. Photo from the PLNP archive

Comparison of average annual flows in the catchment Plitvice Lakes for the period 2002–2017 (Fig. 23) shows that the Matica River is the main water contributor to the lake system (average inflow for the analyzed period is 2.26 m3 /s). Of this water volume, about 67% flows through the Crna rijeka Stream, about 23% through the Bijela rijeka Stream and the remaining about 10% are direct inflows. There is no hydrological station at the outlet of the lake system where flow could be measured. The closest one is at the outlet of Lake Kozjak, where about 2.92 m3 /s flows out. The contribution of the Matica River is about 77%, 15% is the inflow of the Rjeˇcica Stream, and 8% is the contribution of the direct basins of Lake Proš´ce, the Upper Lakes and Lake Kozjak, as well as the inflow of the Sušanj Stream. At the very outlet of the Plitvice Lakes catchment there is no station with a sufficiently long data series (only in 2016 the hydrological station Sastavci was established and in 2021 it was moved downstream at the location Golubnjaˇca), and the analysis was performed for the first downstream station on the Korana River in the Luketi´ci profile. About 2.63 m3 /s flow through this part of the Korana River. Comparison of inflow to the upper reaches of the Korana River (data from Kozjak Bridge and the Plitvica Stream profile) and flow on the Luketi´ci profile shows that significant losses are present on the analyzed section of the Korana River, amounting to 0.964 m3 /s for the period from 2002 to 2017. The largest losses were in 2014 and amounted to 1.68 m3 /s. Only in one year (2010) there were no losses (Radiši´c et al. 2019).

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

51

Fig. 23 Comparison of average annual flows in the catchment area of Plitvice Lakes in the 2002– 2017 period (hydrological stations: 1—Crna rijeka, 2—Bijela rijeka, 3—Matica, 4—Rjeˇcica, 5— Plitvica, 6—Kozjak, 7—Korana-Luketi´ci) (modified from Radiši´c et al. 2019)

A more detailed presentation of the balance relationships can be found in Fig. 24, where a graphical representation of selected input data—precipitation at the Plitviˇcka jezera station, calculated evapotranspiration, water levels of both lakes, water overflow from Lake Kozjak and inflows through the Rjeˇcica Stream and the Matica River, as well as recalculated monthly inflows into both lakes—inflow through fallen precipitation on the lake, inflow from the catchment area (excluding the surface of the lake) and inflow from Lake Proš´ce into Lake Kozjak during the selected characteristic year 2006—is presented. It was found that, as for precipitation, the highest and lowest values of water levels and discharges from both lakes occur in April and October. In the case of inflow in Lake Kozjak it can be seen that the largest amount of water comes from inflows from the Upper Lakes, then follows the inflow of the Rjeˇcica Stream, calculated inflows from the catchment area and a relatively small amount of inflow comes from precipitation directly on the lake surface. Similar relationships are present in the vicinity of Lake Proš´ce, except that in this case the largest amount of water comes from the Matica River. The lakes have a narrow range within the annual water level variation—in Lake Kozjak (Fig. 25) it averages about 70 cm, with the smallest within the annual change between the observed extremes of 36 cm, and the largest of 109 cm. At Lake Proš´ce these differences are much less pronounced. Table 10 shows that during the last 20-year monitoring period, water levels at Lake Kozjak have risen significantly—on average about 16 cm per year, with the highest value in February (22 cm) and the lowest value in the period April-July (12 cm).

52

ˇ I. Canjevac et al.

Fig. 24 Graphical representation of measured precipitation at Plitviˇcka jezera station, calculated evaporation, water levels and overflows, and calculated inflows into lakes Kozjak and Proš´ce (1— calculated inflows of fallen precipitation into the lake, 2—calculated inflows from the catchment— without lake surface, 3—inflows from the Matica River, 4—inflows from the Rjeˇcica Stream, 5—calculated inflows from Proš´ce to Kozjak) during 2006

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

53

Water level (cm) 200 180 160 140 120 100 80 60 40 20 0 1960

1965

1971

1976

1982

1987

1993

1998

2004

2009

2014

2020 Year

Fig. 25 The course of variations of extreme annual water levels in Lake Kozjak (1961–2019)

Table 10 Characteristic monthly and annual water level (cm) at Lake Kozjak for two periods between measurement interruptions 1953–1990 and 2001–2020 I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Year

1953–1990 Avg

83

83

89

99

93

83

75

70

68

72

79

85

82

SD

11

11

13

14

12

10

9

10

12

17

14

15

7

Cv

0.13

0.14

0.14

0.14

0.13

0.12

0.11

0.14

0.17

0.24

0.18

0.18

0.09

Max

98

99

111

132

125

103

99

96

100

122

102

114

98

Min

49

60

49

69

70

63

60

52

44

49

56

51

69

2001–2020 Avg

101

104

110

111

105

96

88

84

85

89

97

103

98

SD

11

12

12

15

12

10

8

9

13

14

12

14

8

Cv

0.11

0.11

0.11

0.14

0.11

0.10

0.09

0.11

0.15

0.15

0.13

0.14

0.08

Max

122

126

127

137

128

118

102

108

131

119

115

123

112

Min

75

72

83

85

84

80

76

70

69

68

66

72

81

Within the annual distribution of mean daily flows at Lake Kozjak hydrological station (outlet of Lake Kozjak), for the period from 1953 to 2019 (excluding 1991– 1999), is given in Table 11. It shows that the average annual flow is 3.24 m3 /s, and that the annual value varies between 1.42 (2011) and 6.38 m3 /s (1962), while within the year the average monthly flows vary between 1.73 m3 /s (in August) and 5.81 m3 /s (in April). The lowest extreme flow value of 0.248 m3 /s was recorded in April 1959, and the highest of 30.6 m3 /s in May 2019.

II

III

12.2

0.716

MAX

MIN

9.10

8.51

0.995

Max

Min

0.49

0.45

Cv

3.54

0.755

15.4

0.960

1.72

3.28

1.49

Avg

SD

0.924

24.0

1.23

12.0

0.52

2.33

4.50

1953–2019 (excluding 1991–1999)

I

0.248

26.2

1.88

19.6

0.56

3.25

5.81

IV

0.943

30.6

1.27

11.0

0.49

2.17

4.38

V

0.758

25.1

0.938

8.12

0.44

1.33

3.00

VI

0.602

10.6

0.775

4.47

0.36

0.746

2.08

VII

0.602

19.2

0.685

5.40

0.53

0.926

1.73

VIII

0.569

25.7

0.678

8.79

0.72

1.27

1.76

IX

0.534

28.9

0.674

12.6

1.02

2.31

2.27

X

0.516

25.1

0.648

7.91

0.62

1.84

2.98

XI

0.572

22.4

0.905

12.5

0.67

2.53

3.77

XII

0.25

30.6

1.42

6.38

0.33

1.07

3.24

Year

Table 11 Characteristic monthly and annual discharge values (m3 /s) at Lake Kozjak hydrological station in the 1953–2019 period (excluding 1991–1999)

54 ˇ I. Canjevac et al.

Hydrology, Hydrogeology and Hydromorphology of the Plitvice Lakes …

55

Fig. 26 Mean annual water levels on Lake Proš´ce and Lake Kozjak in the 1952–2019 period

The course of the mean annual water level (Fig. 26) and discharge recorded only at Lake Kozjak (Fig. 27) are interesting. From the annual trend of mean water levels, it can be seen that in the period until the cessation of monitoring due to the Homeland War (1990), the trend of mean annual levels was ascending, more pronounced at Lake Proš´ce (11.2 cm/10 years) than at Lake Kozjak (3.7 cm/10 years). After the renewal of monitoring, water levels at both lakes have a slight descending trend, more pronounced at Lake Proš´ce (2.4 cm/year) than at Lake Kozjak (1.2 cm/year). One possible reason for this is the increasing occurrence of high waters, which leads to the process of erosion of tufa barriers. Indeed, the lakes change their morphology through the process of growth and erosion of tufa barriers, and such changes generally take place very slowly and with annual changes mostly in the order of 1 cm. The growth process of the tufa barriers is generally faster than the sedimentation process on the lake bottom, and therefore the lake system rises above the base, and the erosional dynamics of the tufa barriers are increasingly affected by the increasing hydrostatic pressure and hydrodynamic effects of the overflowing water. Therefore, periodically (on both a long-term and geological time scale) abrupt changes occur in the form of cracking, fracture and erosion of tufa barriers, followed by significant morphological changes in the geometry of the lake system and changes caused by a range of abiotic and biotic factors. In case of a sudden collapse of high barriers, lake sediment is set in motion, moved and mixed with sediment from lower lakes. On the other hand, less dramatic changes take place during the occurrence of floods in the form of fine erosion of insufficiently consolidated parts of the tufa barriers. Thus, at the end of March and the beginning of April 2018, there was a collapse of tufa barriers at Lake Milino jezero, due to the coincidence of two successive flood peaks only 14 days apart, and whose peak discharges of about 18 m3 /s had the

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Fig. 27 Mean annual discharge on the outlet of Lake Kozjak in the 1953–2019 period

character of about 20-year return period flood (Gradevinski fakultet u Rijeci 2019). In terms of magnitude of impact, this was by far the most significant in relation to all similar events since the establishment of PLNP. It caused the diversion of water flows and their channelization through the area of the broken part of the barrier, where additional intensification of erosion and destruction of the barrier occurred due to the concentration of the flow and increase in the speed of the water flow. On the other hand, part of the previously wet barrier left without water. In the paper Radiši´c et al. (2019), based on the analyzed trends of average annual water levels, it was found that the average lowering of the barrier in the period 2001–2017 was 0.59 cm/year, while for the period from 1952 to 1990 the average increase of the same barrier was 0.56 cm/year (Zwicker and Rubini´c 2005). The probable reason for these changes is the increased values of maximum runoff recorded in recent years, which with their kinetic energy potentiate the erosion processes of both the tufa barriers themselves and the vegetation on them (Radiši´c et al. 2019). In the 2017–2019 period, the project “Development of a methodology for hydromorphological status assessment in lakes and implementation of hydromorphological monitoring” was carried out, within the framework of which the hydromorphological status of Lake Proš´ce and Lake Kozjak was assessed (Vuˇckovi´c et al. 2019b). The assessment and evaluation of the status of individual hydromorphological elements was carried out by desktop and field surveys using an adapted methodology according to the European standard EN 16870: 2017 (EN 2017). Elements affecting the hydromorphological status of lakes are defined by the EU WFD (2000) and include, for lakes, hydrological characteristics (including hydrological regime, water volume, retention time, groundwater connection) and morphological characteristics (including morphometry, bedforms and landforms, substrate, continuity,

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riparian vegetation and land cover). The zones of a lake were determined and were evaluated separately according to the hydromorphological element (Fig. 28). Each hydromorphological category (hydrology and morphology) was divided into several elements, which were evaluated (graded) individually, with a grade from 1 to 5, where 1 represents very good state, and 5 represents very bad state. The average grade from all elements gave the final grade of hydromorphological state of the lake. The same principle was used to determine the average grade for the hydrological and morphological state was given. The hydrological characteristics of both Lake Kozjak and Lake Proš´ce are in a very good condition (Tables 12 and 13). Their retention time, connection to groundwater, stratification and water mixing are not disturbed. Water levels of Lake Proš´ce are not under any anthropogenic influence. Lake Kozjak is under the influence of water abstraction, which does not disturb significantly the conditions in the lake. On average, less than 1.7% of the water is withdrawn from the lake for the communal water supply, in maximum water intake is 52 L/s.

Fig. 28 Lake zones used in the assessment of hydromorphological state

Table 12 Hydromorphological state of Lake Kozjak Lake name

Kozjak

Hydromorphologic element

Lake zone

Grade

Hydrology

Whole lake

1.00

Morphology

Average grade

Open zone

1.0

Lakeshore

1.5

Lakeshore zone

2.5

Watershed

1.5

Whole Lake

1.46

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58 Table 13 Hydromorphological state of Lake Proš´ce Lake name

Proš´ce

Hydromorphologic element

Lake zone

Grade

Hydrology

Whole lake

1.00

Morphology

Open zone

1.0

Lakeshore

1.5

Lakeshore zone

2.5

Average grade

Watershed

1.0

Whole Lake

1.38

1.55

The morphology of both lakes is in good condition. The bedforms and bed-forming processes of the open water zone are not disturbed. The lakeshore and lakeshore zone of both lakes are areas of highest anthropogenic pressure. In this regard, the lakeshores are slightly modified with artificial materials and stone blocks. The lakeshore zones (riparian zones) are moderately modified (Figs. 29 and 30). Due to the existence and maintenance of tourist trails and roads the cross-sections of lakeshore zones are altered, and the riparian vegetation is modified. They are also under the influence of artificial materials such as concrete and asphalt. Such changes affect sediment transport from surrounding areas into the lakes. The watershed area of Lake Kozjak and thus also of Lake Proš´ce is predominantly covered by forests and other natural and semi-natural areas such as meadows, bushes and abandoned agricultural land (National habitat map 2016). The average grade of both Lake Kozjak (1.46) and Lake Proš´ce (1.38) shows that they are both in a very good hydromorphological condition. The hydromorphological state of other lakes is very similar to that of Lake Kozjak and Lake Proš´ce. Hydrology is not disturbed, while tourist trails and interventions on the lakeshore represent the greatest morphological influence and pressure. Tourist trails could also potentially pose a threat to the tufa barriers, as some of the trails are built on them.

6 Conclusion The entire hydrological system of the Plitvice Lakes is supplied with water by surface streams and numerous springs. Water exchange through underground pathways plays very significant and yet not completely understood role (for example the Spring of the Bijela rijeka Stream). General hydrological analysis of the Plitvice Lakes has shown a trend of decreasing quantity of water flowing through the Plitvice Lakes system since 1950s. This is caused by presented general trends of increasing mean annual air temperatures and decreasing precipitation. Changes in the occurrence and duration of snow in the last twenty years because of the temperature increase are the main triggers of change in the discharge regime of streams in the PLNP in

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Fig. 29 Types of lakeshore and lakeshore zones of Lake Kozjak

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Fig. 30 Types of lakeshore and lakeshore zones of Lake Proš´ce

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the recent period. Changes in discharge regimes of rivers feeding the system are evident through the redistribution of discharge within a year, lower spring primary and earlier late autumn/winter secondary maximums which has consequences for the complete Plitvice Lakes environment. In this fragile system interaction of surface and underground hydrological processes presents a key connecting factor so water monitoring and research should be continued and enforced. Hydromorphological state of both streams and lakes is generally very good which is expected for protected areas. Nevertheless, special attention should be payed to longitudinal connectivity in streams which could be reestablished given that most of the mills are out of operation for decades. The influence of human activities on streams and lakes in the PLNP should be considered, assessed and protection measures should be enforced in order to reduce and put to minimum pressure resulting from intensive touristic activity in the area. In addition, consequences of changed physical–chemical properties of water (especially resulting from wastewater) as well as climate elements should be considered and monitored. With climate elements change and expected more often extreme events occurrence, morphological changes of the lake system could be more intensive. This must be considered in future protection and management of this unique natural system.

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Water Chemistry Andrijana Brozinˇcevi´c, Maja Vurnek, and Tea Frketi´c

Abstract Freshwater chemistry is quite variable and depends on many factors, such as the type and hydrological characteristics of the water body, the geological background, the type of climate, and the biological and physico-chemical processes that take place in it. Plitvice Lakes National Park is a unique and complex freshwater barrage lakes system characterized by the deposition of carbonates in the form of tufa and fine-grained lake sediment. Analysis of long-term monitoring data of physicochemical parameters from seventeen sites on the longitudinal profile of the water system of Plitvice Lakes, the main characteristics and spatial and temporal variations of water chemistry were determined. The waters of Plitvice Lakes are oligotrophic with low content of nutrients and dissolved organic matter, dissolved oxygen concentrations around saturation and relatively high values of hardness and alkalinity. The karst springs are characterized by relatively constant values of all monitored parameters without seasonal variations, while some seasonal dynamics were observed at the tributaries depending on air temperature and catchment characteristics. The water chemistry of the lotic biotopes at the tufa barriers and the Korana River differs significantly from the springs and tributaries. Temporal and spatial variations are the result of seasonal changes in air temperature, biological processes, hydrodynamics of the barrage lake system, and intensive deposition of tufa and lake sediment in the lake system. Keywords Plitvice Lakes · Physico-chemical parameters · Tufa deposition · Water monitoring · Barrage lakes · Long-term data

1 Introduction In natural waters, organisms and their abiotic environment are interconnected, interact with each other, and determine how the aquatic ecosystem functions. Therefore, biology and geology are necessary in addition to water chemistry when A. Brozinˇcevi´c (B) · M. Vurnek · T. Frketi´c Public Institution National Park Plitvice Lakes, Josipa Jovi´ca 19, 53231 Plitviˇcka Jezera, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_3

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analysing and interpreting chemical reactions and processes that affect the distribution and circulation of chemical components in natural waters (Stumm and Morgan 1996). Freshwater chemistry is quite variable, in rivers generally more so than in the lakes. Natural spatial variation is mainly determined by the type of rocks available for weathering, the type of climate, and the composition of rainfall, which in turn is influenced by proximity to the sea. Running water chemistry temporally changes under multiple influences e.g. seasonal changes in discharge regime, precipitation intake and biological activity (Allan and Castillo 2007). Flow is the most important factor that distinguishes streams from lakes. Lack of flow enables forming of a vertical gradient in lakes, Streams, on the other hand, have a longitudinal gradient from origin to end (Lampert and Sommer 2007). Lakes are an integral part of the global hydrological system. Because they interact with atmospheric water, surface water and groundwater, lake hydrology is influenced by both their physiographic and climatic characteristics (Winter 2004). The water system of the Plitvice Lakes National Park is very complex, consisting of karst springs, numerous tributaries, and a system of 16 larger lakes and a number of smaller ones, which at their end form again the course of a river, the Korana River. The lake system is interconnected by larger and smaller tufa barriers through which the water flows, forming a special type of lake, the tufa barrage lakes (Kosti´c-Brnek and Brnek-Kosti´c 1974) (Fig. 1). These complex dynamics influence water properties, such as temperature, flow velocity, amount of dissolved gasses and distribution of inorganic and organic substances in the water. The geological background of the lake catchment area consists of carbonate rocks (dolomites and limestones), which significantly influences the chemical composition of the water (Srdoˇc et al. 1985; Biondi´c et al. 2010). Among the many physical, chemical, and biological processes that occur in water, for the Plitvice Lakes system the most significant is the process of precipitation of calcium carbonate, i.e. calcite in the form of tufa and fine-grained lake sediment. Tufa represents the product of calcium carbonate precipitation under the cool water (near ambient temperature) regime and typically contains the remains of micro and macrophytes, invertebrates and bacteria (Ford and Pedley 1996). The deposition of tufa and lake sediments is responsible for the formation, appearance and viability of the entire lake system (Srdoˇc et al. 1985). The interaction of atmosphere, soil, geological substrate and water is necessary for the process of calcite deposition to occur. By percolation through the soil, surface water is enriched with CO2 produced by the decomposition of organic matter in the soil (humus). Therefore, water readily dissolves calcium carbonate from carbonate rocks, forming a solution of calcium and bicarbonate ions. Such groundwater, oversaturated with CO2 , rapidly loses most of its dissolved CO2 when it reaches the surface, resulting in an oversaturation of the water with magnesium and calcium carbonates, which is a precondition for the deposition of calcite (Stumm and Morgan 1996). Studies of the process of tufa deposition at Plitvice Lakes by Srdoˇc et al. (1985) showed that for the deposition of tufa and lake sediment the following basic chemical conditions should be met:

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Fig. 1 Part of the barrage lake system at the Upper Lakes. Photo by Ž. Renduli´c

1. Oversaturation of water with calcium carbonate, i.e. saturation index (Isat ) > 3 (in places where tufa formation is very intense: Isat = 5–7) 2. The pH value of the water above 8.0 (in watercourses where tufa formation is intense, the pH is between 8.2 and 8.4) 3. The concentration of dissolved organic matter in the water lower than 10 mg C dm−3 . Emeis et al. (1987) and Chafetz et al. (1994) showed that the deposition of calcite and tufa in Plitvice Lakes is the result of a chemical process that is biologically induced by the action of algae and cyanobacteria, which through their photosynthetic activity, influence the water chemistry and thus promote deposition. The study by Srdoˇc et al. (1985) also showed that calcite is deposited on all natural substrates (leaves, macrophytes, wood) and on artificial substrates (plastic sheets), but not on copper mesh. Thus, copper, which prevents the growth and development of algae and microbial communities in general, and causally prevents the deposition of calcite. In addition to copper, dissolved organic carbon, which is rich in amino acids that react with calcite, has been found to inhibit calcite deposition. It has also been found that deposition occurs throughout the year, but with much less intensity in winter when the deposition process is inorganic only. It is the combination of carbonate bedrock, physico-chemical properties of water and biota in the Plitvice Lakes area that determines the formation of tufa (Srdoˇc et al. 1985; Plenkovi´c-Moraj et al. 2002). The rate of sediment deposition in surface waters is significantly higher (5–10 times) than sediment deposition in lakes (Chen et al. 2004). Sediment growth shows significant seasonal differences (higher growth in summer months), and

68

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the flow velocity of watercourses where precipitation occurs also has a significant influence. At higher flow velocities (in winter and spring) partial erosion occurs, while at calm waters there is insufficient aeration, which slows down the process of carbonate deposition (Horvatinˇci´c et al. 2014). In such a flow barrage system, the chemistry of the water and the processes occurring at the lake outlet tufa barriers are influenced by vertical gradients and the dynamics of the upstream deep lakes. The two largest and deepest lakes in the system, Lake Kozjak and Lake Proš´ce, develop summer stratification and reverse winter stratification and mix twice a year (in late autumn and early spring), hence we call them dimictic lakes (Habdija et al. 2011; Horvatinˇci´c et al. 2014; Klai´c et al. 2020). During stratification, there is no vertical mixing of water as epilimnion, metalimnion with thermocline and hypolimnion develop, so only the surface water layer or epilimnion water moves (Löffler 2004). Due to the lack of mixing of the water layers, hypoxia and anoxia can occur in mesotrophic and eutrophic lakes due to the increased decomposition of organic matter in the hypolimnion (Lampert and Sommer 2007). Plitvice Lakes, like all aquatic ecosystems, are subject to eutrophication, the process of enriching water with elevated concentrations of limiting nutrient salts. Eutrophication, otherwise a natural process that lasts for millennia, can be significantly accelerated by anthropogenic activities (cultural eutrophication), which can quickly lead to aquatic ecosystem degradation. Nutrients can enter the aquatic system through agricultural land, wastewater, detergents, industry, etc. (Ansari et al. 2011). Eutrophication can also be caused by climate change, which has recently become more common (Dokulil et al. 2010). The whole karstic environment of Plitvice Lakes, including water, sediment, soil and air, is very sensitive to any kind of pollution. Although the Plitvice Lakes area is protected as a national park, the lake system as a tourist destination is under pressure from various aspects of anthropogenic impacts. Unintentional pollution with wastewater from surrounding settlements, tourist facilities or from roads is a constant threat of pollutant input. Heavy metal contaminants or organic pollutants have so far been found in lake sediments, water, and biota in relatively low concentrations or in traces (Mikac et al. 2011; Vukosav et al. 2014; Dvorš´cak et al. 2019). Although the waters of Plitvice Lakes are still oligotrophic and the process of tufa deposition is still active, constant monitoring of the aquatic ecosystem is necessary to detect any possible changes in time and to preserve the process of tufa deposition. In this chapter, we provide an overview and analysis of long-term water monitoring data over the longitudinal profile of the aquatic system Plitvice Lakes. These data represent the longest continuous data series on physico-chemical indicators of water and can provide a quality insight into spatial and temporal variations of water chemistry.

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2 Study Area and Monitored Parameters The surface water system of Plitvice Lakes consists of karst springs, tributaries, a barrage lake system with tufa barriers, and the Korana River. The surface water descends from 720 m a.s.l. (spring of the Bijela rijeka Stream) to 368.6 m a.s.l. (Korana River), crossing the area of Triassic dolomites into an area composed of Cretaceous limestone. The components of such a complex water system have their own hydrodynamics and each of them responds in a specific way to climatic conditions, different environmental and anthropogenic influences, which manifests itself in spatial and temporal variations of water chemistry. Research and monitoring of physico-chemical parameters is the best way to understand these variations as well as the processes occurring in the aquatic ecosystem. In this chapter, we analyse long-term monitoring data of surface water at seventeen sites of the longitudinal profile of the lake system conducted by the Plitvice Lakes National Park Conservation Service. In the longitudinal profile, length ~12 km, the first three sites (1–3) are the main karst springs, the next six sites (4–9) comprise the main tributaries of the lake system, while seven sites (10–16) relate to lotic biotopes at tufa barriers, i.e. lake outlets. The last site (17) is the outlet from the lake system of Plitvice Lakes, the Korana River (Fig. 2).

Fig. 2 Map of the monitoring sites

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Monitoring of physico-chemical parameters at surface waters has been conducted monthly at most sites since 2006, while four sites (3, 8, 9, and 17) were added to the monitoring program in 2012. Mean monthly data were analysed for the period from 2006 (i.e., 2012) to 2020 for each parameter and for each monitoring site. The overall mean long-term monitoring data (for the period from 2006 to 2020) for each parameter at each monitoring site were also analysed to determine the spatial dynamics of each parameter. Therefore, data expressed as mean monthly values and overall mean values were calculated for the entire monitored period. Data were collected through field measurements and laboratory analyses. Field measurements of temperature (T), pH and electrical conductivity (κ) were performed using a digital multimeter (WTW—Wissenschaftlich-Technische Werkstätten, Germany). In the laboratory, the following parameters were analysed: dissolved oxygen (DO), alkalinity (Alk), total hardness (TH), calcium hardness (CaH), chemical oxygen demand with potassium permanganate (COD–KMnO4 ), orthophosphate (PO4 3− – P), nitrate nitrogen (NO3 − –N), nitrite nitrogen (NO2 − –N) and ammonia nitrogen (NH4 + –N). Magnesium hardness (MgH) was calculated as the difference between total and calcium hardness. Laboratory analyses were performed according to standard analytical methods (APHA 2005; Brozinˇcevi´c and Vurnek 2011; Vurnek et al. 2021).

3 Spatial and Temporal Variations of Water Chemistry 3.1 Temperature Stream water temperature depends on latitude, water volume and depth, and the presence of aquatic vegetation. The water temperature value of streams does not vary significantly along the vertical axis of the water column except, for example, in cases where there is a large inflow of groundwater. Similarly, the water temperature values as well as their variations differ from each other between the upper, middle and lower parts of the river flow (Allan and Castillo 2007). According to the mean monthly temperature data, there is a difference in seasonal and temporal variations between different types of sites. The karst springs (1–3) show constant temperature throughout the year, while the temperature data of tributaries (4–9), tufa barriers (10–16) and Korana River (17) show typical seasonal fluctuations characteristic of surface waters of continental areas, with minimum temperature values in winter, gradual increase and decrease in spring and autumn and maximum in summer (Fig. 3). The temperature at the point of emergence of a spring is determined by the mean annual temperature of the drainage area and varies annually by only a few tenths of a degree Celsius. The overall mean long-term temperature for the spring of the Bijela rijeka Stream is 7.97 ± 0.48 °C, for the spring of the Crna rijeka Stream 8.05 ± 0.54 °C and for the spring of the Plitvice Stream 7.55 ± 0.21 °C. Such constant values

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Fig. 3 Mean monthly temperature values at monitoring sites for the monitored period 2006–2020 (see Fig. 1 for site IDs)

are characteristic of karst springs and are an indicator of good mixing of seasonal inflows in the aquifers (Srdoˇc et al. 1985; Ford and Williams 2007). At tributaries, downstream from springs, water temperature is influenced by air temperature and solar radiation. In winter, temperatures at the tributaries are lower than the mean monthly temperatures at the springs, while they increase during the warmer season. The greatest variation among tributaries was measured at Sartuk Stream, for which the lowest mean monthly temperatures were recorded in January (3.62 ± 1.77 °C) and the highest in August (15.87 ± 1.26 °C). Other tributaries show less variation, such as the Crna rijeka Stream which has mean monthly temperatures ranging from 7.3 ± 0.4 °C in January to 9.38 ± 0.69 °C in August, and the Rjeˇcica Stream, from 5.62 ± 1.38 °C in February to 10.82 ± 0.44 °C in August. Such differences between tributaries occur due to various factors such as water depth, flow rate, shading of streams, etc. (Fig. 4). The greatest variations in mean monthly temperatures are found in the downstream flow at the lake outlet tufa barriers and at Korana River. The lowest mean monthly temperatures were recorded in February at the Lake Kozjak outlet tufa barrier (3.85 ± 1.61 °C), where the highest mean temperatures were also recorded in August (21.46 ± 1.17 °C). Such large seasonal variations in water temperature at lake outlet tufa barriers are a consequence of the greater influence of air temperature and solar radiation on large surface layers of lakes compared to streams. Seasonal temperature variations affect the solubility of calcium carbonate, the rate of diffusion of CO2 from the water, and the rate of other chemical and biological processes (Srdoˇc et al. 1985). The location of the tufa barrier below the Veliki slap Waterfall does not show such large fluctuation in mean monthly temperature (from 4.48 ± 2.57 °C in February

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Fig. 4 The Rjeˇcica Stream—dynamic change of water depth, flow rate and shading. Photo by A. Brozinˇcevi´c

to 13.98 ± 1.87 °C in July), but is more similar to the temperature regime of the tributaries Plitvica Stream and Sartuk stream, probably because it is not a lake outlet, but forms at the end of the Plitvice Stream, after a 63 m high waterfall (Fig. 5). Analysis of temporal and spatial variations shows the influence of air temperature on water temperature, especially at downstream sites: tufa barriers and the Korana River. When comparing the temperature data of the two study periods (1981–1986 and 2010–2014), an increase in mean annual water temperatures of 1.5 °C for lakes and 0.4 and 0.6 °C for the springs of Crna rijeka and Bijela rijeka Stream was observed (Sironi´c et al. 2017). An increase in mean annual air temperatures of 0.06 °C per year for the period 1986–2019 was also found (Krajcar Broni´c et al. 2020). On the other hand, statistical analyses of water monitoring data for the period 2006–2017 showed negative trends for springs and streams and positive trend for lotic biotopes, however these trends were not statistically significant (Vurnek et al. 2021). Significant changes in mean water temperatures are expected in the future due to global increases in air temperature and climate change. An increase in water temperature can affect the increase in primary production in lakes, changes in lake trophicity, and a range of physicochemical and ecological processes controlled by temperature (Fritz 1996; Clarke 2009). It is clear that water temperature will need to be monitored in the future to detect changes and better understand trends.

Water Chemistry

Fig. 5 The tufa barriers below the Veliki slap Waterfall. Photo by A. Brozinˇcevi´c

73

74

A. Brozinˇcevi´c et al.

3.2 Dissolved Oxygen Differences in the DO concentration in different seasons result mainly from the dependence of the solubility coefficient on temperature, which is inversely proportional to water temperature (Allan and Castillo 2007). The oxygen content of unpolluted mountain streams is close to 100% relative saturation because of a constant exchange of gases between the atmosphere and the water, enhanced by turbulence (Lampert and Sommer 2007). At springs, where water temperature is relatively constant throughout the year, there is also no major variation in the DO concentration (Fig. 6). Mean monthly concentrations for all three karst springs range from 9.43 to 10.89 mg O2 L−1 . Although there is no major seasonal variation, higher concentrations were measured during the warmer season. This can be explained by the porosity of the terrain, which allows precipitation to enter the groundwater, which is usually more abundant during the winter months. Winter precipitation, especially snow, dissolves large amounts of oxygen and thus infiltrates into the groundwater, enriched with oxygen, which comes to the surface after a certain time. At springs, groundwater that has reached the ground in winter comes to the surface in the spring and summer months, so DO concentrations are somewhat higher at these sites during this period (Srdoˇc et al. 1985). Seasonal changes in DO were also observed at tributaries that exhibit seasonal variation in temperature. Higher concentrations were measured during the winter months, while decreasing during the summer months. As with temperature, the greatest variation was recorded at Sartuk Stream, which had the highest mean monthly concentration in January (11.92 ± 0.99 mg O2 L−1 ) and the lowest in August (9.27 ± 0.56 mg O2 L−1 ). The other streams also show this trend, but with somewhat smaller intervals. In summer, downstream, at the sites of lake outlet tufa barriers and the Korana River, there is a significant decrease in concentration due to a greater warming of the water, while in winter the opposite occurs, i.e., the water is cooled coming from the springs and therefore the oxygen concentration increases. These characteristic changes have been noted in previous analyses (Srdoˇc et al. 1985; Sironi´c et al. 2017; Vurnek et al. 2021). Comparing the overall mean long-term values of DO at some monitoring sites for the period 2012–2020 with the research data from the period 1982–1985 (Srdoˇc et al. 1985), one can observe a decrease in the value of DO in the recent period, especially at the sites of lotic biotopes at the tufa barriers and the Korana River (Table 1), probably as a result of increasing temperatures at these sites. Changes in DO concentration were also recorded in the vertical gradient of Lake Proš´ce. Due to summer stratification in the hypolimnion of the lake, the concentration of dissolved oxygen decreases so that hypoxia occurs, followed by anoxia at the greatest depths. This lake shows the first signs of natural eutrophication and can be classified as an oligotrophic lake transitioning to mesotrophy (Habdija et al. 2011; Horvatinˇci´c et al. 2014).

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Fig. 6 Seasonal variations (mean monthly values for the period 2006–2020) of dissolved oxygen at the monitoring sites

Table 1 Overall mean long-term values of dissolved oxygen for the period 2012–2020 and the period 1982–1985 at monitoring sites (Mean ± σ —overall mean long-term values plus/minus the standard deviation)

Monitoring sites

DO/ mg O2 L−1 2012–2020

1982–1985

Mean ± σ

Mean ± σ

1

10.48 ± 0.69

11.06 ± 0.41

2

10.00 ± 0.79

10.74 ± 0.26

3

10.45 ± 0.75

10.79 ± 0.53

6

10.54 ± 0.78

11.25 ± 0.54

7

10.26 ± 0.84

10.48 ± 0.79

9

10.36 ± 1.12

10.99 ± 1.86

14

9.78 ± 1.20

10.57 ± 1.20

15

9.77 ± 1.25

11.20 ± 1.76

17

10.13 ± 1.27

11.77 ± 1.96

3.3 pH The concentration of hydrogen ions (H+ ), expressed as pH, is very important both chemically and biologically because it determines the acidity of water (Allan and Castillo 2007). The pH of water is also one of the key factors in the process of calcium carbonate precipitation or tufa depositing process. The interdependence of pH and

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CO2 results in a decrease in pH at higher concentrations of dissolved CO2 , i.e., an increase in pH due to the degassing of CO2 from water (Allan and Castillo 2007). Mean monthly water pH values at all monitored sites ranged from 7.38 to 8.5. Similar to temperature and DO, spatial variations are evident with respect to different site types (Table 2). The pH is lowest at karst springs with relatively constant mean monthly values throughout the year (7.38–7.86), i.e. it shows no seasonal variation. Groundwater coming to the surface is enriched with CO2 , which causes low pH values at the springs (Allan and Castillo 2007). In the studies by Srdoˇc et al. (1985) and Kempe et al. (1985), the highest CO2 concentrations were found at springs, corresponding to the lowest pH values at these sites. Due to such low pH values in the spring area, carbonate precipitation does not occur, i.e. the process of tufa deposition does not take place. Very soon after the source, there is rapid degassing of CO2 so the pH of the water at the tributaries increases. Downstream in the system of lakes, tufa barriers and the Korana River, there is a further gradual decline in the CO2 concentration in the water (Kempe et al. 1985; Srdoˇc et al. 1985; Bareši´c et al. 2011). Monitoring data show that pH in all streams increases above 8.0 shortly after the springs and shows no seasonal dependence. Mean monthly pH values for Sartuk Stream were in the range from 8.22 (July) to 8.50 (February), and compared to the other tributaries were higher, which may be a consequence of terrestrial material input i.e. limestone leaching (Bareši´c 2009). There is a further increase in pH in the lake system, downstream from the tufa barriers in the Upper Lakes system, through those in the Lower Lakes system to Korana River (for monitoring sites from 10 to 17). For these monitoring sites mean monthly values for the monitored period range between 8.11 and 8.46. The downstream increase in pH is a result of increasingly intense deposition of carbonates in the form of tufa and fine-grained lake sediment and the release of CO2 from the water. At such sites, some seasonality is observed, as all sites show a very slight decrease in pH in summer, possibly related to increased water temperatures that affect biological activities such as respiration and dissolution processes of calcium carbonate and other minerals, as they can increase the concentration of dissolved carbon dioxide (Allan and Castillo 2007; Lampert and Sommer 2007).

3.4 Alkalinity A measure of the ability of water to absorb OH− and H+ ions without changing the pH, i.e., the buffer capacity of an aquatic system, is called water alkalinity, which is of great importance to aquatic organisms (Allan and Castillo 2007). Aquatic habitats on sedimentary substrates, especially carbonate substrates, are rich in carbonates with good buffer water capacity. Here, alkalinity is defined by the carbonatebicarbonate water balance CO2 –HCO3 − –CO3 2− in which HCO3 − (bicarbonate) is involved as a very important anion. Groundwater oversaturated with CO2 dissolves mineral carbonates and becomes rich in bicarbonate. Often alkalinity is measured as

7.62

8.44

8.50

8.20

8.24

8.35

8.24

7.38

7.50

8.21

7.99

8.12

8.10

8.37

8.41

8.25

8.27

8.29

8.26

8.32

8.35

8.38

8.43

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

8.30

8.24

8.24

8.12

8.13

8.26

8.20

8.27

7.66

7.86

Feb

7.66

Jan

pH

1

Monitoring sites

8.45

8.43

8.40

8.32

8.32

8.32

8.33

8.25

8.43

8.39

8.20

8.13

7.97

8.28

7.66

7.43

7.65

Mar

8.36

8.38

8.38

8.32

8.29

8.24

8.31

8.29

8.45

8.37

8.22

8.16

8.01

8.24

7.64

7.55

7.63

Apr

8.39

8.43

8.34

8.25

8.25

8.24

8.25

8.26

8.39

8.34

8.15

8.12

7.99

8.25

7.68

7.45

7.64

May

8.36

8.42

8.30

8.23

8.21

8.21

8.23

8.24

8.42

8.36

8.12

8.17

8.04

8.26

7.60

7.44

7.65

June

8.29

8.40

8.27

8.21

8.12

8.16

8.18

8.20

8.22

8.23

8.09

8.11

8.08

8.15

7.57

7.48

7.65

July

8.35

8.38

8.26

8.11

8.16

8.14

8.17

8.21

8.39

8.31

8.06

8.05

8.08

8.13

7.57

7.54

7.70

Aug

Table 2 Mean monthly pH values for the period 2006–2020 at monitoring sites (see Fig. 1 for site IDs)

8.37

8.36

8.26

8.17

8.16

8.17

8.21

8.23

8.41

8.32

8.10

8.06

8.09

8.13

7.57

7.50

7.72

Sept

8.34

8.39

8.32

8.22

8.20

8.18

8.26

8.20

8.39

8.30

8.07

8.06

8.06

8.08

7.61

7.59

7.72

Oct

8.37

8.42

8.37

8.27

8.26

8.23

8.27

8.22

8.38

8.36

8.11

8.06

8.03

8.03

7.57

7.44

7.76

Nov

8.45

8.44

8.38

8.29

8.25

8.30

8.32

8.27

8.46

8.41

8.17

8.13

8.13

8.23

7.60

7.58

7.76

Ded

Water Chemistry 77

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A. Brozinˇcevi´c et al.

Fig. 7 Spring of the Crna rijeka Stream. Photo by K. Miculini´c

milligrams per liter of CaCO3 , which measures the acid neutralizing capacity due to carbonate and bicarbonate (Lampert and Sommer 2007). At water pH values of 7.5–8.5, characteristic for the waters of Plitvice Lakes, alkalinity consists almost entirely of bicarbonate ions because concentrations of all other ionic species that make up the alkalinity of the water are negligible (Stumm and Morgan 1996). The hydrological system of the lake is characterized by relatively high values of alkalinity or bicarbonate concentration directly related to the process of tufa deposition, which is most pronounced when the water is a pure bicarbonate solution without an organic matter (Srdoˇc et al. 1985). The highest mean monthly alkalinity values were measured at the Rjeˇcica Stream, ranging from 272.52 mg CaCO3 L−1 in April to 282.19 mg CaCO3 L−1 in July (Fig. 9). The high concentrations at the springs and upper streams are a consequence of the bicarbonate-rich groundwater and the absence of tufa deposition (Fig. 7). This is also consistent with the pH values, which are lower at the springs than in lakes, indicating that there are no physico-chemical conditions for carbonate precipitation here (Srdoˇc et al. 1985). The alkalinity at the spring of the Bijela rijeka Stream, due to higher amounts of dissolved salts, is higher than the alkalinity at the spring of the Crna rijeka Stream, indicating differences in the geological composition of the catchments of these two springs (Deškovi´c et al. 1981; Srdoˇc et al. 1985; Biondi´c et al. 2010). No seasonal variations were observed at the springs and main tributaries. In the downstream course, the concentration of HCO3 − gradually decreases from Matica River and the first lake in the system to Korana River. The decrease in the downstream flow is due to degassing of CO2 from the water, decomposition of bicarbonate forming carbonates (i.e. pronounced calcite precipitation), and uptake of HCO3 − by photosynthetic organisms (Bareši´c et al. 2011). The Rjeˇcica Stream, which flows into Lake Kozjak with its highest alkalinity raises the alkalinity value at the Lake Kozjak outlet tufa barrier compared to the upstream Lake Gradinsko outlet. The lowest concentrations were measured in the downstream, at the Lake

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Fig. 8 Seasonal variations (mean monthly values for the period 2006–2020) of alkalinity at the lake outlet tufa barriers (10–15) and the Korana River (17)

Kaluderovac outlet tufa barrier and at Korana River, where the lowest mean monthly value of 179.39 ± 11.60 mg CaCO3 L−1 was reached in July. A downstream decrease was also observed at the Plitvica Stream, from the spring to the Veliki slap Waterfall tufa barrier due to tufa deposition in the downstream part of the stream. In the lake system, i.e. at the lake outlet tufa barriers, seasonal fluctuations are observed in the form of a decrease in alkalinity in the summer period, especially in the downstream part (Fig. 8). The decrease in bicarbonate concentration in the warmer part of the year indicates more intensive tufa deposition at higher temperatures, as well as the presence of a biogenic factor in the chemical deposition processes (Srdoˇc et al. 1985). Analyses of data for the periods 1981–1986 and 2010–2014 showed that there was an increase in HCO3 − concentrations in the lake system, as a result of increasing concentrations at the main spring (Sironi´c et al. 2017). A statistically significant positive trend was also shown by monitoring data for the period 2006–2017 when observing the overall monitored sites (Vurnek et al. 2021).

3.5 Hardness Water hardness is determined by the concentration of polyvalent metal cations in solution, in natural waters these are most often calcium and magnesium (Stumm and Morgan 1996). In the waters of Plitvice Lakes, total hardness represents the concentrations of calcium and magnesium in the water in the form of bicarbonate

80

A. Brozinˇcevi´c et al.

(Ca-hardness and Mg-hardness). The concentrations are expressed in mg CaCO3 L−1 , from which the molar concentrations of Ca2+ and Mg2+ ions can be calculated, and then from them and other physico-chemical parameters (T, pH, HCO3 − ) the saturation index CaCO3 (Isat ), which is one of the basic indicators of the possibility of calcite or tufa deposition from the water (Srdoˇc et al. 1985). Total hardness shows the same spatial dynamics as alkalinity, which is due to the fact that Ca2+ and HCO3 − are the main chemical particles involved in calcite deposition (Fig. 9). Hardness values are highest in the Rjeˇcica Stream, whose overall mean long-term value is 300.30 ± 15.93 mg CaCO3 L−1 . The Rjeˇcica Stream receives water from numerous permanent and intermittent forest streams and has no distinct karst spring, but seeps from the surface into impermeable layers. This water is enriched with CO2 derived from the biodegradation of organic matter from humus and largely dissolves limestone and therefore contains large amounts of dissolved calcium bicarbonate, reflected in high alkalinity and hardness (Srdoˇc et al. 1985). High values have been measured at springs and other main tributaries, with higher values for the Bijela rijeka Stream than for the Crna rijeka Stream due to the differences in the geological base of the catchments mentioned earlier. Downstream of the springs, at the tributaries, no major differences in concentrations were observed. No seasonal variations were observed either. According to Srdoˇc et al. (1985), the reason for this is Isat , which is close to equilibrium (only 1–2) at the springs of Bijela rijeka, Crna rijeka and Plitvica Stream, and there are almost no seasonal fluctuations because the physico-chemical parameters on which Isat depends are mostly constant. Low Isat values indicate that the physico-chemical conditions required for tufa deposition are not met here. At the tributaries Rjeˇcica Stream and Sartuk Stream the values of Isat are higher. Nevertheless, no tufa deposition was recorded on Sartuk Stream, probably due to increased concentrations of dissolved organic carbon, which is an inhibitor of deposition, and insufficient numbers of calcifying algae and cyanobacteria, which would induce deposition through the process of photosynthesis (Srdoˇc et al. 1985; Bareši´c et al. 2011). Hardness values decrease downstream from Matica River through a system of lakes and tufa barriers to Korana River. Isat here ranges from 3–10 and the process of calcite deposition in the form of tufa is very intense (Bareši´c et al. 2011). According to Kempe et al. (1985), the concentration of calcium carbonate decreases by 27.5 mg CaCO3 L−1 from Lake Proš´ce to Lake Kozjak. In contrast to alkalinity, the lowest mean monthly values were not measured at Korana River, as its hardness is increased compared to the Lake Kaluderovac outlet tufa barrier due to the waters of the Plitvica Stream and the Sartuk Stream with their high concentrations. The lowest mean monthly values of total hardness of 207.26 ± 7.33 mg CaCO3 L−1 were measured at Lake Kaluderovac outlet tufa barrier in August. A decrease in hardness was observed in the lake system during the summer and autumn periods, similar to the alkalinity resulting from more intensive deposition of calcite. From the values of Ca-hardness and Mg-hardness, we gain insight into the concentrations of Ca2+ and Mg2+ ions and their spatial and temporal dynamics at the monitored sites. The dynamics of Ca-hardness in the aquatic ecosystem are consistent with the observed dynamics of total hardness, whereas no major spatial and temporal

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Fig. 9 Spatial dynamics of alkalinity and total hardness (overall mean long-term values for the period 2006–2020) at monitoring sites

changes were observed for MgH. The reason for these dynamics is the fact that calcite deposited in the lake system in the form of lake sediment and tufa, is almost pure CaCO3 in its composition, to which magnesium does not bind in significant concentrations (Srdoˇc et al. 1985). Therefore, Ca-hardness or Ca2+ concentrations in the spring region are relatively high and constant, as is total hardness, because there is no calcite precipitation, i.e. there is no mechanism by which dissolved Ca2+ ions are removed from the water. The highest overall mean long-term values of Ca-hardness were recorded for the spring of Crna rijeka Stream (181.91 ± 11.54 mg CaCO3 L−1 ), Crna rijeka Stream (189.81 ± 12.35 mg CaCO3 L−1 ), Plitvica Stream (179.81 ± 13.27 mg CaCO3 L−1 ) and the spring of the Plitvica Stream (176.44 ± 10.35 mg CaCO3 L−1 ) (Fig. 10). In the lake system, concentrations decrease downstream and show seasonal variations, manifested by a decrease in concentration in the warmer part of the year, since in summer the physico-chemical conditions of calcite deposition are most favourable and photosynthesis, which further enhances deposition, is very intense. The lowest mean monthly values of 117.79 ± 8.68 mg CaCO3 L−1 were obtained at the Lake Kaluderovac outlet tufa barrier for August. The overall mean long-term values of Mg-hardness or Mg2+ concentrations are almost constant from the Matica River to the Korana River, they do not change in the downstream course, and do not show pronounced seasonal dynamics (Fig. 11). Larger differences in Mg-hardenss concentrations exist only between individual springs and tributaries, which is a consequence of differences in the geological composition of the respective catchments. The highest overall mean long-term concentrations for the monitored period are found in the Rjeˇcica Stream (138.20 ± 11.82 mg CaCO3 L−1 ),

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Fig. 10 Box Whiskers plot of calcium hardness for the period 2006–2020 at the monitoring sites (see Fig. 1 for site IDs). The centre line and number stands for overall mean long-term value

Sartuk Stream (121.00 ± 14.24 mg CaCO3 L−1 ), spring of the Bijela rijeka Stream (120.50 ± 12.45 mg CaCO3 L−1 ) and the Bijela rijeka Stream (110.40 ± 12.90 mg CaCO3 L−1 ). This can be explained by the geological structure of this area, where water is filtered mainly through dolomite deposits (Deškovi´c et al. 1981; Biondi´c et al. 2010). As an opposite of the tributaries, the springs of the main streams express lower mean annual concentrations of Mg-hardness. The lowest overall mean long-term concentration of Mg-hardness is at the spring of the Crna rijeka Stream (66.76 ± 15.20 mg CaCO3 L−1 ), which is almost half lower than at the spring of the Bijela rijeka Stream. The overall mean long-term concentration at the spring of Plitvica Stream is also relatively low and is 87.48 ± 14.47 mg CaCO3 L−1 . The spring of the Bijela rijeka Stream derives its water from predominantly limestone deposits and the spring of the Plitvica Stream from dolomite and limestone deposits in approximately the same proportion (Deškovi´c et al. 1981; Biondi´c et al. 2010). Statistical analyses of long-term monitoring data (2006 to 2017) showed a negative trend in total hardness for springs and positive trend for streams and lotic biotopes, while the overall trend for total hardness, Ca-hardness and Mg-hardness was positive (Vurnek et al. 2021). Analyses of data over the past three decades (1981 to 2014) showed a significant increase in Ca2+ concentrations in the Plitvice Lakes water system, while Mg2+ concentrations does not show any temporal change (Sironi´c et al. 2017).

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Fig. 11 Box Whiskers plot of magnesium hardness for the period 2006–2020 at the monitoring sites (see Fig. 1 for site IDs). The centre line and number stands for overall mean long-term value

3.6 Electrical Conductivity The electrical conductivity is a measure of the electrical conductance of water and is expressed in μS cm−1 at a temperature of 25 °C. It is most commonly used as a quick guide to approximate the total dissolved inorganic ions in water (Talling 2009). The ionic content of freshwater (salinity) is defined by the concentrations of the four most common cations (Ca2+ , Mg2+ , Na+ , K+ ) and the four most common anions (HCO3 − , CO3 2− , SO4 2− , Cl− ). The ionic content of water is most influenced by the weathering of rocks, so that due to the dominance of sedimentary carbonate rocks, HCO3 − , CO3 2− and Ca2+ and Mg2+ are the most represented (Allan and Castillo 2007). Electrical conductivity is temperature dependent, and in the longitudinal stream its values can be significantly affected by contributions from various tributaries, exchange with different basal rocks or sediments, and human-introduced pollutants (Talling 2009). Conductivity values in the Plitvice Lakes water system reflect the previously analysed chemical content of waters. Conductivity is stable at the springs and at the tributaries, where no seasonal changes are observed. At the spring of the Bijela rijeka Stream it is high, and the range of mean monthly values is between 480 μS cm−1 in February and 503 μS cm−1 in January, while the springs of the Crna rijeka Stream (418–441 μS cm−1 ) and the Plitvica Stream (430–477 μS cm−1 ) have lower mean monthly values. The chemical composition of the Matica River reflects the

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Fig. 12 Box Whiskers plot of electrical conductivity for the period 2006–2020 at the monitoring sites (see Fig. 1 for site IDs). The centre line and number stands for overall mean long-term value

composition of the Bijela rijeka and Crna rijeka Streams, so that the mean monthly values range from 434 μS cm−1 in April to 454 μS cm−1 in August. In the tributary Rjeˇcica Stream the conductivity is highest (490–512 μS cm−1 ), and relatively high values were also measured at the Sartuk Stream (442–485 μS cm−1 ). No deposition of carbonates in the form of tufa and sediment occurs at the mentioned locations of the springs and most of the tributaries, and the high values of conductivity at the tributaries are a consequence of their passage through forested areas, dissolving larger amounts of salt (Srdoˇc et al. 1985). In the downstream flow, there is a gradual decrease in overall mean long-term conductivity values from the Matica River (441.10 ± 16.59 μS cm−1 ) to the Korana River (362.10 ± 24.78 μS cm−1 ) as a result of increasing calcite deposition (Horvatinˇci´c et al. 2006; Biondi´c et al. 2010) (Fig. 12). At the downstream course, seasonal changes are also observed in the form of a decrease in conductivity in the warmer period and an increase in the colder period of the year. The differences between the highest and lowest mean monthly values at these localities average 35 μS cm−1 , with the lowest mean monthly value of 344 μS cm−1 recorded in September at Korana River. Analyses of monitoring data for the period 2006–2017 showed that electrical conductivity has a positive overall trend that is consistent with trends of hardness and alkalinity (Vurnek et al. 2021).

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3.7 Nutrients Macronutrients required in larger quantities by autotrophic and heterotrophic organisms in the aquatic ecosystem are nitrogen (N), phosphorus (P), potassium, calcium and magnesium, while micronutrients or trace elements such as iron, manganese, zinc, etc. are required in smaller quantities. Often, the availability of certain nutrients in the aquatic system, especially N and P, is less than the demand for them, so they become limiting factors for aquatic habitat productivity (Allan and Castillo 2007). On the other hand, elevated N and P concentrations can be the main cause of degradation of the aquatic ecosystem and its biodiversity, i.e., lead to eutrophication (Ansari et al. 2011). Human activities are the most common cause of increased N and P in water leading to eutrophication of lakes, rivers and coastal zones (Carpenter et al. 1998). Nitrogen occurs in freshwater ecosystems in dissolved inorganic forms as ammonium (NH4 + ), nitrite (NO2 − ), and nitrate (NO3 − ), which are more readily available for biological use, and in the form of organic compounds. These forms of nitrogen enter water through atmospheric deposition, nitrogen fixation, groundwater, or soil leaching. Nitrogen is used as a nutrient in streams by algae, and different groups of algae can take up different forms of nitrogen, which affects their amounts. In lakes, nitrogen can disappear completely from surface layers during periods of strong phytoplankton development (Allan and Castillo 2007). The inorganic forms of nitrogen, NH4 + –N, NO2 − –N, NO3 − –N, were measured and concentrations were expressed as mg N L−1 . Nitrates are present in the highest concentrations and show some spatial variation with regard to site type (Fig. 13). The highest overall mean long-term values were measured at the spring of the Plitvica Stream (1.2 ± 0.19 mg N L−1 ), at the spring of the Bijela rijeka Stream (1.1 ± 0.17 mg N L−1 ), and then consequently at their watercourses. The spring of Crna rijeka Stream has lower overall mean long-term values (0.7 ± 0.15 mg N L−1 ) as well as the Crna rijeka Stream (0.68 ± 0.15 mg N L−1 ). It can be seen that the highest concentrations are associated with spring or groundwater, which may be due to leakage of water into the underground through humus and weathering of rocks. Indeed, studies indicate that some sedimentary rocks contain large amounts of fixed nitrogen, so that their weathering can supply running water with significant amounts of nitrate (Holloway et al. 1998; Williard et al. 2005). Horvatinˇci´c et al. (2006) also associated increased nitrate values at springs with groundwater seepage through the humus layer and transport by surface water from the forested environment. The higher overall mean long-term values of the Sartuk Stream (0.72 ± 0.14 mg N L−1 ) and Rjeˇcica Stream (0.82 ± 0.18 mg N L−1 ) are probably due to surface leaching of the tributaries flowing through the forest area. Precipitation may also contribute to the elevated nitrate concentrations in the Crna rijeka Stream, Rjeˇcica Stream and Sartuk Stream, as the composition of the water at these sites reflects the isotopic and thus chemical composition of precipitation (Babinka 2007). NO3 − –N concentrations at the lake outlet tufa barriers are significantly lower than at the springs and tributaries. Overall mean long-term values do not show further

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spatial dynamics, but range from 0.52 ± 0.16 mg N L−1 at the Lake Kozjak outlet tufa barrier, to 0.55 ± 0.15 mg N L−1 at Lake Proš´ce outlet tufa barrier. It is evident from the mean monthly values that there is a slight decrease in the warmer season (Fig. 14). A similar dynamic was observed in the studies of Habdija et al. (1987), which is interpreted as a result of the assimilation activity of the planktonic community in the lakes, which increases in the warmer part of the year. This leads to depletion of nutrients in the lakes and downstream lotic biotopes. At the Korana River, the overall mean long-term value is 0.60 ± 0.14 mg N L−1 , which is the higher than at the tufa barriers, probably due to the water from the Plitvica Stream, which raises the value in the Korana River with its higher NO3 − –N concentrations. Concentrations of NO2 − –N and NH4 + –N are low at all monitoring sites throughout the monitoring period, often below the detection limits of the methods. Overall mean long-term NO2 − –N concentrations for springs and tributaries range up to 0.002 mg N L−1 , while they range from 0.002 to 0.003 mg N L−1 on the lotic biotopes of the tufa barriers. The observed concentrations do not express spatial and seasonal variations. NH4 + –N concentrations are also low, at the springs and tributaries the overall mean long-terms values range from 0.005 to 0.015 mg N L−1 , while in the lake system from the Lake Proš´ce outlet tufa barrier to the Korana River a downstream decrease in overall mean long-term ammonia concentrations from 0.027 to 0.006 mg N L−1 is observed. At these sites a decrease in concentrations in the summer period was observed, which is consistent with the seasonal dynamics of the nitrate.

Fig. 13 Box Whiskers plot of nitrate concentrations for the period 2006–2020 at the monitoring sites (see Fig. 1 for site IDs). The centre line and number stands for overall mean long-term value

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Fig. 14 Seasonal variations (mean monthly values for the period 2006–2020) of nitrate concentrations at the lake outlet tufa barriers (10–15) and the Korana River (17)

The main pathway by which phosphorus enters surface water is through weathering of rocks and resuspension of sediments, which usually occurs at very low concentrations, so phosphorus is often considered as limiting factor. Phosphorus is present in water in dissolved and particulate forms, and in each of these forms it can be inorganic and organic (Allan and Castillo 2007). The dissolved inorganic form of phosphorus, orthophosphate (PO4 3− ), is the only significant form of phosphorus in water that can be biologically utilized (Lampert and Sommer 2007). The obtained values of PO4 3− –P, expressed as mg P L−1 , are very low at all monitoring sites. Overall mean long-term values for all monitored sites range from 0.006 to 0.012 mg P L−1 , with higher values measured at springs and tributaries than at tufa barriers. Orthophosphates show some temporal variations with regard to seasons, with an increase in spring and late summer/early autumn. This is likely due to co-precipitation of phosphorus with calcite, which is characteristic of many lakes with high pH and high Ca2+ and HCO3 − concentrations (House 2003). This phosphorus immobilization is strongest in spring and summer during photosynthesis (De Vicente et al. 2006). Statistical analyses of monitoring data showed a positive correlation between discharge and nutrients, indicating resuspension of sediments during high discharges (Vurnek et al. 2021). Based on the results obtained, it can be concluded that nutrients enter the lake system mainly from springs and tributaries, as well as from precipitation. Low nutrient levels, corresponding to oligotrophic waters, are a characteristic of the waters of Plitvice Lakes and have been confirmed by previous studies (Habdija et al. 2005; Brozinˇcevi´c et al. 2013). Based on measurements of phosphate concentration and studies of phytoplankton and rotiferes, Špoljar et al. (2007) conclude that the seasonal

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Fig. 15 The Lake Kaluderovac outlet tufa barrier. Photo by N. Marki´c

and spatial distribution of nutrients is most strongly influenced by water overflow across barriers (Fig. 15). Nutrient inputs entering Lake Proš´ce via the Matica River are apparently prevented and reduced at barriers and are also deposited in deeper and calmer lakes (Miliša et al. 2006).

3.8 Dissolved Organic Matter Organic matter in freshwater ecosystems is in the form of dissolved molecules, colloids, or particles and may be allochthonous (mainly from soil), autochthonous (from surface waters or algae or phytoplankton), and synthetic or anthropogenic organic matter (from human activities or industry) in origin. The basic components of dissolved organic matter (DOM) are carbohydrates, proteins, amino acids, lipids, phenols, alcohols, organic acids, and sterols. Humic substances (fulvic acid and humic acid) of terrestrial origin are the dominant fractions in freshwater and coastal ecosystems and are derived from the decomposition of plant material and animal remains from the soil. Dissolved organic matter is involved in various chemical and biological processes in natural waters due to its chemical properties: formation of complex compounds with trace elements, maintenance of acidity and alkalinity, control of nutrient cycling (NH4 + , NO3 − and PO4 3− ), photoinductive and microbial degradation resulting in various degradation products (e.g. dissolved inorganic

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carbon (DIC), CO2 , CH4 , CO, organic acids). All these compounds are very important for the functioning of aquatic ecosystems, and DOMs can provide an important source of energy in the form of C and N (Mostofa et al. 2009). Dissolved organic carbon (DOC) and some polyphosphates inhibit tufa deposition by blocking sites of active crystal growth on the calcite surface. At a temperature of 25 °C and a calcite saturation index of 0.95, calcite deposition is completely inhibited when the DOC concentration is >300 μmol L−1 , and the particle size of calcite crystals decreases from 100 to 18‰) reflecting the specific source conditions during water vapor formation (Gat and Carmi 1970). Ratio of stable isotopes of nitrogen, more abundant 14 N (99.63%) and less 15 N (0.37%), i.e. δ 15 N, is used to study the nitrogen cycle (Robinson 2001). It is the biogeochemical cycle by which nitrogen and nitrogen containing molecules are circulated among atmosphere, terrestrial, and marine ecosystems, through processes of fixation, ammonification, nitrification, and denitrification. Radioactive isotope of cesium, 137 Cs, is a product of nuclear fission of 235 U and other fissionable isotopes in nuclear reactors and nuclear weapons. It is visible in

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sediment layers marking the bomb-peak and events of nuclear power-plant accidents like Chernobyl (1986) and Fukushima (2011). It has a half-life of 30.2 years. (Pennington et al. 1973; Robbins and Edgington 1975; Klaminder et al. 2012). Naturally occurring radioactive isotope of lead, 210 Pb, is a product of decay series 238 of U and is directly produced from gaseous 222 Rn. It can be present as atmospheric fallout, and it can also be formed in situ in sediments. Its half-life is 22.26 years and it is used for dating sediment up to 200 years old (Appleby and Oldfield 1992; Suckow and Gäbler 1997). U/Th dating is based on principle that uranium compounds co-precipitate with carbonates (Ivanovich and Harmon 1982; Schwarcz 1989). Since all uranium isotopes are radioactive, they decay (in series of steps) producing thorium isotopes. In U/Th dating the age is calculated from the degree to which secular equilibrium has been restored between the radioactive isotope 230 Th and its radioactive parent 234 U within a sample. It is used for dating carbonate sediments such as speleothems, tufa and corals of ages between 1 and 500 ka.

3 Setting, Climate and Sampling The Plitvice Lakes are located in the Dinaric karst of central Croatia. They are in the heart of the national park covered with deciduous forest with some conifer stands. The lakes are a system of 16 cascading lakes separated by tufa barrages and connected by waterfalls on vertical distance from 700 to 400 m a.s.l. and total length of about 15 km. The system begins with the Crna Rijeka and the Bijela Rijeka Streams merging into the Matica River that discharges to the first lake, Lake Proš´ce. The system ends with Veliki slap Waterfall where the Plitvica stream merges with lakes that discharge into the Korana River. The lakes are divided into the Upper and the Lower Lakes. The Upper Lakes are settled on dolomite bedrock, and here also two biggest lakes are formed (Lake Proš´ce and Lake Kozjak), while the Lower Lakes sit on limestone bedrock. The annual temperatures at the Plitvice Lakes range from 8.0 to 10.8 °C (1986– 2019), with an average value of 9.2 ± 0.5 °C. January is the coldest month (0 ± 2.3 °C on average) and July is the warmest month (18.4 ± 1.1 °C). Snow falls between November and March; however, reduced snowfall is observed in recent times compared to the older data. Annual precipitation amount at the Plitvice Lakes ranges between 1148 and 2113 mm in the 1986–2019 period. Annual precipitation amounts increase with a slope of 10 ± 7 mm per year, r = 0.31. A wider range in monthly and annual precipitation amount values has been observed, as observed also in data for Zagreb (Krajcar Broni´c et al. 2020a, b). Monthly precipitation at the Plitvice Lakes is distributed relatively uniformly throughout the year, with slight maxima observed in spring and autumn and minimum in summer months. From 1970 to 2010s there have been numerous sampling campaigns for physicochemical parameters and isotope measurement (Fig. 2). The samples of water, soil,

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Fig. 2 a Measuring physico-chemical parameters in situ in winter of 1982, b sampling of water in early spring of 1982, c sampling water for DIC and precipitating in form of BaCO3 in 50 L reservoirs in 1982, d retrieving of 12 m sediment cores on the Lake Kozjak, October 1983, e retrieving of short sediment cores from the lakes by scuba divers in 2003, f retrieving of short cores by gravitational corer in 2011, g sampling of water for stable isotope analyses in 2003, h sampling of aquatic plants from Lake Kozjak in 2011, i retrieving sediment and water from sediment traps from Lake Kozjak in 2011

plants and sediment were collected at different sampling locations that involved the two biggest lakes, main springs and numerous micro locations covering total length of the Plitvice Lakes (Fig. 3).

4 The Plitvice Lakes Environmental Cycle The Plitvice Lakes are dynamic water—karst system settled on limestone and dolomite bedrock with intensive interaction between limestone bedrock, water, soil, atmosphere and biota and water and carbon cycles are mutually dependent and connected through HCO3 − molecule (Fig. 4).

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Fig. 3 Cross-section of the Plitvice Lakes from springs to the Korana River with frequently used sampling locations (abbreviations: BR–spring of the Bijela Rijeka River, CR–Spring of the Crna Rijeka River, Mat–the Matica River, Gra–Lake Gradinsko, Rje–the Rjeˇcica Stream, Kal–Lake Kaluderovac, P–the Plitvica Stream)

Fig. 4 Carbon and water cycles in a karst water system

On the surface of bedrock, vegetation forms a layer of soil rich in decomposed organic matter. Due to root and soil respiration and organic matter decomposition, CO2 in soil is formed. Precipitated water dissolves soil CO2 forming carbonic acid which dissolves carbonate bedrock forming bicarbonate ion HCO3 − . Water containing bicarbonate ion emerges at springs where degassing of CO2 from water also takes place. At the pH range from 7.5 to 8.5, observed at the Plitvice Lakes (Srdoˇc et al. 1985a; Sironi´c et al. 2017), all dissolved inorganic carbonate species (dissolved inorganic carbon, DIC) are in form of bicarbonate ion. Carbon in DIC is delivered from limestone bedrock and from soil CO2 affecting a14 C and δ 13 C values of DIC. This influence of limestone derived carbon on DIC carbon isotope composition is called the hard-water effect. Oxygen atoms originate from atmospheric CO2 , from

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bedrock carbonate, but predominantly from water (precipitated, runoff and surface water) and hydrogen atom solely from precipitation, runoff and surface water. At the waterfalls, intensive exchange between atmosphere and water can take place. Also, aquatic mosses and diatoms are abundant in these areas. The result is intensive photosynthesis and calcite precipitation in form of tufa. Similar process, but less intensive is present in lakes with slower flow where depositing of calcite in form of sediments takes place. Calcite precipitation in still lake waters is biogenically induced by macrophites and cyanobacteria (Leng and Marshall 2004). The karst lake sediment mostly consists of CaCO3 (calcite) with some organic matter mostly originating from lake’s biota (plants). Aquatic plants can also use atmospheric CO2 for photosynthesis, which is especially pronounced in case of aquatic mosses. Precipitation of CaCO3 can also be abiogenic, and it takes place at waterfalls where sprays of water stimulate CO2 degassing and CaCO3 precipitation. Tufa, lake sediment (both inorganic and organic part), aquatic and semiaquatic plants and terrestrial plants during different seasons or physico-chemical conditions (increased or decreased water levels, change in temperature) are also degraded or can be dissolved or carbon from them can be exchanged with carbon in DIC. Therefore, carbon from degraded or dissolved tufa or lake sediments can again be a part of aquatic or semiaquatic plants, or can build lake sediment or tufa.

5 Water Cycle at the Plitvice Lakes As karst contains a labyrinth of channels, its aquifer store large amount of groundwater that can be used for human consumption. Precipitation presents a main input to groundwater which emerges in karst springs, and therefore knowledge on the isotope composition of precipitation is a prerequisite for groundwater studies. The Plitvice Lakes, although protected from direct anthropogenic influence, are not protected from global changes such as climate change that are reflected in air warming ((0.06 ± 0.01) °C per year, Krajcar Broni´c et al. 2020a) and water warming in springs and in surface water (Sironi´c et al. 2017). Though the warming of water bodies favors the tufa precipitation process, increase in temperature contributes to loss of water from the lakes through evaporation (Babinka 2007; Rubini´c et al. 2008; Biondi´c et al. 2010) that can be traced by changes in stable isotope composition.

5.1 Precipitation Monthly precipitation for 3 H activity concentration (A) analyses was collected between 1978 and 1984 (Krajcar Broni´c et al. 1998), while in precipitation samples from 2003 to 2006 A and stable isotopes composition (δ 2 H and δ 18 O) was determined (Krajcar Broni´c et al. 2006, 2020a; Babinka 2007) at the meteorological station at the Plitvice Lakes (altitude 550 m, IAEA-WMO station code 1,432,501).

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Fig. 5 Tritium activity concentration (A) in precipitation in Zagreb and at the Plitvice Lakes for periods 1978–1984 and 2003–2007

Tritium activity concentration in precipitation at the Plitvice Lakes reflected that of precipitation in Zagreb (Fig. 5), showing seasonal pattern with maxima between May and July, and the minima in winter (Krajcar Broni´c et al. 2020a). Data for linear correlation between the two sets decrease in 3 H activity of precipitation. The mean 3 H activity in precipitation at the Plitvice Lakes for 2003–2006 period was 7.4 ± 4.5 TU, with the highest value of 18.8 TU in July 2006, and winter values close to the detection limit. The δ 2 H values in precipitation of the Plitvice Lakes in 2003–2006 period ranged from −132.4 ‰ in winter to −22.3 ‰ in summer, and δ 18 O from −18.3 ‰ to − 4.1‰ (Krajcar Broni´c et al. 2020a). The mean δ 2 H and δ 18 O values in the period from July 2003 to June 2006 were −59.7‰ and −8.3‰, respectively, resulting in difference of 1.9 ‰ in δ 18 O between Zagreb and the Plitvice Lakes. This difference was explained by difference in the mean annual temperatures (Zagreb, 12.3 °C and the Plitvice Lakes, 9.4 °C, temperature gradient 0.33‰ per °C) and the difference in altitudes of the two sites (385 m difference in altitude, gradient −0.28‰ per 100 m) (Vreˇca et al. 2006; Krajcar Broni´c et al. 2020b). Local Meteoric Water Line (LMWL) for the Plitvice lakes precipitation for 2003– 2006 data (n = 36) was described by the following relation obtained by the PWLSR method: δ 2 H = (7.97 ± 0.12)δ 18 O + (13.8 ± 1.3)

(4)

The LMWL slope is close to that of the GMWL, while slightly higher intercept 13.8 ± 1.3 (Fig. 6) indicates higher influence of the Mediterranean precipitation at the Plitvice Lakes than in Zagreb (Krajcar Broni´c et al. 2020a). This conclusion is justified also by the deuterium excess values which are higher at the Plitvice Lake (mean value 14.0 ± 2.2‰, range from 7.7 to 17.9‰) than in Zagreb (mean 8.8 ± 0.8‰, range from 2.3 to 10.7 ‰ in 2003–2006 period) (Krajcar Broni´c

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Fig. 6 Stable isotope composition of the Plitvice Lakes (PL) precipitation and Local meteoric water line LMWLPWLSR (Eq. 4). GWL, SWL and LWL correspond to the Groundwater line, Surface water line and Lake water line (Eqs. 5, 6 and 7, respectively). For comparison, LMWL for Zagreb and the Global Meteoric Water Line (GMWL) are shown

et al. 2020a). Higher d-excess value may be explained partly by higher altitude of the Plitvice Lakes and by more intense influence of the Mediterranean air masses in the area of the Plitvice Lakes. Seasonal variation of the deuterium excess values showed higher d-excess monthly values in autumn–winter precipitation (September– December, 15.2 ± 0.4‰) that during summer (July–August, 12.5 ± 1.0‰). Such a behavior indicates relatively more influence of the Mediterranean air masses during the autumn (Krajcar Broni´c et al. 2020b).

5.2 Groundwater There are three main springs that feed the lakes: springs of the Crna Rijeka and the Bijela Rijeka Stream (in the south) and of the Plitvica Stream, that were used for groundwater analysis. During the whole period 1978–2015, 3 H activity concentration in all three main springs showed general decrease, in accordance with the tritium activity concentration trends in precipitation. However, the seasonal variations were much smaller than those in precipitation. The average 3 H activity values in spring of the Bijela Rijeka Stream were always higher and the relative standard deviations smaller than in the other two springs (Fig. 7) (Krajcar Broni´c et al. 2020a). The ranges of δ 18 O and δ 2 H values in springs of the Crna Rijeka, the Bijela Rijeka and the Plitvica Streams in 1979–1990 and 2003–2019 were less than 1‰ and less than 10‰ respectively, which were much smaller that the corresponding ranges in precipitation, 14.2‰ and 110.1‰, respectively (Krajcar Broni´c et al. 2020a). The average values in the three springs were similar in each of the mentioned periods, while spring of the Bijela Rijeka Stream had the narrowest range in the both periods. All three spring have higher δ 2 H values by 3–5‰ in the second period, 2003–2019,

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Fig. 7 Tritium activity concentration A in springs of the Crna Rijeka (CR), the Bijela Rijeka (BR) and the Plitvica (P) streams in 1978–2015 period. Insert shows individual data for CR and BR in 2015

that can be attributed to the increase in spring water temperature of 1.2–1.9 °C (Sironi´c et al. 2017), which is in agreement with the increase in air temperature of 0.06 °C per year (Krajcar Broni´c et al. 2020a). The δ 2 H and δ 18 O data in the three springs cluster together along the LMWL line for the Plitvice Lakes, and the Groundwater Water Line (GWL) was determined for all the springs: δ 2 H = (5.7 ± 0.7)δ 18 O + (−10 ± 7), n = 31, r = 0.84

(5)

The GWL has both a lower slope and a lower intercept than the LMWL (Eq. 4, Fig. 6), which may be a consequence of water evaporation during its flow in karst aquifers. Using tritium activity concentrations in groundwaters/springs, mean residence time (MRT) of the three springs was determined using the exponential model that supposed a complete mixing of a new recharge with the already existing water in the aquifer (Geyh 1973; Małoszewski et al. 1983; Srdoˇc et al. 1985a; Srdoˇc and Krajcar Broni´c 1986). The shortest MRT of 2 years was obtained for the spring of the Crna Rijeka Stream and the longest MRT of 4 years for spring of the Bijela Rijeka Stream. These data corroborate stable isotope data, the longer the MRT, the smaller the variations in isotope composition.

5.3 Surface Waters A dataset of δ 2 H in surface waters (2003–2005) is in Babinka (2007), and in surface and in subsurface waters (from 2 to 10 m depth) can be found in Krajcar Broni´c et al. (2020a). Water was sampled with a grab sampler at eight sites along approximately

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10 km, from the Matica to the Korana River. Subsurface water (later will be referred as “lake water”) samples was collected from 2012 to 2014 at four sediment traps: two in Lake Kozjak at 6 m and 8–10 m, respectively, one in Lake Proš´ce at 6 m and one in Lake Gradinsko (“Gra” in Fig. 3) at 2 m water depth (Krajcar Broni´c et al. 2020a). The δ 2 H values in surface waters showed the amplitude of less than 10‰ that was much lower than that in precipitated water (approximately 110‰) and reflected the isotope configuration of groundwaters i.e. springs. Seasonal fluctuations were observed with highest amounts in summer and lowest in late winter. The highest values fluctuated and were not observed at all location simultaneously because the water is retained in the lakes (namely Lake Proš´ce and Lake Kozjak) of approximately two months. The average δ 2 H values increased at the downstream locations because of evaporation of surface waters (Krajcar Broni´c et al. 2020a). A similar increase in stable isotope values along the water course was reported also by Yehdegho et al. (2016). They observed a successive isotopic enrichment in δ 18 O and δ 2 H from Matica (−10.7 ± 0.1‰ and −71.4 ± 1.1‰, respectively) to the Korana River (−10.3 ± 0.2‰ and −69.0 ± 1.7‰, respectively). The relation between δ 2 H and δ 18 O in surface waters is described by the Surface Water Line (SWL): δ 2 H = (6.3 ± 0.3)δ 18 O + (−3.4 ± 2.7), n = 87, r = 0.94

(6)

The corresponding relation for subsurface lake water (LWL) was: δ 2 H = (5.8 ± 0.3)δ 18 O + (−9.4 ± 3.0), n = 42, r = 0.95

(7)

Both SWL and LWL had lower slopes and intercepts than the LMWL, in accordance with the GWL (Fig. 6). Such values indicated influence of evaporation of surface waters that was more pronounced in big lakes. The conclusion was justified by comparing slopes and intercept for all individual surface and lake waters from the 2011–2014 sampling campaign. A decrease in both slopes and intercepts in the downstream direction was observed (Krajcar Broni´c et al. 2020a). The δ 18 O values from the complete depth profiles (deep lake waters) in Lake Proš´ce and Lake Kozjak during 2005 were reported by Biondi´c et al. (2010). During the lake turnover time (late autumn–winter) the δ 18 O values were consistent throughout the lakes depth. In contrast, owing to stronger evaporation effects on the surface of lakes, the shallow parts of the lake (0–10 m) were enriched in δ 18 O during summer compared to the deeper parts.

6 Carbon Cycle of the Plitvice Lakes As it was described earlier, at the Plitvice Lakes karst barrage water system carbon is exchanged among atmospheric CO2 , terrestrial plants and soil CO2 . Soil CO2 with water forms carbonic acid that dissolves limestone and dolomite carbonate

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forming bicarbonate ions marked with 14 C in aquifer. This water, enriched in bicarbonate ions, emerges at springs where intensive degassing takes place. Apart from bicarbonate from geochemical process, dissolved inorganic carbon (DIC) in surface water consists of atmospheric CO2 dissolved in water, from CO2 from soil runoff and from organic matter decomposed in lakes. This DIC is incorporated into aquatic and semiaquatic plants and lake sediment or tufa. Global change in isotope composition is reflected in different natural archives like tree rings, lake sediments and tufa. Long term studies on decadal scales are also an important source of data for investigating human impact on the environment. In the following section each of these compartments will be discussed through isotope research at the Plitvice Lakes.

6.1 Atmospheric CO2 , Terrestrial Plants and Soil Anthropogenically undisturbed global atmospheric CO2 had δ 13 C of −6.5‰ (Francey et al. 1999; Graven et al. 2017) and a14 C of 100 pMC (Hua et al. 2013). Due to the Suess effect global δ 13 C reached −8.4‰ in 2015 while a14 C reached the lowest value of 97.5 pMC in 1949–1951 (Hua et al. 2004). Because of another anthropogenic activity—atmospheric thermonuclear bomb tests—a14 C started increasing from 1951 to reach the peak (“the bomb peak”) of 198 pMC in Northern Hemisphere in 1963 (Levin et al. 1985). Current global a14 C levels (in 2016) are 100.9 pMC (at Schauinsland sampling station, Hammer and Levin 2017). The global change in 14 C levels were also present in the atmosphere of the Plitvice Lakes, which is evident from tree rings (Krajcar Broni´c et al. 1998, 2006, 2010). Activities of 14 C of tree rings from the Plitvice Lakes were compared to that of the atmospheric CO2 at the Plitvice Lakes, in Zagreb (Croatia capital), and to data from clean air site for northern hemisphere (Hua et al. 2013; Hammer and Levin 2017) (Fig. 8). Atmospheric a14 C values at the Plitvice Lakes were similar to the values at the global clean air sites and higher than those in Zagreb, urban area that is under influence of the fossil fuel combustion. The δ 13 C values in terrestrial plants at the Plitvice Lakes range from −31‰ to −27‰ (Krajcar Broni´c et al. 1986; Marˇcenko et al. 1989; Sironi´c 2012; Sironi´c et al. 2021), which is expected range for C3 cycle plants. Organic part of terrestrial soil is composed mostly of plant litter. Plant litter was studied in 2005 and 2006 in layers below conifer and evergreen trees (beech, spruce and fir tree, Fig. 9a) (Bareši´c 2009). Plant litter of evergreen trees shows delay of in 14 C activity of about 10 years in regard to the atmosphere, while plant litter from conifer trees reflect atmospheric values with delay of one season at the most. Recent soil layers (deposited after the bomb-peak) showed corresponding increase in a14 C with a delay from atmospheric 14 C level. Surface soil sampled from 1975 to 2011 present delayed response of atmospheric carbon is soil with about 25 years (Fig. 9b) ranging from 120 pMC sampled in 1985 (Krajcar Broni´c et al. 1986; Srdoˇc et al. 1986b) to 110 pMC sampled in 2011 (Sironi´c 2012).

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Fig. 8 Mean annual 14 C activity in atmospheric CO2 at the reference station for the Northern Hemisphere (Schauinsland) and at the stations Zagreb and Plitvice Lakes, and in tree rings at the Plitvice Lakes

Fig. 9 a a14 C of atmospheric CO2 compared to a14 C of plant litter collected in predominantly beech, spruce and fir forests in 2005 and 2006, b a14 C of atmospheric CO2 and of soil surface layer (1974–2011)

Soil is formed mostly from terrestrial vegetation remains, and undisturbed peat formations can be used as natural archives. Dated layers of two peat bogs at the Plitvice Lakes location Luliˇcina Bara showed that the peat was formed continuously for the last 6000 years and it was used to define chronology for corresponding pollen remains—proxy for environment change (Srdoˇc et al. 1985b).

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6.2 Dissolved Inorganic Carbon (DIC) and Aquatic Vegetation Geochemical interaction of soil CO2 , water and carbonate bedrock produces dissolved inorganic carbon (DIC) in aquifer that eventually emerges in spring water and runoff. Its a14 C and δ 13 C values are result of the proportions of the carbonate bedrock and soil CO2 , and of the physico-chemical conditions present during HCO3 − formation. The principle of geochemical dissolution of karts bedrock with carbonic acid from soil in a “closed” system is described by chemical equation: C∗ O2 + H2 O + CaCO3 ⇌ HC∗ O3 − (aq) + HCO3 − (aq) + Ca2+ (aq)

(8)

where carbon atom marked with “*” originates from atmosphere with a14 C reflecting a14 C activity of atmospheric CO2 (about 100 pMC). HCO3 − species contains equal portions of carbon from the atmosphere and from the bedrock (0 pMC). This chemical reaction is a source of radiocarbon reservoir effect, in karst regions known as the hardwater effect. Due to the hard-water effect radiocarbon ages of secondary carbonates are always apparently older than their real age. According to stoichiometry, a14 C of DIC and of secondary carbonate should be 50 pMC, and the apparent age of DIC would be ~5700 years for contemporary sample. In reality, measured a14 C for spring waters are often above 50 pMC, meaning that the limestone dissolution process by carbonic acid takes place in open or semi-open geochemical system. For 14 C analyses DIC was sampled by precipitation as BaCO3 in 1980’s in 50 L containers (Fig. 2c), and when AMS was implemented at RBI in 2008, in one-liter bottle. By meaning of 13 C and 14 C the principle of geochemical dissolution of karst bedrock with carbonic acid from soil was studied using theoretical models for groundwater (Krajcar Broni´c et al. 1986). It was concluded that δ 13 C and a14 C of resulting DIC was not only influenced by soil CO2 alone, but also by atmospheric CO2 leaking through porous karst rocks. Similar study conducted in 2011 found that the proportion of soil CO2 to bedrock carbon varied on the springs of the Crna and Bijela Rijeka Streams, depending on difference in their MRT and on the volume flow rate of water (Sironi´c et al. 2020). From upper locations towards downstream some trends in chemical parameters were observed: pH and calcite saturation index increased and concentrations of dissolved Ca2+ , HCO3 − and CO2 decreased (Srdoˇc et al. 1985a; Sironi´c et al. 2017). Also, downstream increases in a14 C and δ 13 C values in DIC and in recent tufa were observed (Srdoˇc 1986; Srdoˇc et al. 1986b; Horvatinˇci´c et al. 1989; Bareši´c et al. 2011a; Sironi´c et al. 2020). This phenomenon was studied in an attempt to define the initial activity to be used in dating tufa and lake sediments that directly precipitate from DIC (Krajcar Broni´c et al. 1992). On the basis of change in a14 C activity from 1980 to 2010s and changes in a14 C and δ 13 C downstream, it was calculated that the carbon in DIC originated not only from geochemical dissolution of bedrock carbonate with carbonic acid from soil

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CO2 , but also from atmospheric and surface soil carbon (46%: 56%). The amount of atmospheric and soil CO2 mixture increased in total DIC down the flow, causing increase in δ 13 C and a14 C (Sironi´c et al. 2020). The a14 C of DIC ranged from about 60 and 80 pMC (at springs of the Crna and Bijela Rijeka Streams), both in 1980’s and 2010s, to 90 pMC in 1980s and 85 pMC in 2010s at the Korana River (Srdoˇc et al. 1986b; Sironi´c et al. 2020). These ranges were compared to theoretical pre-bomb peak period calculated from a14 C of tufa dated before 1950 (Srdoˇc et al. 1986b). It was found that a14 C DIC is largely influenced by atmospheric CO2 and that 2010’s values were similar to the pre-bomb values—theoretical a14 C values for DIC if they were measured before 1950 (Fig. 10, Sironi´c et al. 2020) The pre-bomb values were calculated from a14 C of tufa dated to before 1950 (Srdoˇc et al. 1986b). Values of δ 13 C ranged from −12.5‰ to −8.5‰, from springs to the Korana River, for both periods. Plants that use DIC for photosynthesis like algae or semiaquatic plants like aquatic mosses are also under influence of the hard-water effect. They also reflect the change in surface DIC carbon isotopic composition (Marˇcenko et al. 1989; Sironi´c et al. 2015, 2021). The a14 C of moss is the same as a14 C of DIC if plant uses only DIC for photosynthesis. If plant uses carbon from DIC and CO2 for photosynthesis, as it is the case of aquatic mosses, the result is a14 C that depends on their ratio. In case of δ 13 C values, fractionation of 13 C isotope during photosynthesis from HCO3 − to plant and from CO2 to plant, significantly influences the end δ 13 C value of plant which is not only the result of mixing ratio of carbon from DIC and atmospheric CO2 , but is also a result of 13 C fractionation during photosynthesis from atmospheric CO2 to plant and DIC to plant. The plants were sampled in 1980’s and in 2010’s in different environmental conditions (Fig. 11). Ranges of δ 13 C values for completely submerged plants were from −35 to − 30‰, for floating and emersed −32 to −27‰, and for aquatic mosses from −48 to −31‰, while a14 C ranged from 60 to 120 pMC depending on plant species, location and sampling period (1980s or 2010s, Marˇcenko et al. 1989; Sironi´c et al. 2015). Comparing a14 C in DIC, atmosphere and aquatic plants measured in 1980s and in 2010s the fraction of atmospheric CO2 in plant was determined: in algae Fig. 10 a14 C in DIC in the recent period (2010s), in 1980s (post-bomb period) and in the pre-bomb (before 1950) period. The shadowed area marks springs of the Bijela Rijeka and the Crna Rijeka Streams

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Fig. 11 Aquatic and semi aquatic plants of the Plitvice Lakes: a floating plant at the Rjeˇcica River tributary to Lake Kozjak, b sedge on the shore of Lake Proš´ce, c aquatic moss (Pallustriela commutata) at the Korana River

(0–20%), floating plants (10–20%), semiaquatic plants (mosses, from 20 to 70%), and emersed plants (60–95%) (Sironi´c et al. 2015). Some species of aquatic mosses contained no atmospheric CO2 , proving that they turn to abiosis during dry periods (Sironi´c et al. 2021). From calculated amount of atmospheric CO2 in aquatic moss, δ 13 C of DIC, atmospheric CO2 and moss, 13 C fractionation factor from DIC to moss was determined. Constant fractionation factor from CO2 to moss was assumed to be −20‰, like in C3 plants. The different values for 13 C fractionation factor from DIC to moss showed differences in metabolic paths of certain groups of mosses and their adaptation to environmental conditions. From relation of share of atmospheric carbon to 13 C fractionation from HCO3 − to moss, it was evident that certain moss species have more intense fractionation with more atmospheric carbon they contain. The specie Palustriella commutata (the most abundant aquatic moss in the Plitvice Lakes) has more-or-less constant amount of atmospheric carbon, while its fractionation depends on water-flow intensity. The difference in fractionation factor of different moss species is evidence of their high ability for adaptation in different environments.

6.3 Tufa and Lake Sediments Both carbonate and organic fractions of tufa and lake sediments contain carbon from different sources so carbon isotope composition, especially δ 13 C can give insight into origin of carbon and of the dynamics of processes involved in formation of carbonates and organics. In addition, δ 18 O point toward physical conditions during carbonate precipitation and δ 15 N toward origin of organic matter. Since tufa and sediments are natural archives, radioactive isotopes of uranium and thorium, as well as 137 Cs and 210 Pb are used to determine chronology. Carbonates in form of tufa and lake sediments in the Plitvice Lakes precipitate by (1) degassing of CO2 that is only temperature dependent—the higher the temperature, the more intense degassing and precipitation, and (2) biomediated by photosynthesis of aquatic plants and cyanobacteria. Due to the hard-water effect 14 C age is not straightforward when it comes to dating, especially for the bomb-peak

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period. However, in combinations with 13 C, 18 O and 15 N, 14 C becomes an indicator of origin of carbon in studied samples and of path of carbon in the carbon cycle. The initial idea that actually kicked off isotope research at the Plitvice Lakes was dating of tufa deposits in order to determine the time of formation of the Plitvice Lakes. When it was found that the hard-water effect influenced secondary carbonates (Srdoˇc et al. 1980), there were more attempts made to determine initial 14 C activity by comparing 14 C activity in carbonate of tufa with 14 C dates in terrestrial matter associated with tufa (Srdoˇc et al. 1983, 1986c; Krajcar Broni´c et al. 1992). Lake sediments of 12 m were retrieved in 1983 (Fig. 2d) and layers were analyzed for a14 C and dated by comparing carbonate lake sediments layer to corresponding organic residue of a terrestrial plant (e.g. wood) when found in a certain layer (Srdoˇc et al. 1986c). The sediments spanned back to 6500 BP (Lake Kozjak) and 8000 BP (Lake Proš´ce) and it was assumed that the initial activity of 14 C was uniform during this period. This approach of dating was successful for sediments older than 200 years, in the period before anthropogenic disturbance of atmosphere with bomb peak 14 C (Krajcar Broni´c et al. 1992), while the recent sediments needed to be dated either by 210 Pb,137 Cs, 14 C dating of terrigenic makrofossils or by linking disturbances in organic matter content in sediment with the known occurrence of extreme hydrological events (Horvatinˇci´c et al. 2008, 2014, 2018). The sedimentation rate of 1.6 mm/yr was determined in the 12-m long sediment of Lake Proš´ce (Srdoˇc et al. 1986c), while it was ~7 mm/yr in the recent sediments (Horvatinˇci´c et al. 2008, 2018). The 12-m long sediments of Lake Kozjak showed disturbance in the layers below about 3 m, but in the upper 2 m of the core the sedimentation rate of 0.8 mm/yr was determined (Srdoˇc et al. 1986c). Sedimentation rate in the recent sediments of Lake Kaluderovac spanned from 3 to 7 mm/yr. As it was presented in the previous section, a14 C in DIC changed from upstream to downstream locations more than 10% which extended the problem further: depending on the location of tufa or sediment, their initial a14 C activity can vary significantly. The a14 C of deposited carbonates (like tufa) correspond to that of DIC at similar location (Horvatinˇci´c et al. 1989). Various experimental techniques were used to define initial 14 C activity (Krajcar Broni´c et al. 1992) by comparing a14 C of precipitated tufa to contemporary organic material of terrestrial origin and to a14 C of DIC from which tufa precipitates. In general, if no other mean of determination of initial a14 C activity is available, for sample from the Dinaric karst, including the Plitvice Lakes area, initial a14 C can be approximated with 85 pMC (Horvatinˇci´c et al. 2003). Apart from the problem of unknown initial activity due to the hard-water effect, tufais difficult to date since its porous structure can contain carbonate deposited after tufa formation (Srdoˇc et al. 1986a). Therefore, tufa samples were dated by both, radiocarbon and U/Th methods and it was shown that radiocarbon dates of Holocene tufa were reliable. However, for the samples dated by radiocarbon method that were older than 30 ka BP, U/Th method showed that they were much older (Srdoˇc et al. 1994; Horvatinˇci´c et al. 2000). By U/Th method it was determined that the Plitvice Lakes were existing for more than 300 ka, but in different form than today (Srdoˇc et al. 1986c; Horvatinˇci´c et al. 2000, 2003).

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For the sediments that present last 200 years, bomb peak reflection was assumed and for the first time shown with a peak in 14 C activity in 30 cm long sediments form Lakes Proš´ce (Srdoˇc et al. 1992). Sediments from lakes Proš´ce, Kozjak, Gradinsko and Kaluderovac were retrieved again in 2003 and in 2011 (Figs. 2e, f and 12) and 137 dated by Cs and 210 Pb showing that carbonate part of sediment reflect 14 C bomb peak with an attenuation and a delay of up to 30 years (Horvatinˇci´c et al. 2008, 2014). Comprehensive summary of δ 13 C and δ 18 O data for tufa is presented in Horvatinˇci´c et al. (2003). Similar conditions were present for tufas from Pleistocene and Holocene eras, and for sediment from Holocene, pointing to warmer conditions that were favorable to tufa growth. This was corroborated with seasonal observation of tufa growth rate that was up to 10 times higher in summer than in winter (Srdoˇc et al. 1985a). Lower δ 13 C for tufa samples from the Plitvice Lakes comparing to speleothems from similar eras and location indicate biogenic carbon is more significant for tufa than for speleothem formation (Horvatinˇci´c et al. 2003). Lake sediments and laminar tufa proved to be good archives of past conditions. Laminar tufa is a good archive since it can be dated by counting layers. Analyses of stable isotopes δ 13 C and δ 18 O in the Korana River laminar tufa that was formed from 1980 to 2004, and recent lake sediments showed trends in δ 13 C (from −9.1 to −8.2‰) and δ 18 O (from −9.9 to −8.0‰) that can be traced to changes in temperature and hydro meteorological parameters (Horvatinˇci´c et al. 2008; Cukrov

Fig. 12 Sediments cores retrieved in 2003 from lakes: a Proš´ce, b Gradinsko, c Kozjak and d Kaluderovac

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et al. 2011). Analyses of a14 C, δ 13 C and δ 18 O from another sample of laminar tufa from the Korana River formed from 1980 to 2001 also showed similar variations in δ 13 C and δ 18 O (−8.9 to −8.7‰ and −10.0 to −8.5‰, respectively), and also showed a peak in a14 C that can be assigned to the bomb peak reflection (a14 C from 75 to 90 pMC, Felja 2009). Though δ 13 C values indicate origin of carbon, they are also under influence of isotope fractionation following dynamic processes during formation of organic and inorganic matter. The first implication that changes in bioproductivity could be monitored by isotopes of carbon at the Plitvice Lakes was presented in Horvatinˇci´c et al. (2008) and in Bareši´c (2009) with observed increase in δ 13 C values closer to surface of 2003 sediments from Lakes Proš´ce, Gradinsko, Kozjak and Kaluderovac. Therefore, in order to define the origin of mineral and organic part of sediments, carbon isotopes were combined with C/N ratio, δ 15 N and carbon and nitrogen content in organic matter of short sediments from for Lake Kozjak and Lake Proš´ce sediments sampled in 2003 by 50 cm long corers (Bareši´c 2009; Bareši´c et al. 2011b). There was a significant difference in a14 C of carbonate and organic fractions in the profile of the sediments, with organic carbon typically having more than 20% higher a14 C values, depending on sampling location at the lake (Bareši´c et al. 2011b; Horvatinˇci´c et al. 2018). Analyses of C/N, δ 15 N and δ 13 C of organic fraction indicated it consisted mostly of aquatic plants residues. However, since aquatic plants consume carbon from DIC they are under influence of the hard-water effect in similar way as the carbonate fraction is. The hard-water effect in organic matter was less pronounced due to partial assimilation of atmospheric CO2 . In the organic fraction of the 2003 sediments from the middle part of Lake Proš´ce and Lake Kozjak, δ 15 N composition followed the change in δ 13 C (Bareši´c 2009; Bareši´c et al. 2011b). The δ 15 N ranging from 1.3 to 2.4‰ implied autochthonous origin of organic fraction in sediments, confirmed by C/N values that ranged from 10 to 15. On the other hand, sediment cores retrieved from Lake Kozjak near the Rjeˇcica Stream confluence to Lake Kozjak, verified influence of allogenic organic matter by δ 13 C values ranging from −30.0 to −27.9‰, δ 15 N from −1.7 to 0.6‰ and C/N from 11.3 to 19.8, which were characteristic to terrestrial plants. This was expected since the influence of the terrestrial input is the most pronounced closest to the mouth of the rivers or streams. Combination of a14 C, δ 13 C, organic matter content and C/N in organic fraction and δ 13 C and a14 C of carbonate fraction of the sediments sampled in 2011 and 2012 from Lakes Proš´ce and Kaluderovac was used to reconstruct the past condition in the last 200 years (Horvatinˇci´c et al. 2018). Lake Proš´ce is one of the largest lakes lying on dolomite bedrock at the beginning of the Lakes, and Lake Kaluderovac is among the smallest, lying on limestone as penultimate of the Lakes. Carbonate fraction δ 13 C in all sediments ranged from −12 to −6 ‰, and a14 C from 50 to 85 pMC, except for sediment core sampled at the mouth of the Matica River with δ 13 C ranging from −6 to 2 ‰, and a14 C from 0 to 50 pMC that implied strong influence of allochtonous carbonate material. Organic fraction δ 13 C ranged from −34 to −26 ‰ and a14 C from 70 to 110 pMC, and in general a14 C and δ 13 C values were lower for Lake Proš´ce sediments than for Lake Kaluderovac. This was a direct consequence 14 13 of the increase in a C and δ C of DIC downstream. These sediments showed that

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specific environment conditions of the two lakes and of different sampling points of sediment cores in them had more influence on a14 C δ 13 C, C/N ratio and carbon content, than difference in sizes of Lake Proš´ce and Lake Kaluderovac. Correlations 14 13 13 of a C to δ C and C/N and δ C of sediments from lake shores indicate mixing of allogenic and antigenic material, while cores retrieved from the deepest sampling point showed in-situ precipitation of calcite and fingerprint of aquatic plants in organic part. In the sediment from the deepest point of Lake Proš´ce, a slight increase in δ 13 C in the last decade, which was also observed in Horvatinˇci´c et al. (2008), could be correlated to increase in water and air temperature (Sironi´c et al. 2017; Krajcar Broni´c et al. 2020b). The response of the sediments to the bomb peak was observed in the sediments with material of mostly authigenic origin (Horvatinˇci´c et al. 2018).

7 Concluding Remarks For more than 40 years, isotope data from water, air, soil, vegetation, tufa and lake sediments have been gathered at the Plitvice Lakes revealing the interactive nature of carbon and water cycles in this karst lake system. The most used were isotopes of carbon and oxygen, supported by isotopes of hydrogen, nitrogen and radioactive isotopes of cesium, lead, uranium and thorium. Initial idea was to use 14 C isotope to date the lakes. By carbon dating of 12 m long sediments from lakes Proš´ce and Kozjak it was determined that the lakes as they are today are up to 8000 years old. However, dating fossil tufa by U/Th method showed that the lakes existed also 300 000 years ago, but in different form. Problems emerged from dating karst sediments initialized usage of other isotope and other proxies, often seeking multidisciplinary coordination, in order to study this complex multi-compartment system. Implementation of improved techniques for measuring 14 C activity enabled easier sample collection, more detailed analyses of sediments and analyses of new kinds of materials. Comprehensive analyses of tritium and stable isotopes 2 H and 18 O, in various water bodies revealed seasonal patterns and trends, which is from precipitation reflected in groundwater and lake waters with a delay. Tritium was used to calculate mean residence time of 2–4 years for the two main springs that feed the lakes. Groundwater and lake water stable isotopes also point to evaporation, which is more pronounced at the bigger lakes. Atmospheric CO2 14 C activity at the Plitvice Lakes showed that the area could be considered as a clean air site, not influenced by local fossil fuel combustion. However, the thermonuclear bomb-peak from 1963 was reflected in all compartments of the Plitvice Lakes showing the global impact on the system. Using the bomb peak, it was revealed that surface soil had a delay in carbon introduction from atmosphere of about 25 years, similarly like tufa and lake sediments (~30 years). DIC is influenced by the hard-water effect, that transfers further to aquatic biota and calcite that precipitates from water in form of tufa and lake sediments. Increase in 14 C and 13 C composition of DIC along the water course of the Plitvice Lakes revealed an interesting phenomenon:

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exchange of carbon from DIC with atmospheric and surface soil carbon. Carbon isotope composition of aquatic mosses reflected their ability to adapt to various environmental conditions. Stable isotopes of carbon, oxygen and nitrogen analyzed in lake sediments were used as indicators of origin of organic and carbonate matter in sediments and of past environmental changes, such are increase in air temperature and enhanced bioproductivity in the lakes in the last 40 years. In example of the Plitvice Lakes, isotope analyses have proved to be a unique tool for extracting information from past ages and in recent times and of processes involved in carbon and water cycles. - Boškovi´c Institute laboratory staff Acknowledgements The authors are thankful to former Ruder members Dušan Srdoˇc, Adela Sliepˇcevi´c, Bogomil Obeli´c, Nada Horvatinˇci´c, Elvira Hernaus, and Božica Mustaˇc and the present technician Anita Rajtari´c and to the staff of the “Dr. Ivo Pevalek” Scientific Station of the Plitvice Lakes who took part in sample and data collection. We appreciate also indispensable contributions of numerous colleagues from different institutions who participated in various projects. The work was partially funded by the Croatian Science Foundation project HRZZ-IP-2013-111623 Reconstruction of the Quaternary environment in Croatia using isotope methods (REQUENCRIM), 2014–2018, and projects with the Plitvice Lakes National Park “Influence of environmental and climate changes on the biologically induced calcite precipitation in form of tufa or lake sediment at the Plitvice Lakes” 2011–2014, and “An investigation of the influence of forest ecosystems of the National Park Plitvice Lakes on the quality of water and lakes” 2003–2006. Earlier research was funded through project with Ministry of Science and Education of the Republic of Croatia, and projects of the European Commission ANTROPOL.PROT, SOWAEUMED and STRAVAL.

References Anderson EC, Libby WF (1951) World-wide distribution of natural radiocarbon. Phys Rev 81:64–69 Appleby PG, Oldfield F (1992) Application of lead-210 to sedimentation studies. In: Ivanovich M, Harman RS (ed) Uranium-series disequilibrium: applications to earth, marine, and environmental science. Oxford University Press Arnold JR, Libby F (1949) Age determination by radiocarbon content: checks with samples of known age. Science 678–680 Babinka S (2007) Multi-tracer study of karst waters and lake sediments in Croatia and BosniaHerzegovina: Plitvice Lakes National Park and Biha´c area. PhD thesis, Rheinischen FriedrichWilhelms-Universität, Bonn, Germany Bareši´c J (2005) Primjena teku´cinskog scintilacijskog brojaˇca u metodi datiranja radioaktivnim ugljikom 14 C, (Application of liquid scintillation counter for radiocarbon dating), Master Thesis, Faculty of Chemical Engineering and Technology, University of Zagreb, Zagreb, Croatia, in Croatian Bareši´c J (2009) Primjena izotopnih i geokemijskih metoda u pra´cenju globalnih i lokalnih promjena u ekološkom sustavu Plitviˇcka jezera (Application of isotopic and geochemical methods in monitoring of global and local changes in ecological system of Plitvice Lakes). PhD thesis, Faculty of Chemical Engineering and Technology, University of Zagreb, Zagreb, Croatia, in Croatian Bareši´c J, Horvatinˇci´c N, Roller-Lutz Z (2011a) Spatial and seasonal variations in the stable C isotope composition of dissolved inorganic carbon and in physico-chemical water parameters in the Plitvice Lakes system. Isotopes Environ Health Stud 47:316–329

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Recent Tufa Deposition Renata Matoniˇckin Kepˇcija and Marko Miliša

Abstract The tufa deposits in Plitvice Lakes form a fluvial barrage system with lakes, cascades and waterfalls of unique beauty. This active process exhibits a characteristic longitudinal trend and chemical evolution, creating diverse habitats that host a variety of aquatic taxa, some of which act as biomediators in tufa deposition process. Artificial substrates enabled the identification of some of these organisms and quantification of their role in precipitation, entrapment, and binding of calcite, particularly in the early stages of tufa formation. Important biomediators on tufa barriers include simuliid larvae with their silk pads, hydropsychid larvae nets, and periphyton. Charophytes are probably the most important biomediators in the formation of lake marl in the littoral zone of lakes. Continuous monitoring and analysis of long-term limnological data is needed to protect tufa deposition, as a fundamental phenomenon of this National Park and World Heritage Site. Keywords Biomediators · Artificial substrata · Periphyton · Plant litter · POM · Hydropsychidae · Simuliidae · Eutrophication · Functional feeding guilds (FFG) · Hyporheic · Energy stock · Nucleation

1 Introduction Tufa develops under certain physicochemical conditions as a freshwater carbonate deposit at ambient temperature and typically contains biological remains (Ford and Pedley 1996). Precipitation of calcium carbonate is driven by a unique combination of physicochemical and biological processes (Pedley 2000), and each tufa-depositing system has its own specificity in terms of morphology, hydrology, water temperature, chemical composition and, consequently, tufa deposition rate (Pentecost 2005). The R. Matoniˇckin Kepˇcija (B) · M. Miliša Divison of Zoology, Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] M. Miliša e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_5

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role of organisms in this process is often debated (e.g., Riding 2000), and it appears that their influence may vary depending on the system studied, but also within the same system (Emeis et al. 1987). According to Golubi´c et al. (2008) organisms associated with tufa shape calcareous deposits and give tufa higher porosity compared to thermal travertine deposits. Intensive tufa deposition can alter the natural course of the river, creating cascades, waterfalls and barriers. River flow thus has an aggregating rather than an erosive effect, creating self-regulating systems that are rarely affected by flooding (Ford and Pedley 1996). Changes in geomorphological forms occur rapidly on the geologic scale and are even noticeable on the human time scale. The Plitvice Lakes are one such example. Over a distance of about 8 km, 16 lakes and many small lentic stretches have formed, with many spectacular waterfalls and cascades (Fig. 1). The depositional environment of Plitvice Lakes is typical of a “fluvial barrage model” characterised by a series of arcuate, transverse phytoherm dams (Ford and Pedley 1996). The hydro-system is fed by several springs, the main being Crna Rijeka and Bijela Rijeka, that originate in the surrounding biologically active karst area. As a result, the spring water appears rich in bicarbonate ions at the surface. Due to continuous CO2 degassing downstream, the water is highly saturated in terms of calcium carbonate. The apparent precipitation of calcite begins about 1.5 km from the springs and continues for about 8 km of the watercourse. Original river valley

Fig. 1 The final cluster of waterfalls in Plitvice Lakes barrage system called Sastavci (Photo by Marko Miliša)

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Fig. 2 The string of Lower Lakes with characteristic reticulate flow of water (Photo by Petar Žutini´c)

is transformed into alternating lakes, dams, waterfalls and terraces ranging in size. The lakes are geomorphologically divided into Upper Lakes (12 lakes), which are dolomite-based, and Lower Lakes (4 lakes), which are cut into a limestone canyon. Hydrologic alteration by dams, waterfalls, and reticulated water flows across barriers (Fig. 2) increase the air–water interface, supporting outgassing of inorganic carbon dioxide outgassing, the major inorganic process in tufa deposition (Chen et al. 2004). Since the process of calcite precipitation is continuous, the entire system is transient in nature and subject to constant change. After the so-called Sastavci (Fig. 1) tufa deposition is less intense, and the Korana River is formed.

2 Longitudinal Trend of Tufa Precipitation There is a longitudinal trend in tufa precipitation along the course of the headwater streams and subsequent lakes, as the Upper Lakes have lower precipitation rates compared to the Lower Lakes (Srdoˇc et al. 1985; Matoniˇckin Kepˇcija et al. 2011). This longitudinal trend in tufa deposition (TD) parallels the downstream chemical trend (Fig. 3) and is consistent with trends observed in other tufa-depositing systems (e.g., Drysdale et al. 2002). The downstream decline in alkalinity and carbonate hardness is most pronounced in summer (Matoniˇckin Kepˇcija et al. 2005; Serti´c Peri´c et al. 2011) because the process of tufa deposition is highly seasonal (Srdoˇc et al. 1985; Pentecost 2005). Tufa deposition rates can be measured directly on natural, semi-natural (Drysdale and Gillieson 1997; Šiljeg et al. 2020) or artificial substrates (Miliša et al. 2006; Matoniˇckin Kepˇcija et al. 2011; Gulin et al. 2022). Artificial substrates in the form of glass slides (26 × 76 mm) have been used in several sampling campaigns in Plitvice Lakes because they are inert and can be easily used as sampling devices on carriers (Fig. 4). These slides can be used for microscopic observations in the

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Fig. 3 Longitudinal gradient of total hardness and alkalinity (average ± SD) between December 2001 and December 2002. SA—source area, UL—Upper Lakes barriers, LL—Lower Lakes barriers (From Matoniˇckin Kepˇcija et al. 2005)

analysis of periphyton as well as for laboratory analysis of chlorophyll, dry matter and organic matter. TD was calculated as the difference in dry matter before and after the dissolution in 16% hydrochloric acid following Pitois et al. (2001). Tufa deposition measured using glass slides ranged from zero in winter to more than 10 mg CaCO3 cm−2 in summer, with clear longitudinal trend (Fig. 5), corresponding to longitudinal chemical changes (Matoniˇckin Kepˇcija et al. 2005). The increase of TD longitudinally in Plitvice Lakes was also observed by Srdoˇc et al. (1985) and corresponds to the typical trends of chemical evolution of tufa-depositing fluvial systems (Drysdale et al. 2002; Chen et al. 2004). Environmental parameters showed statistically significant correlation with TD; positive correlation was found with periphyton formation parameters: chlorophyll

a

b

Fig. 4 Glasss slides on a plexiglas carriers anchored to the brick to resist high current velocity placed on the tufa barrier between Kaluderovac Lake and Novakovi´ca Brod Lake (a) and after four weeks of exposition (b) (Photo by Renata Matoniˇckin Kepˇcija)

Recent Tufa Deposition 14 12 mg CaCO3/cm2

Fig. 5 Tufa deposition (average ± SE, n = 40 for each reach-season) measured using glass slides on three reaches in Plitvice Lakes during 2001–2002. Average exposition time was 3 weeks

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0 Source area Spring

Upper Lakes barriers Summer

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a and organic matter, while a negative correlation was found with flow velocity. Although higher flow velocity is supposed to increase tufa deposition (Pitois et al. 2003; Chen et al. 2004), the opposite results were recorded in Plitvice Lakes also by Matoniˇckin Kepˇcija et al. (2011). This phenomenon could be explained by sedimentation of calcite crystals from the upstream lakes, also places of calcite precipitation (Sviben et al. 2018). Periphyton, also favouring low velocity micro-habitats, possibly trap and bind calcite particles, adding to higher TD in slower flow. Temperature was positively correlated with TD, confirming the strong influence of seasons (Srdoˇc et al. 1985; Golubi´c et al. 2008).

3 The Role of Organisms in Tufa Deposition and Their Imprint on Tufa Morphology Organisms living in or on habitats characterised with active tufa deposition at the same time cope or prosper from TD, and some influence this process (Pentecost 2005), leaving their imprint on tufa morphology. Tufa formation creates heterogenous habitats, supporting diverse communities, both in lentic and lotic parts of Plitvice Lakes hydro-system. However, at sites with high TD calcite can even bury periphytic organisms (Pedley 2000), or negatively affect their attachment (Sviben et al. 2018). The rough surface of tufa provides greater microhabitat area, promoting algal growth (Kock et al. 2006). Tufa on barriers is not only a substrate for rich periphyton growth: this deposit is also embedded in the periphyton matrix, i.e., it reinforces the matrix. Periphyton is one of the communities whose role in the deposition process is often emphasized (Emeis et al. 1987; Primc-Habdija et al. 1997). Emeis et al. (1987) described the stages in tufa formation in Plitvice Lakes and assigned an essential role to epiphytic diatoms and cyanobacteria inhabiting mosses on tufa barriers. The “sticky excretions” serve as traps for micrite from the lakes, and further precipitation occurs on these crystal nuclei. Extracellular mucilage secreted by bacteria, cyanobacteria, and algae may be involved in the formation of so-called microbial carbonates (Winsborough 2000).

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The initial precipitation of tufa on artificial substrates placed on tufa barriers differed along the longitudinal profile of Plitvice Lakes. In the headwaters, there were no or only small amounts of tufa (Fig. 5), often in association with diatoms (Fig. 6). Upper Lakes barriers were characterised by micrite and microspar calcite particles, often clustered together. Pedley (2000) described the different structure of bio-mediated and physicochemical tufa deposits and noted that micrite is formed by organism-mediated precipitation. Different sharp crystals (sparite) were observed on glass slides in the Lower Lakes barriers (Fig. 6). Sparite crystals form by inorganic precipitation on crystal nuclei according to Emeis et al. (1987). However, tufa dominated by algae can be associated with both micrite and sparite (Pentecost 2005), as shown in Plitvice Lakes (Golubi´c et al. 2008). The heterogeneous structure of the initial precipitates probably reflects the influence of the periphytic matrix and the organisms that trap and bind micritic crystals. The initial precipitates change due to diagenetic changes, the nature of which depends on the dominant mosses, cyanobacteria, or algae (Golubi´c et al. 2008). Photosynthetic organisms can also directly affect the saturation index by uptake of carbon dioxide. Variations in diurnal rates of TD and changes in the chemistry of TD systems have been attributed to the effects of photosynthesis. The contribution of photosynthesis to the net TD was estimated to range from 3 to 35%, depending on the system studied and the method used (reviewed by Pentecost 2005). Flow velocity apparently affects the importance of biotic influence on TD. According to Merz-Preiß and Riding (1999), organisms in fast-flowing streams have no impact on supersaturation with respect to calcite as they are only possible nucleation sites, but they could stimulate precipitation in slow-flowing and low-CO2 streams and lakes. The influence of aquatic plants on the increase of TD, but also on the dissolution of calcite within a 24-h cycle has been described in pools but not in the fast-flowing part of tufa-depositing system (Liu et al. 2006). In the classification of tufa facies, mosses and macrophytes are often recognised as influencing deposition process (Pentecost 2005). Animals and their structures have been less studied in terms of TD biomediators, although Matoniˇckin and Pavleti´c (1972) made observations of chironomid- and trichopteran-type of tufa in karst rivers of the Dinarid Mountains. Drysdale (1999) showed that nets and retreats made by hydropsychid caddisfly larvae can increase the rate of TD up to 20-fold. Silk nets, created by larvae primarily for filtration, serve as sites for calcite precipitation and become encrusted and buried over time. In lakes, various Trichoptera families may also contribute to tufa fabrics, as preserved in caddisfly-dominated fossil carbonates (Leggit and Cushman 2001). Hydropsychidae have a strong influence on the TD rate in the tufa barriers of Plitvice Lakes. Their larvae are rheophilous and therefore common on tufa barriers. Our studies with artificial substrates revealed that hydropsychid nets were present on about 10% of the substrates. These silk structures accounted for an average of 71.0% of the total deposited calcite and increased the TD rate by up to forty times (Table 1). Hydropsychid larvae showed a preference for certain flow velocities (Fig. 7), which also affected TD rates in these flow velocity ranges. Because the

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e

f

Fig. 6 Photomicrographs of periphyton and calcite crystals on glass slides after exposition on tufa barriers of Plitvice Lakes during summer months of 2002. a Source area Crna Rijeka Stream (28 days of exposition); b, c Upper Lakes barrier (55 days of exposition); d–f Lower Lakes barrier d and e 40 days of exposition, f 55 days of exposition) (Photo by Renata Matoniˇckin Kepˇcija)

strength of the influence on TD varies with flow velocity, fossilised nets may be a good indicator of paleohydraulic conditions (Matoniˇckin Kepˇcija et al. 2005). The relatively low TD during winter in Plitvice Lakes (Srdoˇc et al. 1985; Matoniˇckin Kepˇcija et al. 2005) provided a glimpse of other biomediators in the fascinating process of TD (Matoniˇckin Kepˇcija et al. 2006). Calcite crystals and diatoms showed a clumped distribution on artificial substrates and were concentrated in mucilaginous structures with characteristic morphology (Fig. 8). These structures were so-called silk secreted by larvae of Simuliidae (Diptera: Insecta) in the form of

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Table 1 Tufa deposition rate (mg dm−2 d−1 ) in Plitvice Lakes in three reaches in different seasons on glass slides with or without hydropsychid silk; n.m. stands for not measurable due to small quantities (from Matoniˇckin Kepˇcija et al. 2005) Area

Hydropsychid silk

Season Winter

Spring

Source area

−

n.m

n.m 0.04 ± 0.03

21.08 ± 16.04

n.m

0.24 ± 0.30

0.45 ± 0.32

0.23 ± 0.25

11.40 ± 7.32

21.96 ± 17.00

12.68 ± 9.71

0.13 ± 0.10

3.37 ± 3.01

13.77 ± 8.21

0.32 ± 0.28

Barriers of upper lakes Barriers of lower lakes

+ − + −

Fig. 7 Proportion of artificial substrates (n = 400) with hydropsychid silk in relation to current velocities after the exposition on tufa barriers in Plitvice Lakes (from Matoniˇckin Kepˇcija et al. 2005)

Summer 0.33 ± 0.30

Autumn 0.09 ± 0.13

20% 15% 10% 5% 0% < 20

20 - 50

50 - 100

> 100

Flow velocity (cm/s)

lines and pads to adhere to a substrate. The silk remains on the substrate and exhibits a characteristic morphology when the larvae are merely settling down, exploring the substrate, or staying there to feed, and has a durability of several weeks (Kiel 1997). The silk pads of simuliids have been shown to have a significant influence on the initial phase of tufa deposition, as these structures trap and bind micrite (Matoniˇckin Kepˇcija et al. 2006). Detailed analysis of the silk structures and calcite crystals showed that the silk covered about 1% of the area of the artificial substrate, but bound about 40–55% of the micrite found after a few weeks of exposure, depending on the velocity conditions. Simuliid larvae live as typical filter-feeders in lotic environments such as tufa barriers, so they further promote the growth of tufa. Compared to the tufa barriers in the Plitvice Lakes hydrosystem (Srdoˇc et al. 1985), the lakes are sites with a lower TD rate. Some macroalgae can promote calcite deposition in lakes, as has been shown for charophytes. Tufa deposits in two different species of the genus Chara in Lake Proš´cansko accounted for 32% (for C. globularis) to 72% (C. subspinosa) of the dry weight of the algae (Sviben et al. 2018). Calcification in charophytes probably serves to generate protons that the alga uses to assimilate bicarbonate and nutrients (McConnaughey and Whelan 1997), so it is definitely controlled by photosynthetic activity. The deposition of so-called “Chara marl” may be similar to TD on barriers (Pentecost 2005), and thus Chara species may be one of the dominant biomediators of TD within lakes in the Plitvice Lakes hydrosystem.

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a

b

c

d

Fig. 8 Accumulation of calcite crystals on silk pads and life-lines created by simuliid larvae a Landing silk pads (LSP) and wandering silk pads (WSP) (54 days of exposition), b, c LSP (20 and 39 days of exposition), d Simuliid drift line binding calcite crystals in linear pattern (20 days of exposition) (From Matoniˇckin Kepˇcija et al. 2006)

4 The Influence of Eutrophication on Tufa Deposition Giving the complexity of inter-relationship between chemistry, hydrology and biota in active tufa-depositing systems, they should be carefully monitored and investigated to preserve TD. Nutrient enrichment in TD hydrosystems can slow calcite growth and phosphorous can even cease it at higher concentrations (Plant and House 2002). Eutrophication was stated as a threat to TD systems, including Plitvice Lakes (Pentecost 2005). Nutrients, particularly phosphorous, were shown to influence tufa deposition and periphyton development in Plitvice Lakes, as shown in the in situ experiment using nutrient diffusing substrata (Matoniˇckin Kepˇcija et al. 2011). The methodology used (Fig. 9) enabled to test the influence of nutrients using small quantities of nutrients to minimize the influence on the whole system. Tufa deposition was (Fig. 10) surprisingly enhanced rather than diminished on nutrient-enriched substrates, most likely due to increased algal accrual on enriched substrates and their binding of calcite crystals. Mild enrichment has been shown to promote deposition in these nutrient-limited systems, while negative effects likely predominate at higher nutrient concentrations (Plant and House 2002). Wiik et al.

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Fig. 9 Nutrient-diffusing substrata placed on a tufa barrier between Gradinsko Lake and Veliki Burget Lake in October 2006 (Photo by Renata Matoniˇckin Kepˇcija)

12

mg/cm2

Fig. 10 Deposited tufa (as calcite) at two study sites (LTD—low tufa deposition, HTD—high tufa deposition: HTD) with two rheotopes and 4 treatments (C—control, P—phosphorus added, N—nitrogen added, P + N—phosphorus and nitrogen added) (Adapted from Matoniˇckin Kepˇcija et al. 2011)

10

C

8

N P

6

P+N

4 2

0 slow flow

fast flow

slow flow

LTD

fast flow

HTD

(2015) presented a series of changes in a marl lake showing its high sensitivity to eutrophication. The authors emphasize that historical and paleolimnological data should be used in such systems. Nutrient concentrations in the Plitvice Lakes water system remain quite low, and there is no significant trend toward increase (Vurnek et al. 2021). However, according to Sironi´c et al. (2017), there have been changes in water temperature that may affect the calcite saturation index. There is a need for analysis of long-term data as well as inclusion of historical data to provide a more accurate picture of possible changes in limnological parameters of this system.

5 Organic Matter Dynamics and Tufa Deposition Allochthonous organic matter, predominantly leaf litter, is an energetic fundament for benthic assemblages in forested headwater streams. In water it is transformed physically by water flow force and abrasion, and also chemically transformed by

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133

hydrolysis and leaching and finally consumed by microorganisms and higher organisms (Minshall et al. 2000). During decomposition of leaf litter higher levels of CO2 , and consequently lowered pH, and increased amounts of dissolved organic matter may obstruct tufa deposition. In addition, leached phosphate (and nitrate) ions may obstruct formation of calcite crystalline lattice further hindering tufa deposition (Plant and House 2002). In contrast, the microbes colonizing leaf litter may provide favorable conditions for crystal nucleation both through exuding complex molecules, and through utilization of CO2 (autotrophic microbes) buffering the pH change. The mosses which are highly abundant at tufa barriers function in the same manner and due to their abundance are the main factor in promoting tufa deposition (along with the temperature and the water flow that cause CO2 outgassing). Conversely, looking from the leaf litter’s point of view tufa crust deposited on leaf surface may slow down the breakdown process (obstructing the access to the leaf matter) but crystals may damage leaves, making them more susceptible to decay. Leafs of different plants decompose at different rates (Boulton and Boon 1991). At Plitvice Lakes riparian vegetation is predominantly beech and aquatic emergent vegetation is butterbur. During the first week probably during the first 2 days, mass loss is not different in respect to the environmental factors with more than half of the mass of butterbur and more than one tenth of mass of beech leaf matter lost. However, breakdown rates after the first week differ with respect to the environmental factors. Temperature, tufa deposition rate (TDR) and flow velocity all promote mass loss (decomposition) of butterbur (soft) leaves while temperature is less important factor in beech (tough) leaf decomposition (Miliša et al. 2010). Since the decomposition in all environmental setups are linear (subsequent to leaching) we conclude that there is no negative effect of tufa deposition on decomposition processes—on the contrary if anything it is promoted by tufa deposition. Tufa deposits in fast depositing habitats of Plitvice Lakes tend to be porous and soft due to the fast growth. This type of tufa probably allows rapid leaching and colonization by microorganisms. We observed that slow flow combined with high tufa deposition extends the persistence time of the tough beech leaves. More compact deposits at slow flow sites (and at lower temperature in comparison with the aforementioned fast-flow tufa) may obstructs both leaching and microbial colonization. The mean persistence time for butterbur leaves i.e. time in which there is any leaf mass remaining is 62 days (range 25–132 days) and 442 days for beech leaves (range 62–1020 days) (Fig. 11). Decomposition is slowest when temperatures are low regardless of other factors. Mass of tufa per gram of leaf mass linearly increases suggesting there is no impeding effect of the decomposition process (Fig. 12). Rather it seems to be promoted by faster decaying processes as almost double the amount of calcite crystals were deposited on soft—fast decomposing leaves (mean: 0.16 ± 0.08 g(tufa) g(leaf) −1 wk−1 ) than on tough—slow decomposing leaves (mean: 0.09 ± 0.06 g(tufa) g(leaf) −1 wk−1 ). The amount of deposited tufa during the first week was significantly higher on butterbur than on beech. Also, more tufa is deposited in fast leaching leaves suggesting that this process in Plitvice lakes probably also promotes crystallization/nucleation of calcite. Less tufa deposited on the tough leaves further corroborates that the decomposition process promotes tufa deposition. Finally, more tufa was

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Fig. 11 Breakdown rates of beech and butterbur leaves at high and low tufa depositing sites. Circlepoints, thick lines and bold text is for breakdown at slow flow habitats and triangle-points, normal line and normal text is for breakdown at fast flow habitats (Miliša et al. 2010)

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135

deposited on the beech leaf’s underside where the stomata and hairs (trichomes) are located and where leaching and microbial colonization start hence creating favorable condition for crystal nucleation and deposition (Canhoto and Graça 1999) (Fig. 13). Due to the beech leaf toughness, leaching and, consequently, microbial colonization is retarded compared with butterbur. Low temperatures, on the other hand hinder tufa deposition as more than twice less is deposited in winter months than in summer. Decomposition processes of organic material (detritus) lead to three main sizefractions of particulate organic matter (POM): coarse (>1 mm; CPOM), fine (1 mm to 50 µm; FPOM) and ultra-fine (< 50 µm; UPOM) particles. These size fractions are food sources for different feeding strategies od macroinvertebrates (Moog 2002). The accumulation of organic particles is a function of stream bed roughness, porosity and morphology, in-stream vegetation, debris dams and filterer fauna (Wanner and Pusch 2001). In tufa habitats, aside from these ‘conventional’ mechanisms, organic matter particles are also overgrown with new layers of deposited calcite. These particles are virtually the sole energy source for interstitial fauna, but also enabling the interstices to act as refugia for benthic fauna such as smaller and younger organisms (Pedley 2000). The role of POM in travertine barrier building-mechanisms is dual. Organic particles as well as the attached organisms and their excretions (microbial extracellular polymers—mucopolysaccharides, dwelling and feeding constructions of insects etc.) are nucleation sites crystals (Fig. 14). Conversely, decomposition processes CO2 is produced which may acidify water on a micro-scale thus disabling crystallization and even resulting in a dissolution of already deposited calcite. Plitvice Lakes are characterized by extensive calcite precipitation and the formation of porous stream bottoms with historically proven amounts of up to 10,000 tons of CaCO3 annually per total lake area (Kempe and Emeis 1985), with a vertical growth of the travertine barriers of 13.5 mm per year (Srdoˇc et al., 1985). Generally, mass of deposited POM in Plitvice decreases along the system, with approximately 22–15 g dm−3 of AFDM (ash free dry mass) in the tufa profile of 10 cm. The main reason is that the lakes between tufa barriers serve as sinks for most of the particles and only a smaller part is transferred while larger proportion settles in lake bottom and input of new POM is point-source from the vegetation at the barriers. Most of the organic debris deposits are retained in the moss mats (some 1–2 cm thick), and virtually all of CPOM was found there. In the deeper layers of tufa, FPOM and UPOM fractions dominated. Obviously, the pores in the tufa interstices/caverns are too small for larger particles to be deposited there so only the overgrown particles were and only in the first 3 cm layer of tufa while deeper layers had no coarse particles. Conversely, UPOM particles are relatively scarce in the moss layer as the moss stems do not provide a mesh that would keep extra small particles and they are thus readily flushed downstream by the water flow.

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Fig. 12 Tufa deposition rates on tough (beech) leaves and soft (butterbur) leaves during summer and winter at habitats with slow and fast flows (Miliša et al. 2010)

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a)

137

b)

Fig. 13 Tufa deposited on beech leave’s a upper and b under side (Photo by Marko Miliša)

a)

b)

c)

Fig. 14 a Calcified moss leaves and a non-biting midge (Chironomidae) case (red arrow), b partly calcified organic debris trapped in slightly calcified Neureclipsis nets, c leaf fragment lodged at the top of a Hydropsyche net (Photo by Marko Miliša)

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5.1 Hyporheic Energy Stock and Water Flow In total, almost 50% more POM is accumulated in the moss mats than in the tufa layers below which is much less than in gravel bottom streams. In non-precipitating streams showed moss mats accumulate ten times more than the gravel stream bed (Suren 1991), so overgrowing is supposedly a very strong mechanism of vertical (downward) POM ‘transfer’. However, amounts of organic particles decrease exponentially with depth, while in streams with gravel beds where more POM is found in deeper layers (Mathieu et al. 1991). This is probably the result of the differences in interstitial architecture with few and small longitudinally connected spaces in tufa compared to larger and more numerous longitudinal spaces in gravel bed. Flow velocity at tufa barriers in Plitvice Lakes (and elsewhere at barrier sites/outflows) is higher than in comparable headwater streams as relatively large quantities of water overflow from the lake in a relatively thin overflow layer. In these habitats flow velocities lower than 50 cm s−1 can be considered as slow flow habitats. In turn, at these habitats normally most of the POM is accumulated. UPOM content is always negatively correlated with the flow velocity as very small particles, are as already mentioned, readily flushed and only a small portion is deposited. CPOM on the other hand is readily deposited at slower flows. Also, CPOM can be dominant both in total amounts and proportionally in the top layer of tufa and moss mats (and other vegetation) at fast and very fast flows (50–100 cm s−1 and > 100 cm s−1 ). Due to the force of fast flowing water, these currents carry more particles per unit of volume and naturally more water overflows per unit of time. Any obstacles in these habitats entrap particles from the water flow. FPOM behaves partly as CPOM and partly as UPOM in fast and slow flow respectively. At slow flow finer particles are deposited while at fast flow due to moss architecture moss mats can still entrap the larger FPOM particles. Larger particles get lodged in the patchwork of vegetation, animal constructions and debris and subsequently are calcified, overgrown by tufa and consequently ‘buried’ together with the moss stems by the fast-flow tufa (Fig. 15). Surface flow ceases to be an important factor of accumulation of POM in layers deeper than 7 cm. At sites with the surface flow of 120 cm s−1 (and elsewhere in tufa deposits), the water in the interstices slows significantly and seeps in all directions. When 50 cm3 of red dye is introduced at depth of 4 cm in the tufa substrate, after one hour it is noted approximately 3 cm upstream of the site where the dye was administered, 4 cm in both lateral directions and 9.5 cm downstream (Fig. 15 d). In the hyporheic zone alone, overall ratio of organic matter mass is CPOM: FPOM: UPOM = 1: 2.2: 1.6. CPOM amounts are significantly lower at slow-flow sites. Oppositely, UPOM was more abundant there and FPOM proportion is constant at all sites. Even though the CPOM standing stock was least in respect to mass in the tufa interstices it was the most important as an energy source providing more than 40% of energy i.e. food resources. Mean total energy stored in the POM in hyporheic zone of tufa habitats was 163.4 kJ dm−3 . Mean energy stocked within the POM size categories differed significantly. Energy stock of CPOM, FPOM and

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a)

c)

139

b)

d)

Fig. 15 a Tufa deposited on moss stems, calcified moss leaves are visible (white arrow points to the part of moss stem that died off), b Calcified animal and plant material, red arrow points to the space where leaf was buried (now decomposed), black arrow point to the caddisfly cases, c close-up of a calcified caddisfly case with visible silk threads (red arrow), d a block of tufa with injected dye and syringe with a needle, calcified moss stems are visible to some 4 cm depth (Photo by Marko Miliša)

UPOM was 16.975, 7.496 and 4.360 kJ g(dry matter) −1 . Reduction of energy amount from larger to smaller particles is likely caused by the time that respective particle was in the water. The smaller the particle, the longer it was exposed to decomposition processes included digestion by macroinvertebrates—so more energy was exploited in that course. Also, the abundant filterer fauna ingests smaller particles and convert them to fecal pellets to larger FPOM and transport them vertically in deeper layers (Wotton and Warren 2007). The cumulative energy ratio among the three categories

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of the accumulated POM in the hyporheic zone was CPOM: FPOM: UPOM = 1: 0.9: 0.4.

5.2 Functional Composition of Macroinvertebrates in Tufa Hyporheic Six functional feeding guilds FFGs are present in Plitvice lakes tufa interstices: shredders, grazers, detritus feeders (collectors), passive filterers, active filterers and predators. Cumulative average abundance of macroinvertebrate fauna is approximately 190 individuals dm−3 , predominantly detritus feeders 76% and grazers 14%. Predators and passive filterers each represented around 4% while active filterers and shredders each accounted for around 1% of total fauna (Table 2). A high variation in fauna abundance suggests both patchy distribution and high migration level of the interstitial dwellers. Grazers were more abundant at habitats with slow flow while passive filterers and collector gatherers were more abundant at habitats with fast flow. Shredders were more abundant at sites with low TDR (tufa deposition rate) and predators do not exhibit changes in abundances among the habitats. The most influential environmental factors in functional structuring of macroinvertebrate assemblages were predatory pressure and flow velocity. Abundance of predators negatively affects the abundance of detritus feeders. These are mostly soft bodied organisms and/or young insect instars. Flow was also a major factor negatively correlated especially with the abundance of grazers. Amount of POM food resources has less effect on functional structuring with CPOM positively affecting shredder abundance and UPOM and FPOM positively affects grazer and detritus feeder abundances respectively. In similar but non-precipitating (outflow) systems, food resources normally have higher importance (Brand and Miserendino 2012). Tufa substrate is different in that it is stable compared to gravel/pebble/cobble substrates and also deposited amounts of POM are stable and ample for relatively few macroinvertebrates that dwell in tufa interstices. Food being plentiful, predators become a more pronounced factor for assemblage structuring—their pressure is more notable on scarce interstitial fauna (Xu et al. 2012). Animals use this habitat as a refuge. In addition, during their migrations they probably seek favorable surface conditions which fast flow habitats offer hence surface flow becomes an important factor for hyporheic structuring of assemblages, especially for rheophilous (fast flow preferring) taxa e.g. passive filterers but also mayflies, stoneflies, and caddisflies in general. They all can move to the top layers for feeding and take refuge in deeper layers. Grazers apply different tactics as their food are biofilm and UPOM, they occupy slow flow habitats (Ishikawa and Finlay 2012). Grazers dislike habitats with rapid tufa deposition as their food resources get rapidly overgrown, similarly shredders do not favor the same habitats.

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Table 2 Abundances (individuals dm−3 ) of each taxa allocated to their respective FFG, at 4 habitat types: slow versus fast flow × high versus low tufa deposition rate (TDR). Note that several taxa are attributed to more than one FFG according to Moog (2002) Flow: TDR: Shredders

Grazers

Passive filterers

Fast High

Slow High

0.0

0.3

0.0

0.5

0.0

6.1

0.0

0.9

0.0

Leuctra sp.

4.2

Micrasema sp.

4.9

Shredders total

9.1

6.6

0.9

0.9

Gastropoda

0.0

16.8

1.0

29.3

Amphinemura sp.

0.0

0.0

0.9

0.0

0.5

4.2

0.5

0.5

0.0

Riolus cupreus

10.5

18.1

0.0

1.7

Hydropsiche sp.

0.0

0.0

0.7

0.0

Micrasema sp.

4.9

6.1

0.0

0.9

Tanytarsini

5.0

0.3

0.7

0.3

Orthocladinae

0.0

0.7

2.6

1.4

Grazers total

24.6

42.6

6.4

33.7

Ephemera sp.

0.0

3.5

0.0

0.0

Tanytarsini

2.5

0.2

0.3

0.2

Orthocladinae

0.0

0.2

0.6

0.3

Active filterers total

2.5

3.8

1.0

0.5

Ecnomus sp.

0.2

0.0

0.2

0.0

Hydropsiche sp.

0.0

0.0

1.7

0.0

Polycentropus sp.

0.2

0.0

0.0

0.0

Philopotamus sp.

0.0

0.0

0.0

1.7

Wormaldia subnigra

7.0

3.5

8.7

0.0

Tanytarsini

5.0

0.3

0.7

0.3

Passive filterers total Detritus feeders

Slow Low

Amphinemura sp.

Leuctra sp.

Active filterers

Fast Low

12.4

3.8

11.4

2.1

Gastropoda

0.0

11.2

0.7

19.6

Oligochaeta

175.3

64.6

168.1

80.4

Paraleptophlebia sp.

0.0

0.0

1.7

0.0

Caenis sp.

0.0

2.1

0.0

0.0

Amphinemura sp.

0.0

0.0

0.5

0.0

Leuctra sp.

5.6

0.7

0.7

0.0

Scirtes sp.

14.0

0.0

0.0

0.0

Tanytarsini

12.6

0.9

1.7

0.9

Orthocladinae

0.0

0.9

3.2

1.7

Tanypodinae

0.3

0.3

0.2

0.5

Hemerodromia sp.

0.0

0.9

0.9

0.0 (continued)

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Table 2 (continued)

Predators

Total

Flow: TDR:

Fast Low

Detritus feeders total

Slow Low

Fast High

Slow High

207.8

81.2

177.8

103.1

Ecnomus sp.

1.6

0.0

1.6

0.0

Hydropsiche sp.

0.0

0.0

1.0

0.0

Polycentropus sp.

1.6

0.0

0.0

0.0

Tanypodinae

3.1

3.1

1.6

4.7

Ibisia sp.

0.0

0.0

5.0

5.2

Sphaeromias sp.

1.7

0.0

0.0

0.0

Hemerodromia sp.

0.0

0.9

0.9

0.0

Predators total

8.0

4.0

10.1

10.0

264.4

142.0

207.6

150.3

6 Conclusion Tufa-depositing system of the Plitvice Lakes, protected as a national park since 1949, show complex and still under-studied patterns of recent calcite precipitation. Although this unique dam system is famous for its spectacular beauty, it should be carefully managed and protected to preserve its fundamental phenomenon: tufa deposition in lakes and on tufa barriers. Climate change, pollution, and other humaninduced activities can potentially have a negative impact on tufa-deposition. This system not only provides us with a privileged view of a rather rapid geological phenomenon, but also harbours high biodiversity of aquatic organisms deserving its high conservation status.

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Pitois F, Jigorel A, Bertru G (2001) Colonization dynamics of an encrusting cyanobacterial mat in a hardwater river (Eaulne, France). Geomicrobiol J 18(2):139–155 Pitois F, Jigorel A, Bertru G (2003) Development of cyanobacterial build-up and evolutionof river bed morphology in the chalk stream Eaulne (Upper-Normandy, France). Biodiv Conserv 12:621– 636 Plant LJ, House WA (2002) Precipitation of calcite in the presence of inorganic P. Colloids Surf A 203:143–153 Primc-Habdija B, Habdija I, Plenkovi´c-Moraj A, Špoljar M (1997) The overgrowth capacity of periphyton on artificial substrates exposed at vertical profile in the Lake Visovac. Period Biol 99(3):403–408 Riding R (2000) Microbial carbonates: the geological record of calcified bacterial-algal mats and biofilms. Sedimentology 47(Suppl 1):179–214 Serti´c Peri´c M, Miliša M, Matoniˇckin Kepˇcija R, Primc-Habdija B, Habdija I (2011) Seasonal and fine-scale spatial drift patterns in tufa-depositing barrage hydrosystem. Fund Appl Limnol 178(2):131–145 Sironi´c A, Bareši´c J, Horvatinˇci´c N, Brozinˇcevi´c A, Vurnek M, Kapelj S (2017) Changes in the geochemical parameters of karst lakes over the past three decades–The case of Plitvice Lakes, Croatia. Appl Geochem 78:12–22 Srdoˇc D, Horvatinˇci´c N, Obeli´c B, Krajcar I, Sliepˇcevi´c A (1985) Procesi taloženja kalcita u krškim vodama s posebnim osvrtom na Plitviˇcka jezera (In Croatian). Krš Jugoslavije 11:101–204 Suren AM (1991) Bryophytes as invertebrate habitat in two New Zealand alpine streams. Freshw Biol 26:399–418 Sviben S, Matoniˇcki Kepˇcija RM, Vidakovi´c-Cifrek Ž, Serti´c PM, Kruži´c P, Popijaˇc A, Primc B (2018) Chara spp. exhibit highly heterogeneous light adaptation, calcite encrustation and epiphyton patterns in a marl lake. Aquat Bot 147:1–10 Šiljeg A, Mari´c I, Cukrov N, Domazetovi´c F, Roland V (2020) A multiscale framework for sustainable management of tufa-forming watercourses: a case study of National Park “Krka.” Croatia. Water 12(11):3096 Vurnek M, Brozinˇcevi´c A, Matoniˇckin Kepˇcija R, Frketi´c T (2021) Analyses of long-term trends in water quality data of the Plitvice Lakes National Park. Fund Appl Limnol 194(3):155–169 Wanner SC, Pusch M (2001) Analysis of particulate organic matter retention by benthic structural elements in a lowland river (River Spree, Germany). Arch Hydrobiol 23:475–492 Winsborough BM (2000) Diatoms and benthic microbial carbonates. In: Riding RE, Awramik SM (eds) Microbial sediments. Springer-Verlag, Berlin, pp 76–83 Wiik E, Bennion H, Sayer CD, Davidson TA, McGowan S, Patmore IR, Clarke SJ (2015) Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water. UK. Freshwater Biol 60(11):2226–2247 Wotton RS, Warren LL (2007) Impacts of suspension feeders on the modification and transport of stream seston. Fundam Appl Limnol 5:231–236 Xu MZ, Wang ZY, Pan BZ, Na ZH (2012) Distribution and species composition of macroinvertebrates in the hyporheic zone of bed sediment. Int J Sediment Resv 27(2):129–140

Energy and Matter Dynamics Through the Barrage Lakes Ecosystem Marko Miliša, Maria Špoljar, Mirela Serti´c Peri´c, and Tvrtko Dražina

Abstract In this chapter, we present the results of seston, macroinvertebrate drift and meiobenthos across several lentic and lotic stretches within the Plitvice Lakes hydrosystem. The insight into the micro-distribution and transport processes of organic and inorganic particles, and organisms in benthos and water column within this hydrosystem greatly contributes to understanding the circulation of matter, energy flow and biocenoses structure in this unique and extensive network of lentic and lotic habitats. We conclude that the seston and drift patterns within Plitvice Lakes hydrosystem are greatly affected by hydromorphological features (discharge, flow velocity, substrate type) of different (micro)habitats as well as by seasonal changes in the moss-coverage of the tufa substrate. Thus, the energy and matter dynamics through the Plitvice barrage lake ecosystem greatly depends on the “sequencing” of the lotic and lentic reaches within the hydrosystem as well as on the efficiency of moss to trap particles and organisms (i.e., macrozoobenthos, meiofauna) and/or to serve as their transport agent/mediator. Our results further indicate that the small-scale distribution and transport of aquatic invertebrates inhabiting fast-flowing barriers and slow-flowing pools within the Plitvice Lakes hydrosystem greatly depend on the development stage, and morphological and functional traits of the individual plankton, macroinvertebrate and meiofaunal taxa. Keywords Particulate organic matter · Seston · Drift · Meiofauna · Hydromorphology · Rotifers · Cladocerans · Copepods · Nematodes · Insect Larvae · Bryophyte traps · Functional traits M. Miliša (B) · M. Špoljar · M. Serti´c Peri´c · T. Dražina Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] M. Špoljar e-mail: [email protected] M. Serti´c Peri´c e-mail: [email protected] T. Dražina e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_6

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1 Introduction Food-web ecology integrates community (species, individuals and populations) and ecosystem ecology (energy, biomass, and nutrients) through the feeding interactions, organic mass distribution and energetic flows (Thompson et al. 2012). Understanding of ecological stoichiometry and trophic network can predict ecosystem alterations, and in turn ecosystem resilience (Woodward et al. 2010). Thus in this chapter survey of longitudinal flux of energy and matter is presented on the spatial macroscale (seston transport through the Upper Lakes) and microscale (macroinvertebrate drift and meiofauna through the lotic stretch over one barrier, in the Upper and Lower Lakes, respectively). Vertical flux of energy and matter is also elaborated in the analysis of macroinvertebrate drift through the different layers of the water column and in the research of different microhabitats of meiofauna from the bryophyte covered bottom (surface) until tufa layer. Nutrient concentrations (nitrate, phosphate) enhance phytoplankton biomass and trigger growth of primary consumers and further links in the food chains and network, and consequently lake productivity or internal load, recognized as one of the crucial element in functioning of this flow-through system (Špoljar et al. 2007a,b). Hydrological features (discharge, water current), tributaries inflow, bottom configuration and coverage (e.g. bryophytes), allohtonous carbon subsidies or external load, i.e., leaf litter indicated as main drivers in flux of the energy and matter cycling on the Plitvice Lakes (Benceti´c Klai´c et al. 2018). All components of aquatic community through the grazing or predation interactions, and long-run, after decomposition as detritus, are integrated in the ecosystem functioning. Hereby, we will take in consideration zooseston, macroinvertebrate drift and meiofauna and accompanying detritus or particulate organic matter (POM). Seston represents food (sensu stricto particle size 0.5–50 µm) for invertebrates in plankton and benthos. The term seston (rheoseston) sensu lato refers to organic and inorganic particles of autochtonous and allochtonous origin, both dead or alive, present in the current of running waters (Breitig and Tuempling 1982). Within zooseston, could be recognized species from plankton (among protozoans, Rotifera, Cladocera, Copepoda, Ostracoda) and benthos (among Turbellaria, Hydrozoa, Nematoda, Gastrotricha, Oligochaeta, Tardigrada, Insecta larvae). In the running waters seston is main food supplier for downstream aquatic community. Generally, benthic macronvertebrates eneter in the water current are considered as drift. Depending on the temporal drift patterns, mechanisms and reasons for the entry of organisms into the water column, there is a distinction between passive and active drift (Williams and Feltmate 1992) and day and night drift (Ramirez and Pringle 1998; Huhta et al. 2000; Céréghino et al. 2004). Passive drift occurs mostly due to changes in environmental (abiotic) conditions (e.g., changes in temperature, chemical properties of water, flow conditions, pollution, etc.) (e.g., Saltveit et al. 2001). Due to the environmental changes, a certain habitat may become unsuitable for the organisms, which are thus forced to enter the drift in a search of more suitable downstream habitats. Active drift is caused by biotic factors such as the presence of

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predators (fish and predatory invertebrate taxa) and/or competition for food, habitat and shelter. It represents the “voluntary” (i.e., active) entry of organisms into the water column and it is often referred to as behavioral drift. By behavioral drift, individuals gain the ability to find new habitat or food, i.e., flee from predators or avoid overpopulation of the original microhabitat (Peckarsky 1979, 1980; Flecker 1992; Williams and Feltmate 1992; Allan 1995). Meiofauna (small metaozoans ranging approximately from 50 µm do max. 1000 µm), are important consumers of seston, especially their microfilter—feeder functional guild. They are numerous on moss covered tufa barriers, especially bdelloid rotifers. In specific benthic habitats of the Plitvice Lakes, moss covered tufa barriers, diverse invertebrate assemblage dwell, represented by both meiobenthos (for details see Section Meiofauna) and macrozoobenthos. From matter dynamics and trophic perspective, most important are filter-feeding animals, as they directly consume organic particles from seston. Both these groups (meio- and macrozoobenthos) have numerous and dense populations of microfilter-feeders: meiofaunal sized rotifers and larger caddisflies and simuliid larvae (Matoniˇckin Kepˇcija et al. 2006; Dražina et al. 2013). In the Plitvice Lakes, karst porous hydrosystem, productivity in the lentic habitats, i.e., lake epilimnion and on the bottom of lotic habitats, particularly presented by bryophyters over barriers, epifiton, seston and drift traps within bryophytes, support metabolism of aquatic biocoenoses in the deeper aphotic lake layers as well as among substrate particles and in the hyporheic zone.

2 Seston as Mediator in Diversity and Food Subsidies Distribution In this review, organic matter in seston and zooseston as its faunistic component are assessed, as both of these components are of particular importance in food webs and dynamics of energy flux and transport of matters. The majority of the zooseston organisms originate from lake littoral and benthos (bed or periphyton), and/or from plankton (from upstream lakes, accumulations). Contrary to other studies, where zooseston from long lotic sectionsflows into a short lentic area (lake, dam, reservoirs) (Welker and Walz 1998; Zimmerman-Timm et al. 2007), within the Plitvice Lakes barrage hydrosystem the opposite ratio of habitats appears—the majority of lentic (lake) habitats are connected with short lotic stretches (channels, cataracts, waterfalls). In the downstream transport of organic seston along the longitudinal profile of the Plitvice Lakes, as well as along other flow-through hydrosystems (i.e., Jankovac acummulations, Papuk Nature Park, Croatia), the seston is considered as the main food source for benthic organisms, e.g., insect larvae, molluscs (Kuˇcini´c 2002; Serti´c Peri´c et al. 2011, 2018; Špoljar et al. 2012a, b; Miliša et al. 2014; Ridl et al. 2018). Moreover, organisms entering seston contribute to the species dispersal (Serti´c Peri´c

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et al. 2011; Špoljar et al. 2012b). In this karst-protected area, control of carbon cycling, particularly from organic sources, is requested from two reasons: first, to sustain recent calcite precipitation and tufa formation (> 10 mg C dm−3 hamper precipitation; Srdoˇc et al. 1985), and second, to prevent increase of the lakes’ productivity and possible eutrophication. Moreover, high amount of organic matter can enhance rooting of vascular plants, including trees, and dislodge bryophytes, as a key factor in tufa deposition on Plitvice Lakes. Generally, there are few particularities in the flux of matter and energy in this lentic-lotic hydrosystem: due to low discharge, amounts of transported matter are much lower than in running waters with usually high discharge, and mainly epilimnion layer from upstream lake contributes in the outflow and further seston flux. For studying the transport of organic component of seston, particularly zooseston, three stretches of the karst barrage Plitvice Lakes (Croatia) were selected, in the area of Upper Lakes (cf. Špoljar et al. 2007a, b): 1. BL1–BL2 channel on the barrier with low inclination of 1% (Barrier Labudovac 1, BL1—Barrier Labudovac, outflow of Proš´ce Lake, BL2—Barrier Labudovac 2, end of Labudovac channel). At the Proš´ce Lake outflow/inflow in lotic stretch, Labudovac channel is covered with submerged vegetation, and the littoral zone of the channel with emerged vegetation, and on the last section of the channel bryophytes (Bryum, Cratoneurum) cover bottom; 2. BVJ–BLG water flow through a deep lake (Barrier Veliko jezero, BVJ—Barrier Lake Galovac, BLG). From the starting point, BVJ, water flows over a high barrier (inclination 3.6%), and through the deep Galovac Lake (maximum depth 25 m, area 0.13 km2 ); 3. BM1–BM2 a channel with cascades and sharp barrier inclination 7.6% (Barrier Milanovac 1, BM1—Barrier Milanovac 2, BM2). The starting point, BM1, was situated at the outflow of water from the largest and deepest lake in the Plitvice hydrosystem, Kozjak Lake (maximum depth 46.4 m, area 0.82 km2 ), and a large area of the littoral zone was overgrown with the emergent species Typha latifolia and Cladium mariscus. At the beginning of the stretch, bed inclination was low, while about 100 m after, the barrier dropped relatively steeply. In this area, after sharp incline the ending point, BM2, was situated, trees shade the channel, and bryophytes coverage over barrier (Bryum, Cratoneurum). In transport of total suspended matter (TSM), contribution of particulate inorganic matter ranged mainly between 60 and 90%, and percentage of particulate organic matter (POM, size range 25–50 µm) in TSM was the highest in summer, ranging from 33 to 46%. In this lentic-lotic hydrosystem, POM and discharge negatively correlated, which was likely influenced by theee three factors: (1) due to the large lake surface area, POM increase occurs during the summer, i.e., low discharge period(s), indicating importance of lake production in the POM transport, (2) positive correlation between longer retention time and higher production in the lake, and (3) erosion processes and increased inorganic particles in seston dominating at higher discharges (Welker and Walz 1998; Špoljar et al. 2007b). The majority POM content in seston belonged to detritus, while zooseston share in POM ranged between 10 and 40% (Fig. 1; Špoljar 2003). In terms of energy, benthic organisms on barriers gain a

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significant energy impulse by increasing POM concentrations in seston (i.e., BM1– BM2), supporting benthic filterers and a high percentage of caddis larvae from genus Hydropsyche (Kuˇcini´c 2002). Accordingly, large amount of organic particles enters the seston by erosion of the substrate and enriches it in terms of energy in further flux through the Plitvice Lakes. Inflow of POM in lentic stretch undergo sedimentation and enters the energy distribution among the benthic and planktonic communities of lakes (i.e., BVJ–BLG). The analysis of zooseston assemblage oscillations included the size categories of microzooseston (50–500 µm) and mesozooseston (500–1000 µm). Within zooseston, representatives of the following groups were identified: protozoans, Turbellaria, Hydrozoa, Rotifera, Nematoda, Gastrotricha, Oligochaeta, Tardigrada, Crustacea (Cladocera, Copepoda, Ostracoda) and Insecta (larvae). Organisms in zooseston differed in the habitat selection traits, and were classified as planktonic, littoral and benthic taxa (Kuczy´nska-Kippen et al. 2020). Diversity and functional traits (feeding and habitats requirements) of dominant zooseston constituents— rotifers, cladocerans and copepods—are shown in Table 1 (Chap. 11). At all sampling points of the three investigated stretches, zooseston reached maximum abundance in September (up to 147 ± 153 ind. m−3 ). Rotifers dominated in both zooseston abundance (58–80%) and diversity (66 taxa), followed by crustaceans (23 taxa), cladocerans and copepods. Zooseston abundance was highest at the beginning of longitudinal profile, at sampling point BL1 (34 ± 47 ind. m−3 ) and the lowest at the end of the profile, at sampling point BM2 (18 ± 26 ind. m−3 ). Lake Proš´ce, located at the beginning of the longitudinal profile, generally shows higher productivity in comparison to downstream Kozjak Lake, most likely due to the nutrient enrichment of the confluence tributaries. Rotifers were most abundant

Fig. 1 Annual flux of the POM and zooseston dry mass and their energy contribution (POM E, Zooseston E) at the studied stratches BL1–BL2, BVJ–BLG, BM1–BM2 along the longitudinal profile of the Plitvice Lakes

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at sampling point BLG (25 ± 32 ind. m-3 ) and least abundant at BM2 (13 ± 20 ind. m-3 ). Crustaceans were most abundant at BM1 (672 ± 419 ind. m−3 ) and in lowest abundance at BVJ (7 ± 12 ind. m-3 ). Their abundance quickly increased in summer and decreased afterwards. The resistance of rotifers in water current in comparison to cladocerans, which are very sensitive to the hydraulic stress, and copepods that avoid water current, resulted in extremely low shares of crustaceans in habitats with higher water flow, i.e., habitats located after waterfalls (BVJ, BM2; Špoljar et al. 2012b). The maximum abundance of zooseston coincides almost at all stations with the maximum concentration of chlorophyll a (chl-a), i.e., indicating algae as important food resources. Contrary, heterotrophic bacteria exhibited opposite trend, indicating the grazing effect. These relationships reflect patterns among zooplankton, algae and bacteria in the upstream lakes. Along the investigated stretches, planktonic organisms were dominant in zooseston with a relative abundance of 50 to 95%. The following zooseston species reached the highest share in abundance: Polyarthra spp., Keratella cochlearis (Gosse, 1851), Kellicotia longispina (Kellicott, 1879), Trichocerca similis (Wierzejski, 1893), Ascomorpha ovalis (Bergendahl, 1982), Synchaeta tremula (Müller, 1786), Gastropus stylifer Imhof, 1891, Collotheca mutabilis (Hudson, 1885) and Asplanchna priodonta Gosse, 1850 (Rotifera); Thermocyclops oithonoides (G. O. Sars, 1863) (adults and nauplii), Ceriodaphnia quadrangular (O. F. Müller, 1785) (Crustacea); Tintinnopsis lacustris Entz (Protozoa). From littoral species, in zooseston dominated the rotifer, Lecane lunaris (Ehrenberg, 1832). Lotic stretches, i.e., BL1–BL2 and BM1–BM2, more strongly influenced the abundance and biomass increase of benthic and littoral organisms than the decrease of planktonic organisms (Figs. 2 and 3). The stretch that includes flow through lentic area, BVJ-BLG, showed the greatest differences in the increase of planktonic organism abundance and biomass, when compared to the two other lotic stretches (Figs. 2 and 3). Among planktonic organisms, differences could be discerned regarding the avoidance of lotic habitats (Špoljar 2007b, 2012b, 2018). Biomass of oligochaets, insect larvae, nematodes and total zooseston showed negative correlations with discharge, and were most responsible for the increase of the zooseston biomass flux through the lotic stretches. Biomass of Cladocera, Copepoda, T. birostris, Polyarthra spp., K. cochlearis and rotifers is positively correlated with food resources such as chl-a and POM, which is mainly related to the upstream lake metabolism and functioning. Further research of microhabitats could reveal great biodiversity of meiofaunal and littoral species, although lake zooplankton studies in up to date literature were mainly introduced with functioning of Proš´ce and Kozjak Lake, further analyses demand research of entire hydrosystem.

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3 Drift Distribution of particles and organisms within the water column and their significance for the energy of aquatic ecosystems. Freshwater benthic macrofauna is most densely distributed between the fifth and twentieth centimeter of the substrate (Giller and Malmqvist 1998; Miliša et al. 2006a). One of the causes of such a distribution is the influence of the flow velocity, which decreases within the substrate. Thus, the organisms living below or between the substrate particles (within the so-called flow refugia) are less exposed to water flow than those living on the surface of the substrate (Lancaster and Hildrew 1993a, b). However, food sources for lotic invertebrates are usually available in the surface layers of the substrate, so benthic organisms migrate within and above the substrate, establishing daily patterns of their activity. Due to the activity, the hydrodynamic forces within and above the substrate strongly influence the organisms’ movements

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and their ability to avoid the water flow (Statzner 1988; Allan 1995). Besides hydromorphological factors and daily activity patterns, the possibility of downstream transport of benthic macrofauna is influenced by the organisms’ size (developmental stage), predator exposure, competition for food and shelter, and density of refugial structures (e.g., aquatic moss and macrophyte vegetation, fine sediment deposits) within the hydrosystem. Moss cover has been recognized as a very significant factor in the retention of organic matter and organisms in karst streams (i.e., withing Plitvice Lakes hydrosystem; Figs. 4 and 5) (Miliša et al. 2006a; Serti´c Peri´c et al. 2011), indirectly affecting the availability of food sources and the biocenological composition of benthic fauna within karst hydrosystems. The downstream transport of particles and organisms is an inevitable phenomenon in all lotic systems. This phenomenon has a great impact on the heterogeneity of lotic (micro)habitats as well as on many ecological processes at different levels within lotic ecosystems (e.g., Palmer and Poff 1997). Transported particles indirectly, by deposition, affect the morphogenesis of benthic habitats, and their further transfer through the water column greatly affects the energy status and stability of aquatic biocenoses (Cummins et al. 1983; Waringer 1992; Allan 1995). Insight into the composition of particles suspended and transported within a water column greatly contributes to understanding of the circulation of matter, energy flow and biocenoses structure within lotic habitats (Vannote et al. 1980; Minshall et al. 1985; Wotton 1990). Inorganic particles suspended in the water column most

Fig. 4 Moss-covered tufa barrier within the Upper Plitvice Lakes. Photo by M. Serti´c Peri´c

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Fig. 5 Moss-covered tufa substrate within the Upper Plitvice Lakes. Photo by M. Serti´c Peri´c

often originate from the fragmentation of the substrate due to erosion, particulate organic matter (POM) originates from detritus (i.e., decomposing plant and/or animal residues), whereas organisms enter the water column passively or actively, mainly due to changes in environmental conditions. Suspended organic matter in lotic hydrosystems can be of different origins. Autochthonous organic matter originates from the watercourse itself, whereas allochthonous organic matter consists of particles entering the watercourse from other sources, most often from riparian vegetation. Furthermore, suspended organic matter is classified into several size categories (Lamberti and Gregory 2006; Wallace et al. 2006): coarse particulate organic matter (CPOM; particles > 1 mm); fine particulate organic matter (FPOM; particles of 0.45 µm–1 mm); dissolved organic matter (DOM; particles < 0.45 µm). Allochthonous CPOM is a major source of energy in lotic ecosystems (Vannote et al. 1980; Cummins et al. 1983). After entering a watercourse, CPOM particles are usually transported downstream. If they are retained at the substrate, these particles are consumed, decomposed or (if the flow conditions change) re-enter the water column. The outcomes of the deposition and retention of CPOM particles are crucial for the further particle processing and for the energy balance within aquatic ecosystems (Cummins and Klug 1979). The dynamics of the transport, deposition and retention of CPOM particles depends on a number of hydrolomorphological conditions. All phenomena that increase the roughness of the substrate and decrease water

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flow velocity enhance removal of the CPOM particles from the water column, representing effective retention conditions (e.g., Smock et al. 1989). The Plitvice Lakes hydrosystem is rich in such phenomena, which is a consequence of a great substrate heterogeneity, extensive moss cover and the cascading alternation of lentic and lotic sections spread over numerous travertine barriers and water channels (Miliša et al. 2006a, b; Špoljar 2007a, b). The quantity and qualitative composition of FPOM greatly differ across various spatial scales, which is affected by the heterogeneity of aquatic (micro)habitats, i.e., their retention capacity for FPOM. FPOM particles are removed from the water column mainly by sedimentation, coagulation and flocculation processes, which are enhanced by increasing particle size, i.e., by increasing deposition rate. The largest amounts of FPOM most often accumulate in lakes and areas of low flow velocity, i.e., in (micro) habitats with abundant moss and macrophyte cover or debris dams (e.g., Smock et al. 1989). The downstream transport of organisms caused by the water flow, better known as drift, is the most frequently studied phenomenon related to the movement and transport of benthic fauna within lotic systems (Waters 1972). The transport of organisms among different aquatic (micro)habitats is a consequence of daily patterns of activity and behavior of benthic organisms, i.e., their movements, dispersion and (re)colonization. Drift is a very important mechanism for the distribution of benthic invertebrates in running waters (Williams and Hynes 1976). The term drift is most often associated with benthic invertebrates (insect larvae and crustaceans), but it can also include periphyton organisms, meiofauna and/or fish and amphibian larvae (Allan 1995). There is no drift fauna per se (Waters 1972), i.e., any individual belonging to the lotic benthic community can be caught in drift at some point. Invertebrates that enter the water column mainly originate from the surrounding substrate and generally stay in drift for a very short time (Allan 1995). Some taxa are particularly common and numerous in drift, such as insects belonging to the orders Ephemeroptera, Plecoptera, Trichoptera and Diptera (especially Simuliidae) and representatives of Amphipoda and Isopoda (Peckarsky 1980, 1991; Peckarsky et al. 1994; Allan 1995; Elliott 2002a, b, c).

3.1 Drift Phenomenon Drifting organisms often have a limited ability to swim (Allan 1995). In lotic systems, there is always a certain number of organisms in the water column easily noticed in the water column during the day (background drift) (Waters 1972). It is assumed that the background drift is caused by accidental dislodgement and can affect any benthic organism (e.g., Huhta et al. 2000). In general, it is difficult to prove the true causes and ways of organisms entering drift, but it is known that the occurrence of drift is influenced by numerous abiotic and biotic factors. Thus, depending on the temporal drift patterns, mechanisms and reasons for the entry of organisms into the

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water column, there is a distinction between passive and active drift (Williams and Feltmate 1992) and day and night drift (Ramirez and Pringle 1998; Huhta et al. 2000; Céréghino et al. 2004). Passive drift occurs mostly due to changes in environmental (abiotic) conditions (e.g., changes in temperature, chemical properties of water, flow conditions, pollution, etc.) (e.g., Saltveit et al. 2001). Due to the environmental changes, a certain habitat may become unsuitable for the organisms, which are thus forced to enter the drift in a search of more suitable downstream habitats. Active drift is caused by biotic factors such as the presence of predators (fish and predatory invertebrate taxa) and /or competition for food, habitat and shelter. It represents the “voluntary” (i.e., active) entry of organisms into the water column and it is often referred to as behavioral drift. By behavioral drift, individuals gain the ability to find new habitat or food, i.e., flee from predators or avoid overpopulation of the original microhabitat (Peckarsky 1979, 1980; Flecker 1992; Williams and Feltmate 1992; Allan 1995). Besides passive and active drift, many authors have observed diurnal drift periodicity, i.e., differences between day and night drift (Huhta et al. 2000; Elliott 2002b; Céréghino et al. 2004). The number of individuals and the diversity of species present in drift are usually higher at night and most often reach maxima soon after sunset and just before sunrise (i.e., at dusk and before dawn). The increased nocturnal drift is an adaptation of benthic invertebrates to the pressure of visual (day) predators (Elliott 1968). Due to the increased predatory pressure during the day, most benthic invertebrates became active at night that increased the possibility of organisms being “washed away” from the substrate (Flecker 1992). There is also a seasonal periodicity of drift. It mainly depends on seasonal differences in temperature, the amount of organic matter within the hydrosystem and the composition of benthic invertebrate community as well as on the life cycles of dominant invertebrate taxa. The largest number of individuals in drift occurs in periods of rapid growth of aquatic insects’ larvae and in the periods before the pupation and emergence (Saltveit et al. 2001).

3.2 Significance of Studying Particle Transport and Benthic Invertebrate Drift in Karst Hydrosystems Karst water systems are widespread throughout the world, being recognized for the richness of endemic and relict life forms inhabiting them, specific geological structure, hydrological features and ecological processes crucial for their sustainability (World Bank 2008). Nevertheless, there is a very scarce information on the importance and spatio-temporal patterns of transport processes of organic and inorganic particles, and organisms in karst hydrosystems (Massei et al. 2006; Špoljar et al. 2007a, b; Serti´c Peri´c et al. 2011, 2014, 2015, 2020). However, it is very important to gain insight into these processes, as they represent food, energy and bed-sediment

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movements within karst hydrosystems that further affect the energy basis for spatial and temporal dynamics as well as for stability and sustainability of karst aquatic habitats. Most previous drift studies have been focused on the patterns of drift in lowland streams, mountain, alpine (glacial) and tropical streams, lake outlets, rocky-, graveland sandy-bottomed streams (e.g. Elliott and Tullett 1977; Waringer 1992; Rader and McArthur 1995; Ramirez and Pringle 1998, 2001; Robinson et al. 2001, 2002; Saltveit et al. 2001; Jacobsen and Bojsen 2002; Hansen and Closs 2007). However, there are only several studies dealing with the downstream transport of particles and organisms in karst and tufa-forming hydrosystems (e.g., Pendergrass 2006; Špoljar et al. 2007a, b; Serti´c Peri´c et al. 2011, 2014, 2015, 2020). Here we present the most interesting findings on the drift patterns at the hydromorphologically different mesohabitats within the Plitvice Lakes hydrosystem.

3.3 Drift Patterns Between Two Mesohabitat Types (Barriers and Pools) Within Plitvice Lakes Hydrosystem We examined the quantitative and qualitative invertebrate drift composition and the associated particulate organic and inorganic matter (APOIM) between barriers and pools throughout the annual (2006–2007) cycle, comparing: (a) spatial drift patterns between two mesohabitat types (barriers and pools) and (b) temporal (seasonal and diel) drift patterns at these mesohabitat types. Our study reach was located within the Upper Lakes, between Burgeti Lake (553 m a.s.l.; area: 1 ha; depth: 5 m) and Kozjak (535 m a.s.l.; area: 81.5 ha; depth: 47 m). It is a ca. 10 m long stretch placed closely downstream of the Burgeti Lake consisting of two tufa barriers and a shallow pool in-between (Fig. 6). Water along the study reach flows over the first barrier (i.e., sampling site B1), creates ca. 1 m-high waterfall and feeds the shallow pool (i.e., sampling sites P1, P2), which ends up with another barrier (i.e., sampling site B2) and a series of waterfall cascades (Fig. 7) that ends up at the Lake Kozjak. The four sampling sites (B1, B2, P1 and P2) represented replicate units of the two differing mesohabitat types, i.e., barriers (B; fast-flowing mesohabitats) and pools (P; slow-flowing mesohabitats). The distance between the individual sampling sites was ca. 3–4 m, enabling the small-scale assessment of drift. Triplicate drift samples were collected monthly from October 2006 to September 2007 (with the exception of August 2007), during midday and dusk at the four sampling sites, i.e., at the two mesohabitat types (barriers and pools). Drift samples consisted of drifting invertebrates, moss, and associated particulate organic and inorganic matter (APOIM). All constituents of the drift samples were analysed. Our drift study revealed that barriers have significantly higher invertebrate drift densities than pools (Mann–Whitney U-test, p < 0.0001) (Fig. 8). The same pattern was observed for moss and APOIM (i.e., CPOM excluding moss; TIM, i.e., total inorganic matter and FPOM) (Mann–Whitney U-test, p < 0.05), whose amounts

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Fig. 6 Spatial arrangement of the four sampling sites along the study reach. Photo by M. Serti´c Peri´c

at barriers were up to fivefold higher than in pools (Fig. 8). In general, for most measured parameters, drift was significantly higher at barriers during the day (Fig. 8). An exception was invertebrate drift within pools, which did not yield significant diel differences (Fig. 8). Such result suggested that within pools, invertebrate drift is largely aperiodic. The increased midday and/or aperiodic drift (between barriers and pools, respectively) are likely a consequence of the lack of fish between barrier- and pool-mesohabitats along the drift study reach within the Plitvice Lakes hydrosystem (Flecker 1992; Hammock et al. 2012; Worischka et al. 2015). These results also indicated that the barriers are a certain “hotspots” of intense day drift of invertebrates and APOIM transport within the Plitvice Lakes hydrosystem. Although some studies suggest that low invertebrate drift densities within pools result from low flow velocities at those mesohabitat types (e.g., Lancaster et al. 2011; Brooks et al. 2017), drift studies conducted within the Plitvice Lakes hydrosystem evidenced that there was no significant correlation between flow velocity and the drift densities (Serti´c Peri´c et al. 2011, 2014, 2015). But, a highly significant positive correlation between the amounts of moss and other drift parameters (i.e., invertebrate drift densities, and the amounts of CPOM excluding moss and TIM in drift samples) suggested that drift patterns within the Plitvice Lakes hydrosystem are greatly influenced by seasonal changes in the moss-covered tufa substrate and its efficiency to

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Fig. 7 A series of waterfall cascades downstream of the drift study reach. Photo by M. Serti´c Peri´c

trap particles and organisms (Serti´c Peri´c et al. 2011, 2014, 2015, 2020). This conclusion was additionally supported by the finding that at both mesohabitat types, mean invertebrate drift densities were generally significantly higher (>100 ind. m−3 ) in autumn and summer than in spring and winter (< 90 ind. m−3 ) (Kruskal–Wallis test, p < 0.001) (Fig. 9). Further analysis (Serti´c Peri´c et al. 2011, 2014, 2015, 2020) revealed that the observed invertebrate drift seasonality was significantly correlated with seasonal drift patterns of aquatic moss. The peaking moss and FPOM drift densities at barriers during late autumn suggested that aquatic moss within our study system is most vulnerable to flow alterations during seasons of aquatic vegetation (i.e., moss) die-off in temperate regions. In addition, the peaking autumn moss amounts in drift samples can be a consequence of the increased moss fragility resulting from high contents of tufa precipitated on moss substrate at that time of year, due to which the moss can become more prone to crack (Serti´c Peri´c et al. 2011, 2020). The increased moss and particulate matter quantities in autumn drift samples were accompanied by the drift density peaks of Oligochaeta, Coleoptera, some Diptera and Trichoptera that represent common moss-dwelling taxa. In spring and summer, drift samples contained higher numbers of Nematoda, Cladocera, Copepoda, Ephemeroptera, Simuliidae and Chironomidae, which was probably

160 Fig. 8 Mean (+ SE, i.e., standard error) invertebrate drift densities (at the top) and the amounts of associated particulate organic and inorganic matter (APOIM), i.e., (from top to bottom): moss, coarse particulate organic matter (CPOM) excluding moss, fine particulate organic matter (FPOM) and total inorganic matter (TIM) in midday and dusk drift samples taken at the two mesohabitat types (B – barriers, P – pools) from October 2006 to September 2007. Asterisks indicate significant differences between midday and dusk based on Mann–Whitney U-test: *p < 0.05, **p < 0.01. Note different scaling of y-axes

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Fig. 9 Seasonal differences in the mean (± SE, i.e., standard error) invertebrate drift densities along the study reach, considering both mesohabitat types (barriers and pools) during the study period (October 2006–September 2007). Asterisks indicate significantly higher mean drift densities during the study period based on Kruskal–Wallis test: ***p < 0.001

a consequence of the seasonal distributional shifts of the aquatic insects’ larvae (i.e., Ephemeroptera, Simuliidae, Chironomidae), moss-dwelling meiofauna (i.e., Nematoda) and/or plankton organisms (i.e., Cladocera, Copepoda) originating from the upstream Burgeti Lake (Serti´c Peri´c et al. 2011, 2014). Thus, the observed spatio-temporal drift patterns within the Plitvice Lakes hydrosystem are likely greatly influenced by the ontogenetic shifts in drift periodicity (i.e., shifts depending on the development stage and morphological characteristics of the individual taxa) as well as by the substrate-specific benthic distribution of invertebrate taxa. These results further indicate that aquatic invertebrates inhabiting fast-flowing barriers and slow-flowing pools within the Plitvice Lakes hydrosystem most likely exhibit “passive drift” mediated by transport agents such as water flow and dislodged aquatic vegetation.

4 Meiofauna 4.1 The Position of Meiofauna in the Matter Dynamics of Plitvice Lakes Ecosystem Meiobethos or meiofauna is a heterogenous group of sediment-associated organisms roughly defined by their size, representing animals that can pass through a 1000 µm sieve (although some authors use a 500-µm upper size limit), but are retained on a 44 µm or even 31 µm sieve (Giere 2009). According to the life cycle, two meiofaunal groups can be distinguished: permanent (metazoans that remain within the meiofaunal size range throughout their entire life cycle) and temporary (small stages of metazoans that start as meiofauna and grow into macrofauna). As stated above, the definition of meiofauna is not unambiguous and sometimes it is difficult to distinguish the size categories of microzoobenthos, meiofauna and macrozoobenthos in the

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marine and freshwater habitats (Giere 2009). In the sediment of freshwater ecosystems, the meiofauna consists of the following taxonomic groups: Rotifera, Nematoda, Gastrotricha, Turbellaria, Oligochaeta, Hydrachnidia, Tardigrada, Ostracoda, Cladocera, Copepoda and small insect larvae (Palmer et al. 2006). In lotic habitats 60−80% of all species are meiofaunal size (Robertson 2000), reaching density up to 6000 ind. cm−3 (Palmer 1990). Being highly abundant in some habitats, meiofauna is important from trophic perspective. Meiofauna is a link between bacteria and higher trophic levels in the food webs (Schmid-Araya et al. 2002; Stead et al. 2005), significantly affecting benthos metabolism. Meiofaunal grazing has a stabilizing effect on periphyton and bacterial populations, by keeping it in a state of logarithmic growth (Traunspurger et al. 1997; Schmid and Schmid-Araya 2002). Filter-feeding meiofauna, especially rotifers consume considerable amounts of UPOM, comparable to bivalves (Kathol et al. 2011; Dražina et al. 2013). There is a growing evidence for top down effects on meiofauna, as numerous macroinvertebrates and juvenile fish feed primarily on meiobenthos (Spieth et al. 2011; Ptatscheck et al. 2020). One of the most important phenomena of the Plitvice Lakes ecosystem are moss covered tufa barriers. Mosses (or bryophytes) represent important functional habitats in lotic and lentic ecosystems. Suren (1992) even coined the term “biotic hyporheic zones” for mosses, as they harbour large quantities of detritus and enormous numbers of invertebrates, with meiofaunal densities being by one or two orders of magnitude higher than in the surrounding gravel bed or fine sediment (Suren 1992; Dražina et al. 2017). In the Plitvice lakes, meiofauna was considered as important part of drift (Serti´c Peri´c et al. 2014), but also as important part in ecology of tufa barriers (Dražina et al. 2013, 2014, 2017). Here we will focus on permanent meiofaunal density and functional feeding guilds in vertical profile of moss-covered tufa substrate. Also, we will discuss most important abiotic factor in these habitat—water flow velocity (FV). For this investigation, we chose the Lower Lakes and the last tufa barrier and Lake (44°54' 11”N; 15°36' 36”E), which divides the Lake Kaluderovac Novakovi´ca Brod, just before formation of the Korana River. Three sampling sites (microhabitats) were chosen: S—slow FV < 50 cm s−1 , M—medium FV 50– 100 cm s−1 and F—fast FV > 100 cm s−1 . At each sampling site, samples of tufa sediments and bryophytes were taken using metal core samplers (inner diameter 4 cm) in triplicate, and each core was divided into three layers: surface bryophyte mats (B) and two deeper hyporheic layers, a porous tufa layer with encrusted bryophyte remains (R) and a layer of solid tufa (T). Samples were collected monthly, from February 2009 to January 2010. In the laboratory meiofauna together with food resources i.e., particulate organic matter (POM) were analyzed. We used two different sieves (1000 µm and 44 µm) to separate macrozoobenthos and meiobethos, but also to obtained three POM size-fractions: coarse (CPOM, >1000 µm), fine (FPOM, 1000–44 µm) and ultrafine (UPOM, 10%). Achnanthidium pyrenaicum is represented with the highest relative abundance (66.42%), followed by C. affinis var. angusta and P. costei with 43.38% and 30.26%, respectively. When the criteria for rare taxa with occurrence at one or two sampling sites are applied to the taxa list, 29 of them are found to be rare at tufa barriers in Plitvice Lakes National Park. Those are Aneumastus stroesei, Brachysira vitrea, Caloneis cf. silicula, Cymatopleura apiculata,

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Cymbella cymbiformis, Cymbella laevis, Cymbella simonsenii, Cymbopleura subaequalis, Diploneis petersenii, Encyonema ventricosum, Encyonopsis microcephala, Epithemia parallela, Eucocconeis flexella, Fragilaria vaucheriae, Gomphonema auritum, Gomphonema lateripunctatum, Surirella birostrata, Luticola cf. frequentissima, Meridion circulare, Navicula hintzii, Navicula densilineolata, Navicula notha, Navicula pseudobryophila, Navicula tripunctata, Geissleria sp., Nitzschia gessneri, Nitzschia pura, Stephanodiscus medius and Ulnaria capitata. Besides being present at only one or two sampling sites, all of them have very low relative abundance 5% to the similarity between the Upper Lakes samples are A. affine, A. pyrenaicum, E. minuta, A. bryophila, C. plitvicensis and P. costei. Cymbella affinis var. angusta, P. costei, D. delicatulus, E. minuta, A. affine, B. neglectissima, G. clavatum and U. delicatissima are characteristic taxa for Lower Lakes, while for the barrier below Veliki slap Waterfall those are A. pyrenaicum, D. tenius, D. delicatulus and E. minuta.

3.2 Floristic Composition of Non-diatoms A total of 72 taxa of algae were identified on tufa barriers, which were not diatoms and are referred to as non-diatoms in the text (Table 3). Because the water on the barriers is mostly lake surface water, 15 of these are classified as planktonic and 57 as benthic. The most diverse and taxonomically rich group is Cyanobacteria (41). Other benthic algae belong to the group of Streptophyta (10), Chlorophyta (4) and Rhodophyta (2). Macroscopic and microscopic images of selected taxa are shown on Figs. 6 and 7. Abundance of non-diatom photoautotrophs was estimated in the field and in the microscope, which gave a unique number for abundance on the scale of 1–5 (CEN– EN 15708 2009). Then the abundance was cubed, and the relative abundance is shown in Fig. 8. Among the non-diatom groups of benthic algae, cyanobacteria dominate in most cases. Chlorophyta are most abundant on the Kozjaˇcki mostovi Barrier, while taxa from the Streptophyta group are characteristic of the tufa barriers of the Upper Lakes and below the Veliki slap Waterfall. Rhodophyta are characteristic of the late

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Table 2 List of diatoms identified at tufa barriers from Plitvice Lakes in June and September 2016, with Omnidia code (OC), frequency of occurrence in samples (Freq.) and maximum abundance (Max.) Taxon

OC

Freq. (%)

Max.

Achnanthidium affine (Grunow) Czarnecki

ACAF

100

20.75

Achnanthidium caledonicum (Lange-Bertalot) Lange-Bertalot

ADCA

69

3.26

Achnanthidium minutissimum (Kützing) Czarnecki

ADMI

56

4.49

Achnanthidium pyrenaicum (Hustedt) H.Kobayasi

ADPY

100

66.42

Achnanthidium straubianum (Lange-Bertalot) Lange-Bertalot

ADSB

69

2.47

Achnanthidium trinode Ralfs

ADTR

19

2.25

Adlafia bryophila (J. B. Petersen) Lange-Bertalot

ABRY

81

16.36

Amphipleura pellucida (Kützing) Kützing

APEL

38

2.39

Amphora inariensis Krammer

AINA

19

7.73

Amphora meridionalis Levkov

AMDN

19

0.24

Amphora pediculus (Kutzing) Grunow

APED

38

2.31

Aneumastus minor (Hustedt) Lange-Bertalot

ANMI

38

0.69

Aneumastus stroesei (Østrup) D. G. Mann

ANSS

6

0.23

Brachysira liliana Lange-Bertalot

BLIL

25

0.49

Brachysira neglectissima Lange-Bertalot

BNEG

75

15.15

Brachysira neoexilis Lange-Bertalot

BNEO

50

2.56

Brachysira styriaca (Grunow) R.Ross

BSTY

31

0.5

Brachysira vitrea (Grunow) R. Ross

BVIT

6

0.25

Caloneis alpestris (Grunow) Cleve

CAPS

31

0.68

Caloneis cf. silicula (Ehrenberg) Cleve

CSIL

6

0.25

Caloneis lancettula (Schulz) Lange-Bertalot and Witkowski

CLCT

19

8.18

Caloneis tenuis (Gregory) Krammer

CATE

19

0.25

Chamaepinnularia cf. soehrensis (Krasske) Lange-Bertalot and Krammer

CHSO

44

2.25 14.59

Cyclotella plitvicensis Hustedt

CPTV

81

Cymatopleura apiculata W. Smith

CYAP

6

0.23

Cymbella affinis var. angusta (Krammer) W. Silva

CAFG

63

43.38

Cymbella cymbiformis C. Agardh

CCYM

6

0.24

Cymbella laevis Nägeli

CLAE

6

0.24

Cymbella lancettula (Krammer) Krammer

CLTL

25

0.25

Cymbella neoleptoceros Krammer

CNLP

25

1.12

Cymbella simonsenii Krammer

CSMO

6

0.24

Cymbella subhelvetica Krammer

CSBH

63

2.56

Cymbella vulgata var. plitvicensis Krammer

CVPL

50

5.11

Cymbopleura amphicephala (Nägeli) Krammer

CBAM

19

0.25 (continued)

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Table 2 (continued) Taxon

OC

Cymbopleura diminuta (Grunow) Krammer

CBDM

Freq. (%) 31

Max. 1.57

Cymbopleura subaequalis (Grunow) Krammer

CSAE

13

0.25

Delicatophycus delicatulus (Kützing) M. J. Wynne

DPDE

100

11.95

Denticula tenius Kützing

DTEN

69

20.6

Diatoma ehrenbergii Kützing

DEHR

75

3.99

Diatoma moniliformis (Kützing) D. M. Williams

DMON

25

5.59

Diploneis calcicolafrequens Lange-Bertalot and A. Fuhrmann

DCFQ

38

0.46

Diploneis petersenii Hustedt

DPET

Diploneis plitvicensis Lange-Bertalot, Fuhrmann and Werum Diploneis praetermissa Lange-Bertalot and A. Fuhrmann

DPRT

6

0.22

25

1.59

56

2.36

Diploneis sp.

DIPS

38

2.73

Encyonema caespitosum Kützing

ECAE

44

2.47

Encyonema ventricosum (C. Agardh) Grunow

ENVE

19

0.75

Encyonopsis cesatii (Rabenhorst) Krammer

ECES

75

1.85

Encyonopsis krammeri E. Reichardt

ECKR

75

12.68

Encyonopsis microcephala (Grunow) Krammer

ENCM

6

0.5

Encyonopsis minuta Krammer and E. Reichardt

ECPM

100

Encyonopsis subminuta Krammer and E. Reichardt

ESUM

81

20.95 6.22

Epithemia parallela (Grunow) Ruck and Nakov

EPHP

6

0.22

Eucocconeis flexella (Kützing) Meister

EUFL

19

0.46

Eucocconeis laevis (Østrup) Lange-Bertalot

EULA

69

3.11

Eunotia arcubus Nörpel and Lange-Bertalot

EARB

81

1.85

Fallacia subhamulata (Grunow) D. G. Mann

FSBH

38

2.95

Fragilaria austriaca (Grunow) Lange-Bertalot

FCAU

38

0.75

Fragilaria perminuta (Grunow) Lange-Bertalot

FCPE

19

0.48

Fragilaria vaucheriae (Kützing) J. B.Petersen

FVAU

13

0.25

Geissleria sp.

GESP

6

0.68

Gomphonema auritum A. Braun ex Kützing

GAUR

13

0.25 11.27

Gomphonema clavatum Ehrenberg

GCLA

100

Gomphonema elegantissimum E. Reichardt and Lange-Bertalot

GELG

19

0.5

Gomphonema lateripunctatum E. Reichardt and Lange-Bertalot

GLAT

13

0.24

Gomphonema lippertii E. Reichardt and Lange-Bertalot

GLIP

25

0.72

Gomphonema occultum E. Reichardt and Lange-Bertalot

GOCU

63

7.16

Gomphonema tenoccultum E. Reichardt

GTNO

31

0.47

Gomphonema vibrio Ehrenberg

GVIB

25

0.99 (continued)

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Table 2 (continued) Taxon

OC

Luticola cf. frequentissima Levkov, Metzeltin and A. Pavlov

LFRQ

Freq. (%) 6

Max. 0.23

Mastogloia lacustris (Grunow) Grunow

MLAC

25

5.62

Meridion circulare (Greville) C. Agardh

MCIR

6

0.24

Navicula cryptotenella Lange-Bertalot

NCTE

81

11.35

Navicula cryptotenelloides Lange-Bertalot

NCTO

81

3.12

Navicula dealpina Lange-Bertalot

NDEA

38

0.49

Navicula densilineolata (Lange-Bertalot) Lange-Bertalot

NDSL

6

0.23

Navicula hintzii Lange-Bertalot

NHIN

44

1.18

Navicula notha J. H. Wallace

NNOT

6

0.24

Navicula pseudobryophila Hustedt

NPBY

6

0.9

Navicula radiosa Kützing

NRAD

38

0.49

Navicula subalpina E. Reichardt

NSBN

88

4.44

Navicula tripunctata (O. F. Müller) Bory

NTPT

13

0.46

Nitzschia dissipata (Kützing) Rabenhorst

NDIS

38

0.72

Nitzschia fonticola (Grunow) Grunow

NFON

31

1.12

Nitzschia gessneri Hustedt

NGES

6

0.47

Nitzschia lacuum Lange-Bertalot

NILA

19

0.96

Nitzschia media Hantzsch

NIME

13

0.47

Nitzschia pura Hustedt

NIPR

6

0.24

Nitzschia semirobusta Lange-Bertalot

NSRB

19

0.93

Pantocsekiella costei (J. C. Druart and F. Straub) K. T. Kiss and E. Ács

PCOS

88

30.26

Pseudostaurosira parasitica (W. Smith) Morales

PPRS

19

1.85

Sellaphora stroemii (Hustedt) H. Kobayasi

SSTM

94

2.2

Simonsenia delognei (Grunow) Lange-Bertalot

SIDE

25

0.46

Stephanodiscus medius Håkansson

SMED

6

0.25

Surirella birostrata Hustedt

SBIR

6

0.22

Tabellaria flocculosa (Roth) Kützing

TFLO

25

1.75

Tryblionella angustata W. Smith

TANG

38

0.5

Ulnaria capitata (Ehrenberg) Compère

UCAP

6

1.12

Ulnaria delicatissima (W. Smith) Aboal and P. C. Silva

UDEL

50

17.04

Ulnaria ulna (Nitzsch) Compère

UULN

50

0.73

spring/early summer period and they are present in the entire Plitvice Lakes where the environmental conditions are optimal, especially the faster water flow (Necchi 2016). Maximum abundance on a scale of 1–5 was rated 4 only in September and covered 75% of the barrier with Oocardium stratum, Spirogyra sp. and Oscillatoria simplicissima. Taxa Rivularia haematites, Microspora amoena var. gracilis, Mougeotia sp.,

Benthic Algae on Tufa Barriers

197

Fig. 2 LM micrographs of diatoms: 1. Cymbella dorsenotata, 2. Cymbella simonsenii, 3. Cymbella cymbiformis, 4. Cymbella subhelvetica, 5. Cymbella vulgata var. plitvicensis, 6. Cymbella lancettula, 7. Cymbella neoleptoceros, 8. C. affinis var. angusta, 9. Cymbella laevis, 10. Encyonema ventricosum, 11. Delicatophycus delicatulus, 12. Encyonema caespitosum, 13. Amphora meridionalis, 14. Amphora inariensis, 15. Amphora pediculus, 16. Cymbopleura amphicephala, 17. Cymbopleura subaequalis, 18. Cymbopleura diminuta, 19. Geissleria sp., 20. Fallacia subhamulata, 21. Encyonopsis cesatii, 22. Encyonopsis subminuta, 23. Encyonopsis krammeri, 24. Encyonopsis minuta, 25. Encyonopsis microcephala, 26. Pantocsekiella costei, 27. Cyclotella plitvicensis, 28. Stephanodiscus medius, 29. Aneumastus minor, 30. Aneumastus stroesei

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Fig. 3 LM micrographs of diatoms: 31. Navicula radiosa, 32. Navicula dealpina, 33.–34. Navicula hintzii, 35. Navicula tripunctata, 36. Navicula densilineolata, 37. Navicula gottlandica, 38. Navicula subalpina, 39. Navicula notha, 40. Luticola cf. frequentissima, 41. Adlafia bryophila, 42. Navicula pseudobryophila, 43. Sellaphora stroemii, 44. Navicula cryptotenelloides, 45. Navicula cryptotenella, 46. Brachysira vitrea, 47. Brachysira neglectissima, 48. Brachysira neoexilis, 49. Brachysira styriaca, 50. Brachysira liliana, 51. Eucocconeis flexella, 52. Eucocconeis laevis, 53.–54. Achnanthidium trinode, 55. Achnanthidium straubianum, 56. Chamaepinnularia cf. soehrensis, 57.–58. Achnanthidium pyrenaicum, 59. Achnanthidium affine, 60. Achnanthidium minutissimum, 61.–62. Achnanthidium caledonicum, 63.–64. Mastogloia lacustris

Benthic Algae on Tufa Barriers

199

Fig. 4 LM micrographs of diatoms: 65. Gomphonema tenoccultum, 66. Gomphonema elegantissimum, 67. Gomphonema occultum, 68. Gomphonema auritum, 69. Gomphonema clavatum, 70. Gomphonema lippertii, 71.–73. Gomphonema lateripunctatum, 74. Gomphonema vibrio, 75. Caloneis cf. silicula, 76. Caloneis lancettula, 77. Caloneis tenuis, 78. Caloneis alpestris, 79. Cymatopleura apiculata, 80. Surirella birostrata, 81. Diploneis praetermissa, 82. Diploneis petersenii, 83. Diploneis sp., 84. Diploneis plitvicensis, 85. Diploneis calcicolafrequens

Batrachospermum gelatinosum, Tolypothrix distorta, Cladophora cf. glomerata and Oedogonium sp. are rated 3, covering up to 50% of the tufa barriers from Plitvice Lakes. Apart from O. stratum, the composition of the non-diatom part of the photoautrotrophic community does not contain any highly specific taxa. On the other hand, in the results of Stankovi´c et al. (2017) the cyanobacterial part of the community does not contain Phormidium incrustatum, which is normally common in tufa

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Fig. 5 LM micrographs of diatoms: 86. Amphipleura pellucida, 87. Diatoma ehrenbergii, 88. Diatoma moniliformis, 89. Nitzschia media, 90. Nitzschia pura, 91. Nitzschia gessneri, 92. Tryblionella angustata, 93. Simonsenia delognei, 94. Denticula tenius, 95. Nitzschia lacuum, 96. Nitzschia semirobusta, 97. Nitzschia fonticola, 98. Nitzschia dissipata, 99. Eunotia arcubus, 100. Meridion circulare, 101. Tabellaria flocculosa, 102. Epithemia parallela, 103. Fragilaria austriaca, 104. Ulnaria delicatissima, 105. Ulnaria ulna, 106. Fragilaria vaucheriae, 107. Fragilaria perminuta, 108. Pseudostaurosira parasitica, 109. Ulnaria capitata

communities (Arp et al. 2010; Berrendero et al. 2016). Since P. incrustatum is part of the tufa deposits of Plitvice Lakes (Golubi´c et al. 2008; Matoniˇckin et al. 1971), the results suggest that this species is difficult to identify in the surface algal mats, as it is carbonate-encrusted species. Both O. stratum and P. incrustatum produce extracellular polymers as locus of intense calcification, supporting tuff formation (Golubi´c et al. 2008).

Benthic Algae on Tufa Barriers

201

Table 3 List of non-diatoms identified at tufa barriers from Plitvice Lakes in June and September 2016, with given code, frequency of occurrence in samples (Freq.) and maximum abundance (Max.) Code

Freq. (%)

Max.

Chlamydomonas sp.

Chsp

19

1

Cladophora sp.

Clsp

6

3

Coelastrum sphaericum Nägeli

*

6

1

Crucigeniella irregularis (Wille) P.M.Tsarenko & D.M.John

*

6

1

Lemmermannia triangularis (Chodat) C.Bock & Krienitz

*

13

1

Microspora amoena var. gracilis (Wille) De Toni

Mica

31

3

Oedogonium sp.

Oesp

38

3

Oocystis marssonii Lemmermann

*

6

1

Pseudopediastrum integrum (Nägeli) M. Jena & C. Bock

*

6

1

Sphaerocystis planctonica (Korshikov) Bourrelly

*

13

1

Sphaerocystis schroeteri Chodat

*

6

2

Tetradesmus obliquus (Turpin) M.J.Wynne

*

6

1

Anathece smithii (Komárková-Legnerová & Cronberg) Komárek, Kaštovský & Jezberová

*

6

1

Aphanocapsa incerta (Lemmermann) Cronberg & Komárek

*

6

1

Chroococcus minutus (Kützing) Nägeli

Cmin

19

1

Chroococcus obliteratus Richter

Cobl

6

1

Chroococcus turgidus (Kützing) Nägeli

Ctur

13

1

Cyanobium cf. parvum (Migula) Komárek, Kopecký & Cepák Cpar

13

2

Komvophoron minutum (Skuja) Anagnostidis & Komárek

Kmin

25

1

Komvophoron schmidlei (Jaag) Anagnostidis & Komárek

Ksch

31

2

Leptolyngbya mucicola (Lemmermann) Anagnostidis & Komárek

Lmuc

25

2

Limnothrix meffertae Anagnostidis

Lmef

13

1

Limnothrix redekei (Van Goor) Meffert

Lred

6

1

Lyngbya sp.

Lysp

6

1

Merismopedia glauca (Ehrenberg) Kützing

Mgla

25

1

Microcoleus amoenus (Gomont) Strunecký, Komárek & J.R.Johansen

Mamo

6

1

Microcoleus autumnalis (Gomont) Strunecký, Komárek & J.R.Johansen

Maut

13

2

Microcoleus cf. setchellianus (Gomont) Strunecký, Komárek & Johansen

Mset

19

2

Nodosilinea bijugata (Kongisser) Perkerson & Kováˇcik

Nbij

6

1

Taxon Chlorophyta

Cyanobacteria

(continued)

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I. Stankovi´c et al.

Table 3 (continued) Taxon

Code

Oscillatoria ornata Kützing ex Gomont

Oorn

Freq. (%) 6

Max. 1

Oscillatoria proboscidea Gomont

Opro

13

1

Oscillatoria simplicissima Gomont

Osim

31

4

Oscillatoria sp. 1

Ossp1

25

1

Oscillatoria sp. 2

Ossp2

6

1

Oscillatoria tenuis C. Agardh ex Gomont

Oten

31

1

Phormidium ambiguum Gomont

Pamb

6

1

Phormidium cf. grunowianum (Gomont) Anagnostidis & Komárek

Pgrun

6

1

Phormidium chlorinum (Kützing ex Gomont) Umezaki & Watanabe

Pchl

6

1

Phormidium griseoviolaceum (Skuja) Anagnostidis

Pgri

19

1

Phormidium irriguum (Kützing ex Gomont) Anagnostidis & Komárek

Pirr

6

1

Phormidium retzii Kützing ex Gomont

Pret

13

2

Phormidium sp. 1

Phsp1

25

1

Phormidium sp. 2

Phsp2

6

1

Phormidium subfuscum Kützing ex Gomont

Psub

6

1

Phormidium terebriforme (C.Agardh ex Gomont) Anagnostidis & Komárek

Pter

6

1

Pseudanabaena biceps Böcher

Pbic

6

1

Pseudanabaena limnetica (Lemmermann) Komárek

Plim

13

1

Pseudanabaena minima (G.S.An) Anagnostidis

Pmin

6

1

Pseudanabaena sp.

Pssp

38

1

Pseudanabaena starmachii Anagnostidis

Psta

6

1

Rivularia haematites C.Agardh ex Bornet & Flahault

R hae

38

3

Scytonema cf. myochrous C.Agardh ex Bornet & Flahault

Smyo

13

1

Tapinothrix varians (Geitler) Bohunická & J.R.Johansen in Bohunická, Johansen & Fuˇcíková

Tvar

6

2

Tolypothrix distorta Kützing ex Bornet & Flahault

Tdis

13

3

Chrysocapsella planctonica (West & G.S.West) Bourrelly

*

13

2

Dinobryon crenulatum West & G.S.West

*

6

1

Dinobryon divergens O.E.Imhof

*

38

3

Dinobryon sertularia Ehrenberg

*

0

0

Dinobryon sociale (Ehrenberg) Ehrenberg

*

25

1

Batrachospermum gelatinosum (Linnaeus) De Candolle

Bgel

25

3

Batrachospermum sp.

Basp

6

Ochrophyta

Rhodophyta 2 (continued)

Benthic Algae on Tufa Barriers

203

Table 3 (continued) Code

Freq. (%)

Max.

Cosmarium botrytis Meneghini ex Ralfs

Cbot

13

1

Cosmarium crenatum Ralfs ex Ralfs

Ccre

6

1

Cosmarium formosulum Hoff

Cfor

6

1

Cosmarium granatum Brébisson ex Ralfs

Cgra

25

1

Cosmarium margaritatum (P.Lundell) J.Roy & Bisset

Cmar

6

1

Cosmarium pseudoornatum B.Eichler & Gutwinski

Cpse

13

1

Mougeotia sp.

Mosp

63

3

Oocardium stratum Nägeli

Ostr

13

4

Spirogyra sp.

Spsp

13

4

Zygnema sp.

Zysp

19

1

Taxon Streptophyta

4 Diversity Ecosystem diversity or biodiversity is one of the key factors for healthy habitats and for sustainable life on Earth. Dudgeon et al. (2006) listed the threats to global freshwater biodiversity under five headings: overexploitation, water pollution, flow modification, destruction or degradation of habitat, and invasion by exotic species. Although in National Parks these threats are minimized, it is important to understand the diversity of benthic algae if we are to understand the ecology of travertine systems such as Plitvice Lakes for even better future conservation. Alpha diversity (local species diversity) in tufa barriers from Plitvice Lakes, based on species number (S) and Shannon diversity index (H' ), shows high differences between diatoms and non-diatoms (Fig. 9). Both species number and Shannon diversity index are high for diatoms on the Upper and Lower Lakes barriers. Despite the low number of non-diatom taxa, the diversity index is high on the Upper Lakes barriers and part of the Lower Lakes. The tufa barrier at the outlet of Lake Gradinsko has almost exclusively diatoms as benthic algae. The Kozjaˇcki mostovi Barrier as the outlet barrier of the largest Lake Kozjak also shows low species number and diversity on both sides, left and right. The algal composition on the tufa barrier below the Veliki slap Waterfall reflects anthropogenic influence, which we can see in the high nitrate concentration, showing a lower number and diversity of diatoms and a rich community of non-diatoms. For diatoms, neither species richness nor Shannon diversity index show remarkable differences between the two sampling campaigns, while Shannon diversity of non-diatoms on the tufa barrier at the outlet of Lake Batinovac is quite low in September compared to June. Interhabitat diversity, or beta diversity in the case of benthic algae at tufa barriers from Plitvice Lakes, is shown in the dendrogram plot generated by cluster analysis based on Bray–Curtis Similarity between samples for diatoms and non-diatoms

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I. Stankovi´c et al.

Fig. 6 Photos of selected non-diatoms at different substrates: 1. Spirogyra sp., 2. Microspora amoena var. gracilis, 3. Oocardium stratum, 4. Oscillatoria simplicissima, 5. Oscillatoria tenuis, 6. Microcoleus cf. setchellianus, 7. Rivularia haematites, 8. Cyanobium cf. parvum

Benthic Algae on Tufa Barriers

205

Fig. 7 LM micrographs of selected non-diatoms: 1. Tolypothrix distorta, 2. Rivularia haematites, 3. Oscillatoria proboscidea, 4. Oscillatoria simplicissima, 5. Oscillatoria tenuis, 6. Microcoleus cf. setchellianus, 7. Komvophoron schmidlei, 8. Microspora amoena var. gracilis, 9. Zygnema sp., 10. Spirogyra sp., 11. Merismopedia glauca, 12. Chroococcus turgidus, 13. Cosmarium formosulum, 14. Limnothrix redekei, 15. Oocardium stratum, 16. Batrachospermum gelatinosum

(Fig. 10). Based on their similarity, the diatom communities can be divided into three main groups depending on the location of the barrier: the Upper Lakes, the Lower Lakes and barrier below Veliki slap Waterfall. Cluster analysis of the taxonomic composition of the non-diatom community shows a similar separation with the exception of a few samples: the samples collected in September from the barrier at the inlet of Lake Novakovi´ca brod, which are more similar to the samples from the Upper Lakes than to those from the Lower Lakes, and the samples collected in June

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I. Stankovi´c et al.

Fig. 8 Proportion of relative abundance of non-diatoms at tufa barriers from Plitvice Lakes in June and September 2016. See Table 1 for sampling site codes. The number next to the code indicates the month of sampling

Fig. 9 Species number and Shannon diversity index of algae at tufa barriers from Plitvice Lakes shown separately for diatoms and non-diatoms in June and September 2016. June = white bars, September = grey and black bars

from the barrier at the inlet of Lake Novakovi´ca brod and Labudovac Barrier, which show a relatively high similarity to the samples from the barrier below the Veliki slap Waterfall. The present results suggest that the diversity of benthic algae at the tufa barriers of Plitvice Lakes is strongly determined by spatial location, possibly due

Benthic Algae on Tufa Barriers

207

Fig. 10 Dendrogram plot of hierarchical cluster analysis based on Bray-Curtis Similarity of relative abundance of algae at tufa barriers from Plitvice Lakes shown separately for diatoms and nondiatoms in June and September 2016

to different water properties. In an extensive limnological study, Matoniˇckin et al. (1971) indicate that higher shading of the Upper Lakes than the Lower Lakes has a significant effect on the photoautotrophic community of the tufa barriers of Plitvice Lakes.

5 Ecology and Conservation Status 5.1 Benthic Algae and Their Relation to Environmental Parameters All aquatic organisms, including benthic algae, are defined by the physical and chemical properties of the water and the morphology of the habitat (Wetzel 2001). Because habitat morphology is quite similar in the case of the tufa barriers at Plitvice Lakes, this chapter focuses on the environmental variables that have the greatest influence on benthic algae (Table 4). Water temperature, pH, conductivity, and saturation have an increasing trend from the barriers in the Upper Lakes to those in the Lower Lakes, while alkalinity, nitrates, and orthophosphates show the opposite trend. The tufa barrier below the Veliki slap Waterfall is rather unique as it has almost two times colder water and three times higher nitrate concentration, which is a consequence of inflowing water from the anthropogenically influenced Plitvica Stream, in contrast to other barriers that are under the influence of the lake surface water. To explain the relationship of benthic algae and environmental variables, PCA analysis was performed. Different environmental variables are important for diatoms and non-diatoms, but this is expected due to their different metabolism and ecology (Wehr et al. 2015).

8.08–8.31

8.08–8.31

9.1–20.1

VJ

8.09–8.31

8.14–8.36

8.24–8.56

9.6–22.1

9.5–21.9

6.1–14.6

KD

NB

VS

8.15–8.35

8.14–8.30

9.0–20.1

9.6–22.1

GJ

KL

8.07–8.32

9.2–20.1

8.6–19.8

LB

pH

Temperature [°C]

BA

Sampling site

372–478

316–358

323–365

320–364

322–360

343–380

343–382

348–391

Conductivity [µS cm−1 ]

95.2–102.5

95.2–101.5

96.7–107.0

98.2–105.9

87.1–102.1

91.4–99.2

86.6–101.2

91.5–104.5

Saturation [O2 %]

205.2–247.5

193.7–214.3

197.7–219.3

201.2–218.8

188.1–219.8

196.7–224.3

209.3–230.4

222.3–238.9

Alkalinity [mgCaCO3 L−1 ]

1070–1330

280–740

290–730

310–710

300–720

360–700

330–700

360–700

Nitrates [mg N L−1 ]

0–11

0–12

3–15

4–21

0–14

2–15

1–14

9–20

Ortophosphates [mg P L−1 ]

Table 4 Range of physical and chemical parameters of water at tufa barriers of Plitvice Lakes for monthly sampling between April and October 2016

208 I. Stankovi´c et al.

Benthic Algae on Tufa Barriers

209

For diatom species, PCA with environmental parameters (Table 3) singled out alkalinity and saturation on axis 1 (intra-set correlations: −0.519 and −0.520, respectively) and nitrates on axis 2 (intra-set correlations 0.750) as the most important, yielding eigenvalues for the first two axes of 0.924 and 0.947, respectively. It explained 50.3% of the total variance on the first two PCA axes. PCA showed a clear spatial separation based on diatom composition (Fig. 11) into three groups: Veliki slap Waterfall, the Upper and Lower Lakes, as did the cluster analyses. Higher concentrations of orthophosphates and alkalinity affect most diatom communities on the tufa barriers of the Upper Lakes with dominant representatives A. affine and C. plitvicensis. The diatom community at the tufa barriers of the Lower Lakes is under the influence of higher temperature and saturation and low nutrient concentration with dominant representatives D. delicatulus, C. affinis var. angusta, E. minuta, G. clavatum and P. costei. The dominant taxa A. pyrenaicum and D. tenuis with the rest of the community at the tufa barrier below the Veliki slap Waterfall are affected by low water temperature and high nitrate concentration, alkalinity and conductivity. For the non-diatom species, PCA performed for the same environmental parameters (Table 4) singled out nitrates and pH on axis 1 (intra-set correlations: 0.7598 and 0.8812, respectively), and conductivity and saturation on axis 2 (intra-set correlations: −0.5539 and 0.6308 respectively), yielding eigenvalues for the first two axes of 0.988 and 0.828, respectively. It explained 63.9% of the total variance on the first two PCA axes.

Fig. 11 PCA triplot showing environmental data, dominant diatom taxa, and samples from tufa barriers from Plitvice Lakes. Diatom codes are the Omnidia codes (OC) listed in Table 2. T = temperature, pH = pH, Cond = Conductivity, Alk = Alkalinity, Sat = Saturation, PO4 3− = orthophosphates, NO3 − = nitrates

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Fig. 12 PCA triplot showing environmental data, non-diatom taxa (dominant in bold letters), and samples from tufa barriers from Plitvice Lakes. Taxa codes are listed in Table 3. T = temperature, pH = pH, Cond = Conductivity, Alk = Alkalini-ty, Sat = Saturation, PO4 3− = orthophosphates, NO3 − = nitrates

PCA showed a clear separation of the barrier below the Veliki slap Waterfall where high nitrate concentration, high conductivity and high pH influenced most of the non-diatom communities (Fig. 12). Oocardium stratum, Spirogyra sp. and T. distorta were dominant taxa. Although samples from the Upper and Lower Lakes barriers were closely spaced, the non-diatom taxa of the Upper Lakes tufa barriers were clearly under the influence of higher temperature and higher concentration of orthophosphates with O. simplicissima, C. glomerata, and Oedogonium sp. as dominant taxa. The non-diatom community at the Lower lake tufa barriers was mainly represented by B. gelatinosum, M. amoena var. gracilis and Mougeotia sp. and was strongly influenced by colder water and higher saturation.

5.2 Benthic Algae as Indicators for Environment and Conservation Status of Diatoms Ecology of diatoms on the species levels is best classified by Van Dam et al. (1994) which is still widely used and accepted as a concept (Lai et al. 2020; SalinasCamarillo et al. 2020). Based on this classification, high number and relative abundance of alkalibiontic, neutrophilic and alkaliphilic taxa corresponds to alkaline habitat (pH = 8.07–8.56), such as tufa barriers of the Plitvice Lakes (Fig. 13). Oxygen requirements successfully classified 59 taxa where most of them belonged to polyoxybiontic and oxybiontic categories that require high oxygen saturation that

Benthic Algae on Tufa Barriers

211

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 13 Proportion of relative abundance of diatoms at tufa barriers at the Plitvice Lakes classified according to the indicators of Van Dam et al. (1994) a pH, b Oxygen requirements, c Saprobity, d Nitrogen uptake, e Trophic state and f German Red List (2018). Numbers in brackets indicate number of taxa classified to a certain category

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is achieved in the Plitvice Lakes throughout the whole year round (86.6–107.0%). Most abundant taxa for nitrogen uptake are taxa tolerant of very low concentrations of organically bound nitrogen. Saprobic indicators have dominant oligosaprobic taxa, while oligotrophic and mesotrophic taxa characterize trophic status of identified diatoms at tufa barriers of the Plitvice Lakes. For other taxonomic groups of benthic algae there is no unique system, but their individual ecology is well known for many taxa. One of the dominant taxa O. simplicissima is known to inhabit freshwater unpolluted and stagnant waters while K. schmidlei also prefers stagnant and flowing clean waters (Komárek and Anagnostidis 2005). On the other hand, the planktic taxa recorded on the barriers show that the water quality in lakes is decreased. The nutrient load from the watershed is a considerable thereat for lakes as well as barriers ecosystems. Characteristic of also dominant O. stratum is to form tiny calcareous incrustations in fast running water of high calcium content (Wehr et al. 2015). Another organism typical for calcareous streams is Rivularia haematites forming hard hemispherical colonies recognizable even with naked eye, which may fuse together forming a crust. According to the German Red List (Hofmann et al. 2018), one third of taxa are in the categories Threatened of extinction, Highly endangered and Endangered with only Achnanthidium trinode is in the first category. Relative abundance of all those taxa do not exceed much over 25% (Fig. 10). Majority of taxa do not have estimated risk or are not in any category with priority for conservation. In an additional phytobenthos sample collected from NB tufa barrier, two species, Cymbella dorsenotata Østrup and Navicula gottlandica Grunow, were identified that cannot be found in the other samples and based on the German Red List can be classified as Endangered at unknown extent and Highly endangered, respectively. The two species are presented on Figs. 1 and 2, however, that additional sample was not involved in the statistical analyses as only one composite sample per sampling site was collected. Application of indicator values and German Red List to diatom composition of tufa barriers of Plitvice Lakes suggest that diatoms are excellent organisms as indicators of environmental condition. Individual ecology of non-diatom taxa is in line with habitat characteristics, therefore benthic algae on the tufa barriers of Plitvice Lakes are good indicators of water quality and habitat protection.

6 Conclusion The Plitvice Lakes is a unique lake-system, but remarkable heterogeneity of benthic algal communities that inhabit tufa barriers can be observed. In addition to the expected temporal variability in composition, both the benthic diatom and nondiatom communities show a distinct spatial structure consistent with changes in environmental properties along the lake-system. As far as local diversity is concerned, the spatial difference is more pronounced for the non-diatoms, but on average they are represented with a lower diversity compared to the diatoms. It is worth noting that no unusual non-diatom taxa were recorded at

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the time of data collection, there is a great potential for the occurrence of rare or even new taxa in the future, therefore the entire National Park, including the tufa barriers, deserve a maximum level of protection. Acknowledgements We thank Prof. Zlatko Levkov for revising the diatom identification and Mirela Šušnjara for the Omnidia calculations. We thank Dr. Maja Vurnek for help with project implementation and physical and chemical analysis of water. To her and to Dr. Andras Abonyi goes acknowledgments for help with field work. The study of benthic algae at tufa barriers from Plitvice Lakes was financed by the NP Plitvice Lakes.

References Arp G, Bissett A, Brinkmann N et al. (2010) Tufa-forming biofilms of German karstwater streams: microorganisms, exopolymers, hydrochemistry and calcification. In: Pedley HM, Rogerson M (eds) Tufas and speleothems: unravelling the microbial and physical controls. Geological Society Special Publications, vol 336. Geological Society, London, pp 83–118 Berrendero E, Arenas C, Mateo P et al (2016) Cyanobacterial diversity and related sedimentary facies as a function of water flow conditions: Example from the Monasterio de Piedra Natural Park (Spain). Sediment Geol 337:12–28 CEN–EN 13946 (2014a) Water quality - Guidance for the routine sampling and preparation of benthic diatoms from rivers and lakes (EN 13946:2014). European Committee for Standardization (CEN) Brussels, Belgium CEN–EN 14407 (2014b) Water quality - Guidance for the identification and enumeration of benthic diatom samples from rivers and lakes (EN 14407:2014). European Committee for Standardization (CEN) Brussels, Belgium CEN–EN 15708 (2009) Water quality - Guidance standard for the surveying, sampling and laboratory analysis of phytobenthos in shallow running water (EN 15708:2009). European Committee for Standardization (CEN) Brussels, Belgium Dudgeon D, Arthington AH, Gessner MO et al (2006) Freshwater biodiversity: importance, threats, status and conservation challenges. Biol Rev 81:163–182 Golubi´c S, Crescenzo V, Plenkovi´c-Moraj A et al (2008) Travertines and calcareous tufa deposits: an insight into diagenesis. Geol Croat 61:363–378 Hammes F, Verstraete W (2002) Key roles of pH and calcium metabolism in microbial carbonate precipitation. Rev Environ Sci Biotechnol 1:3–7 Hanson PC, Bade DL, Carpenter SR et al (2003) Lake metabolism: Relationships with dissolved organic carbon and phosphorus. Limnol Oceanogr 48:1112–1119 Hofmann G, Lange-Bertalot H, Werum M et al. (2018) Rote Liste und Gesamtartenliste der limnischen Kieselalgen (Bacillariophyta) Deutschlands. In: Metzing D, Hofbauer N, Ludwig G et al. (eds) Rote Liste der gefährdeten Tiere, Pflanzen und Pilze Deutschlands, vol 7. Pflanzen. Bundesamt für Naturschutz – Naturschutz und Biologische Vielfalt, Bonn, pp 601–708 Komárek J, Anagnostidis K (2005) Süßwasserflora von Mitteleuropa, Bd. 19/2: Cyanoprokaryota: Oscillatoriales. Spektrum Akademischer Verlag, Heidelberg Lai GG, Padedda BM, Ector L et al (2020) Mediterranean karst springs: diatom biodiversity hotspots under the pressure of hydrological fluctuation and nutrient enrichment. Plant Biosyst 154:673–684 Matoniˇckin I, Pavleti´c Z, Tavˇcar V et al (1971) Limnološka istraživanja reikotopa i fenomena protoˇcne travertinizacije u Plitviˇckim jezerima. Prirodoslovna Istraživanja, Acta Biol 7:1–88 Muñoz I, Real M, Guasch H et al (2001) Effects of atrazine on periphyton under grazing pressure. Aquat Toxicol 55:239–249

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Necchi O (2016) Red Algae (Rhodophyta) in Rivers. In: Necchi O (ed) River Algae. Springer International Publishing, Cham, pp 65–91 Nelson JR, Eckman JE, Robertson CY et al (1999) Benthic microalgal biomass and irradiance at the sea floor on the continental shelf of the South Atlantic Bight: Spatial and temporal variability and storm effects. Cont Shelf Res 19:477–505 Pevalek I (1938) Biodinamika Plitviˇckih jezera i njena zaštita. Zaštita Prirode 1:1–22 Pusch M, Fiebig D, Brettar I et al (1998) The role of micro-organisms in the ecological connectivity of running waters. Freshwater Biol 40:453–495 Roman AM, Sabater S (1999) Effect of primary producers on the heterotrophic metabolism of a stream biofilm. Freshwater Biol 41:729–736 Sabater S, Artigas J, Corcoll N et al (2016) Ecophysiology of River Algae. In: Necchi O (ed) River Algae. Springer International Publishing, Cham, pp 197–217 Salinas-Camarillo VH, Carmona-Jiménez J, Lobo EA (2020) Development of the Diatom Ecological Quality Index (DEQI) for peri-urban mountain streams in the Basin of Mexico. Environ Sci Pollut Res 28:14555–14575. https://doi.org/10.1007/s11356-020-11604-3 Seckbach J, Kociolek JP (2011) The Diatom World. Cellular Origin, Life in Extreme Habitats and Astrobiology, vol 19. Springer, Dordrecht Sheath RG, Cole KM (1992) Biogeography of stream macroalgae in North America. J Phycol 28:448–460 Smol JP, Stoermer EF (2010) The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge University Press, Cambridge Stankovi´c I, Szabó B, Miˇceti´c Stankovi´c V (2017) Sastav i znaˇcaj bentiˇckih algi na sedrenim barijerama Plitviˇckih jezera (Composition and significance of benthic algae on tufa barriers of Plitvice Lakes). Croatian Botanical Society, Zagreb Stevenson RJ (1996) The Stimulation and Drag of Current. In: Stevenson RJ, Bothwell ML, Lowe RL (eds) Algal Ecology. Academic Press Elsevier, San Diego, pp 321–340 Van Dam H, Mertens A, Sinkeldam J (1994) A coded checklist and ecological indicator values of freshwater diatoms from The Netherlands. Neth J Aquat Ecol 28:117–133 Wehr JD, Sheath RG, Kociolek RP (2015) Freshwater Algae of North America: Ecology and Classification, 2nd edn. Academic Press, Amsterdam Wetzel RG (2001) Limnology: Lake and River Ecosystems. Academic Press, San Diego Wolfstein K, Colijn F, Doerffer R (2000) Seasonal Dynamics of Microphytobenthos Biomass and Photosynthetic Characteristics in the Northern German Wadden Sea, Obtained by the Photosynthetic Light Dispensation System. Estuar Coast Shelf Sci 51:651–662 Woodruff SL, House WA, Callow ME, Leadbeater BSC (1999) The effects of biofilms on chemical processes in surficial sediments. Freshwater Biol 41:73–89

The Plitvice Lakes—An Interplay of Moss, Stonewort and Marshland Vegetation Antun Alegro, Anja Rimac, Vedran Šegota, and Nikola Koleti´c

Abstract In terms of plant biodiversity, the Plitvice Lakes are one of the most valuable areas in Croatia. This natural barrage lake system of 16 larger and several smaller lakes, separated with tufa barriers in form of numerous waterfalls of different size, height and water velocity, stretches along 8 km, representing the largest habitat of the tufa forming moos communities (Cratoneurion commutati) in Croatia. Indeed, the Plitvice Lakes are the largest area in Europe supporting the development of these communities. Furthermore, the Lakes are valuable as the largest habitat of stonewort communities (Charetea intermediae) in Croatia. On the other hand, karst terrain is not favourable for the development of marshland vegetation (PhragmitoMagnocaricetea), and therefore this vegetation type covers only small areas, being mostly confined to lake margins. The recent spread of marshland vegetation could indicate the eutrophication of the lake system. Keywords Plitvice Lakes · Croatia · Marshland vegetation · Aquatic vegetation · Tufa waterfall vegetation · Phragmito-Magnocaricetea · Salicetea purpureae · Potametea · Charetea · Cratoneurion

A. Alegro (B) · A. Rimac · V. Šegota · N. Koleti´c Divison of Botany, Faculty of Science, Department of Biology, University of Zagreb, Maruli´cev trg 20/II, 10000 Zagreb, Croatia e-mail: [email protected] A. Rimac e-mail: [email protected] V. Šegota e-mail: [email protected] N. Koleti´c e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_9

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1 Introduction This chapter is an overview of all previous research on marshland, aquatic and waterfall vegetation of the Plitvice Lakes. Although the Plitvice Lakes are one of the oldest national parks in Croatia, systematic survey of the vegetation as well as a regular monitoring are lacking. This does not allow us to make any conclusions considering vegetation changes, succession, dynamics and similar topics. The exception is partially waterfall vegetation, which was studied in late 1950s and 1960s, (Matoniˇckin and Pavleti´c 1960, 1961a, b, 1962, 1963, 1965, 1967a, b; Pavleti´c 1957; Matoniˇckin et al. 1966) and then recently between 2016 and 2018 (Alegro 2019a, b). The results are only partially comparable because the first research was of smaller extent and only bryophytes were considered. Aquatic vegetation was mainly studied in the period from 1983 to1990 (Blaženˇci´c and Blaženˇci´c 1990–1991, 1992, 1994, 1995; Blaženˇci´c et al. 1991), and the marshland vegetation in the period 2004–2006 (Stanˇci´c et al. 2010), with some additional recent surveys in the lakes Proš´ce and Kozjak in 2019. In this overview only the lake system is included, without rivers and streams. The nomenclature was accorded to Euro + Med Plantbase for vascular plants, to Hodgetts et al. (2020) for bryophytes, to AlgaeBase for algae, and to Mucina et al. (2016) for the phytosociological system.

2 Marshland Vegetation and Willow Scrub (Phragmito-Magnocaricetea and Salicetea Purpureae) The marshland vegetation of the class Phragmito-Magnocaricetea is developed in numerous, rather fragmented belts and patches around the lakes, nowhere dominating the landscape. It has been studied by Horvat et al. (1974), Blaženˇci´c and Blaženˇci´c (1990–1991), Šegulja (2005) and Stanˇci´c et al. (2010). The latter paper is the most comprehensive one on the subject and it was used as a basis for this overview, complemented with the recent observations and research results of the authors. Considering the overall distribution in the Plitvice Lakes, marshland vegetation is more abundantly developed along the Lower Lakes. The Upper Lakes are mainly surrounded by forest vegetation reaching to the lake margins and thus not leaving much space for the development of marshland vegetation. However, this vegetation type is occasionally present along the Upper Lakes, especially surrounding the larger waterfalls and other more open habitats. Marshland communities that can be encountered in the area are species-poor monodominant stands with a low proportion of accompanying species. Hence, they are easily recognizable in the landscape. The most widespread marshland community in the area of the Plitvice Lakes are monodominant stands of tall sedge Cladium mariscus (L.) Pohl. The association Cladietum marisci P. Allorge 1922 (Fig. 1) is characteristic of marshland vegetation along the Croatian Adriatic coast, while in the inland part of the country, it occurs

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Fig. 1 Cladietum marisci P. Allorge 1922, the most widespread marshland community in the Plitvice Lakes. Photo by Antun Alegro

exclusively in the Plitvice Lakes (Stanˇci´c 2007; Stanˇci´c et al. 2010). This community is observed along the whole lake system, with much higher occurrence in the middle and lower parts. Very often it covers surfaces of several to several dozen square meters, rarely occupying closed areas of more than 100 m2 . It grows on lake edges, along the margins, in smaller water pools and along some lower barriers, usually in shallow, up to 60 cm deep water. The community is always built as a very dense stand of C. mariscus with the scarce occurrence of other species. Some of the frequent accompanying species are Mentha aquatica, Petasites kablikianus, Eupatorium cannabinum, Solanum dulcamara, Lycopus europaeus and occasionally young shoots of Salix purpurea, indicating an initial stadium of succession toward willow shrubbery. Cladietum marisci is a community of carbonate-rich, oligotrophic, welloxygenated waters (Ellenberg 1996; Wilmanns 1998; Pott and Remy 2000), sensitive to water level fluctuations and drying out if it is cut under water level (Ellenberg 1996). It has low concurrency and could be drifted by Phragmites australis (Ellenberg 1996), although some authors stress its adaptability to the changing environmental factors (Balátová-Tuláˇcková 1991). In Europe, the community was more frequent in warmer postglacial periods (Grosse-Brauckmann and Dierssen 1973; Wilmanns 1998), and therefore the community variant with Sphagnum palustre was named “fossil-community” (Görs 1975). This is a relict vegetation type, which used to be more common in lowland fens in the early Holocene (Sáldo 2011). All other marshland communities are more limited in their distribution within the Plitvice Lakes or occupy significantly smaller areas. One of the most limited communities, confined to a single locality is Equisetetum fluviatilis Steffen 1931. The community occurs on the confluence of the Matica River into Lake Proš´ce (Fig. 2),

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covering an area of several hundred square meters. This is the largest known Croatian locality of this community, which is extremely rare in the country. It grows in a depression flooded only a part of the year, in stagnant and up to 50 cm deep water (Stanˇci´c 2007). The community is the monodominant stand of Equisetum fluviatile with few other species such as Mentha aquatica, Lythrum salicaria, Myosotis scorpioides, Myriophyllum verticillatum, Potamogeton sp. and some others. It is a community of mesotrophic and eutrophic waters and could represent the first stage toward terrestrial plant communities (Mertz 2000). On particular locality, it is partially shaded by surrounding forest, which provides leaf litter ensuring a higher trophy level in comparison to the lake water. Another community developing at the sites where organic matter is depositing is Sparganietum erecti H. Roll 1938. It is a rare community occupying small areas near the Equisetetum fluviatilis locality, as well as several shallow lagoons of Lake Kozjak. Here, the water depth does not exceed 40 cm, and besides the dominant Sparganium erectum some other helophyte species occur, such as Lycopus eurpaeus, Lythrum salicaria, Eupatorium cannabinum, Mentha aquatica etc. More frequent community on habitats characterised by the accumulation of organic matter, but restricted to very small patches rarely exceeding several dozen square meters is Typhetum latifoliae A. G. Lang 1973 (Fig. 3). In some cases, it is questionable if these stands can be considered a separate community or rather

Fig. 2 Equisetetum fluviatilis Steffen 1931, a rare plant community occurring only on the confluence of the Matica River into Lake Proš´ce. Photo by Antun Alegro

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Fig. 3 Typhetum latifoliae A. G. Lang 1973 on the northern edge of Lake Proš´ce. Photo by Antun Alegro

they represent the mosaically developed more widespread neighbouring community, e.g. Phragmitetum australis or Caricetum rostratae. In this community, water depth usually does not exceed 50 cm. Spreading of this marshland community could indicate water eutrophication. The following three communities are more or less restricted to Lake Proš´ce, more precisely to the shallow, slowly flowing or even stagnant water of its southern and northern edge. The community Caricetum elatae W. Koch 1926 is present in the littoral zone of these edges (Fig. 4), while very small stands and individual tussocks can be found sporadically along the lake margins as well. The individual tussocks can be sporadically found in other lakes, but here they do not form the community. The name-giving and dominant species Carex elata is easily recognizable by massive tussocks formed out of leaf remnants and dead and active roots, whose height corresponds to the mean water level (Wilmanns 1998). The water regime required by this community is characterized by deep flooding in spring with fluctuations in the water table during the rest of the year. The more nutrient-rich habitat is preferred compared with the habitats of other associations of the alliance Magno-Caricion elatae (Hájková et al. 2011). Caricetum rostratae Rübel 1912 is another sedge community that can be found in the littoral zone of the edges of Lake Proš´ce (Figs. 4 and 5). Lake Proš´ce is the first locality of this community ever reported from Croatia, followed by several more localities situated within the Plitvice Lakes National Park (Stanˇci´c et al. 2010). The community develops in about 20 cm deep water, in immediate contact with other marshland communities occurring in deeper water, such as Caricetum elatae and Typhetum latifoliae. The community prefers nutrient-poor water but is known to tolerate eutrophication as well (Mertz 2000).

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Fig. 4 Mosaic of Caricetum rostratae Rübel 1912, Caricetum elatae W. Koch 1926 (tussocks) and Typhetum latifoliae A. G. Lang 1973 (in front) on the northern edge of Lake Proš´ce. Photo by Antun Alegro

The third community occurring in the littoral zone of Lake Proš´ce is Scirpetum lacustris Chouard 1924. It is one of the most widespread communities in the karst waters of Croatia, which can be developed in many variants in which Scirpus lacustris is associated with different emergent and submerged macrophyte species. In Lake Proš´ce, the community is represented with the species-poor variant with Myriophyllum spicatum and occasionally Chara spp. Here, Scirpus lacustris comes in loose and patchy stands, in contrast to the margins of water bodies with the higher trophic level across Croatia, where it forms continuous belts. Several patches of this community are present also in Lake Kozjak. In all these sites, the water depth does not exceed 1 m. The species Phragmites australis occurs in many sites in the Plitvice Lakes, usually occupying only small areas. Sometimes it is spreading into other communities, especially Cladietum marisci or it forms dense stands which can be considered a community Phragmitetum australis Schmale 1939 (Fig. 6). The stands grow in the water of varying depths, usually less than 1 m deep, but also in humid habitats outside the water. The species, as well as the associated community, can be regarded as good colonists spreading into other communities and on other suitable habitats in the process of vegetation succession towards terrestrial communities, which is driven by eutrophication. Furthermore, Salix pupurea plays an important role in vegetation succession in the Plitvice Lakes as well (Fig. 7). This species can form dense scrub over lower barriers,

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Fig. 5 Carex rostrata Stokes. The community formed by this species in Croatia is known only from the Plitvice Lakes. Photo by Antun Alegro

in the littoral zone and smaller pools, especially in the Lower Lakes. Those stands form community Salicetum purpureae Wendelb.-Zelinka 1952. In some localities, a smaller proportion of Salix eleagnos can be intermixed within Salix purpurea stands, especially where the water flow is faster. Distinguishing those stands as separate community Salicetum eleagni-purpureae Sillinger 1933 is almost impossible in the field due to the lack of other diagnostic species and continuous variation between them. Regardless of the community name, those scrub is fast spreading, suppressing other marshland and moss communities, slowing the water flow and pushing water ecosystems toward the terrestrial ones. Besides willows, the presence of Molinia caerulea, Arctium lappa and other tall herbs of nutrient-rich soils in these stands indicates succession as well. Some authors (Stilinovi´c and Futaˇc 1989; Stilinovi´c 1998) suggested that the observed increase of the area occupied by marshland communities around the lakes is a result of man-induced eutrophication. In order to reduce the consequences of eutrophication, stands of Cladium mariscus, Phragmites australis, and Scirpus lacustris were experimentally removed (Pavlus and Novosel 2006). One or two years after

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Fig. 6 A dense stand of Phragmites australis (Cav.) Steud. invading waterfall vegetation at Novakovi´ca brod Lake. Photo by Antun Alegro

Fig. 7 Dense scrub of Salix purpurea L. covering the waterfall at Novakovi´ca brod Lake. Photo by Antun Alegro

the removal, it was observed that the population of Cladium mariscus was considerably reduced, the population of Scirpus lacustris had diminished only insignificantly, while the population of Phragmites australis was even more luxuriant than before. An increase in water flow over the waterfalls was also observed following the removal

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of vegetation. A similar project of removal of marshland vegetation and especially willow scrub has started in 2019 at Novakovi´ca brod Waterfall (Alegro 2019b). Stanˇci´c et al. (2010) concluded that marshland vegetation in the Plitvice Lakes shows both positive and negative effects. It has an undoubtful positive effect through the enrichment of biodiversity. On the other hand, the progress of at least some marshland communities regarding the area they occupy within the National Park is related to the eutrophication of oligotrophic karstic lakes. Vegetation succession, if not prevented, could eventually have a negative impact on biodiversity and cause a permanent change of this valuable and unique landscape. Besides described marshland communities, the marshland fern Thelipteris palustris should be mentioned as well (Fig. 8), as the species is very rare in Croatia. In the Plitvice Lakes it forms small stands, usually under a square meter, confined to the narrow area of the shallow littoral zone of some lakes, in only several centimetres deep water.

Syntaxonomical scheme Class Phragmito-Magnocaricetea Klika in Klika et Novák 1941 Order Phragmitetalia Koch 1926 Alliance Phragmition communis Koch 1926 Ass. Cladietum marisci P. Allogre 1922

Fig. 8 Stand of rare fern species Thelipteris palustris Shott. Photo by Antun Alegro

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Ass. Equisetetum fluviatilis Steffen 1931 Ass. Sparganietum erecti H. Roll 1938 Ass. Typhetum latifoliae A. G. Lang 1973 Ass. Scirpetum lacustris Chouard 1924 Ass. Phragmitetum australis Schmale 1939 Order Magnocaricetalia Pignatti 1953 Alliance Magnocaricion elatae Koch 1926 Ass. Caricetum elatae Koch 1926 Ass. Caricetum rostratae Rübel 1912 ex Osv. 1923 em. Dierß Class Salicetea purpureae Moor 1958 Order Salicetalia purpureae Moor 1958 Alliance Salicion albae Soó 1951 Ass. Salicetum purpureae Wendelb.-Zelinka 1952 Alliance Salicion eleagno-daphnoidis (Moor 1958) Grass 1993 Ass. Salicetum eleagni-purpureae Sillinger 1933

3 Aquatic Vegetation (Potametea and Charetea) The first and so far the most comprehensive study of aquatic vegetation was made mainly during the period between 1983 and 1990 (Blaženˇci´c and Blaženˇci´c 1990– 1991, 1992, 1994, 1995; Blaženˇci´c et al. 1991). Following that, aquatic vegetation was recently surveyed only in the two largest lakes (Proš´ce and Kozjak) as a part of the monitoring of ecological status in 2019 and to only a limited extent as a part of the research of the vegetation of waterfalls and barriers (Alegro 2019a). Results of all these studies are compiled and presented here. Vegetation of rooted floating-leaf macrophytes is in general absent from the Plitvice Lakes. Namely, there are no developed belts or defined communities of floating-leaf plants in any of the lakes. Yet, in some locations, individual plants or small populations of Potamogeton natans and P. nodosus can be found, although covering only small and restricted areas. In some of the shallow lagoons of Lake Proš´ce, Lemna minor occurs occasionally but only with a low number of scattered individuals, not forming free-floating duckweed vegetation. In the contact zone between the marshland and submerged stonewort swards vegetation, the vegetation of submerged vascular plants can be developed, regularly in water not deeper than one meter. This vegetation consists mainly of submerged

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Fig. 9 Submerged stand of Mentha aquatica L. Photo by Antun Alegro

forms of helophyte species such as Mentha aquatic (Fig. 9), Lycopus europaeus and Veronica anagallis-aquatica, the latter being quite rare. These mats are mosaically distributed covering small areas, rarely exceeding 10 square meters. The species Myriophyllum spicatum is also rather rare in the whole area of the Plitvice Lakes, but its submerged stems can reach impressive lengths of several meters, especially in deep, calm lagoons of the Lakes Proš´ce and Kozjak. These large plants do not form communities, but rather occur more or less individually. On the contrary, in other parts of Lake Kozjak, this species forms even larger mats, but the plants are of the usual size. These mats are especially well developed on the very end of Lake Kozjak, where the water current is stronger just before the outflow into the Lower Lakes. On the other hand, the species Myriophyllum verticillatum is a common inhabitant of the Plitvice Lakes, usually forming swards in continuous zones in some of the lakes, e.g. in Lake Galovac. The community Myriophylletum verticillati Gaudet ex Šumberová in Chytrý 2011 (Fig. 10) is present in several lakes at different depths: Lake Proš´ce (1.5–6 m), Lake Ciginovac (1–6 m), Lake Batinovac (3–5 m), Lake Veliko Jezero (2–4 m), Lake Vir (1.5–5 m), Lake Galovac (2–5 m) and Lake Gradinsko jezero (up to the bottom at 4 m). Interestingly, in Lake Vir, the only lake not inhabited by the stoneworts, community Myriophylletum verticillati is the dominant vegetation type forming continuous belt. In the Lower Lakes, it does not form a community but occurs more or less individually within stonewort swards. According to Šumberová (2011), M. vericillatum occurs in mesotrophic and naturally eutrophic, but clear waters, in habitats in an advanced stage of terrestrialization. However, the Plitvice Lakes are not meso- nor eutrophic, thus special attention should

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be paid to this community and its spread should be monitored and considered as an indicator of the changes in the ecosystem induced by eutrophication. By far the most important type of aquatic vegetation in the Plitvice Lakes, regarding the spatial extent and biomass, are stonewort swards (Charetea). They are present in all lakes except for Vir. These swards consist of eight species (Table 1), which makes the Plitvice Lakes one of the most important stonewort diversity hotspots in Croatia. The most widespread stonewort species is Chara contraria. It is present in all lakes except Lake Malo jezero, Lake Vir, Lake Gradinsko jezero and Lake Novakovi´ca brod. It forms monodominant association Charetum contrariae Corillion 1957 or less frequently swards in codomination with Myriophyllum verticillatum, informally named Myriophyllum verticillatum-Chara contraria community. Chara contraria forms belts in two zones. In the shallow water of the littoral zone, especially in lagoons of the Lakes Proš´ce and Kozjak, it forms smaller or larger carpets, often with other species, such as Ch. hispida, Ch. vulgaris, Ch. subspinosa and Stuckenia pectinata. However, its major area of distribution is in deeper water, where it can form more or less continuous belts along the lakes. This stonewort vegetation occupies varying depths in different lakes: 6–9 m in Lake Proš´ce and Lake Ciginovac, 5–7 m in Lake Veliko jezero, 5–14 m in Lake Galovac, 2–7(–9) m in Lake Kozjak and 1–8 m in Lake Milanovac. Furthermore, the bottom of the whole Lake Gavanovac is completely covered in this vegetation, from the shallowest to 10 m deep parts, as well as the northern, deeper part of Lake Kaluderovac. In Lake Okrugljak, it forms already mentioned mixed community with Myriophyllum verticillatum from the littoral zone up to 5 m depth. Another very frequent stonewort species in the Plitvice Lakes is Ch. globularis. It has almost the same distribution as Ch. contraria, being present in almost all lakes,

Fig. 10 Myriophylletum verticillati Gaudet ex Šumberová in Chytrý 2011 in Lake Veliko jezero. Photo by Antun Alegro

+

+

+

Chara virgata Kützing

Chara hispida Linnaeus

Chara subspinosa Ruprecht

Nitella opaca (C.Agardh ex Bruzelius) C.Agardh

+

+

+

Chara vulgaris Linnaeus

+

+

+

Chara globularis Thuiller

Chara aspera Willdenow

+

+

Chara contraria A.Braun ex Kützing

+

+

+

+ +

+

+

+

+

+

+ +

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Proš´ce Ciginovac Okrugljak Batinovac Veliko Malo Vir Galovac Gradinsko Veliki Kozjak Milanovac Gavanovac Kaluderovac Novakovi´ca Jezero Jezero Jezero Burget Brod

Table 1 Distribution of stonewort species in the Plitvice Lakes (adjusted after Blaženˇci´c and Blaženˇci´c 1994)

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except Lake Veliki Burget. Similarly like Ch. contraria, it occurs in shallow waters of the littoral zone accompanied by other species, but it can also form monodominat swards of association Charetum globularis Zutshi ex Šumberová et al. 2011. In Lake Batinovac, it completely overgrows the bottom (the maximum depth is 5.5 m) and represents the main and most prominent community of the lake. In Lake Veliko jezero, it spreads in direction of water currents, occupying the zone from 2 to 9 m. It forms more or less continuous belts in three lakes at different water depths: 9–14 m in Lake Galovac, 10–15 m in Lake Milanovac and 3–19 m in Lake Kozjak, with the most luxuriant swards developed between 8 and 11 m. Another quite widespread species is Ch. vulgaris. It is much more common in the Upper Lakes, being absent only from Lake Batinovac, Lake Vir, Lake Gradinsko jezero and Lake Veliki Burget. In the Lower Lakes, it is present only in Lake Gavanovac, but not forming a community because the lake is characterized by widespread swards of Ch. contraria. Lake Vir is the only lake where this species is a dominant stonewort, forming swards of the association Charetum vulgaris Corillion 1957. Its occurrence to the extent observed in this lake probably indicates an increased level of organic matter (Blaženˇci´c and Blaženˇci´c 1995). Usually, it occurs accompanied by other stonewort species with the highest frequency in 5–7 m deep water, but it is quite regularly present in shallow littoral waters as well, again accompanied by several other species. Chara virgata (Ch. delicatula) is present in four of the Upper Lakes—Lake Proš´ce, Lake Galovac, Lake Gradinsko jezero, and Lake Kozjak and its abundance is gradually increasing in the downstream direction. It the two largest lakes, Lake Proš´ce and Lake Kozjak, it does not form monodominant swards, however, in Lake Galovac it comes in both monodominant swards and mixed with C. globularis in the northern part of the lake in 6–12 m deep zone. Monodominant stands of this species occupy the eastern part of Lake Gradinsko jezero as well, extending to a depth of 10 m. Regarding the Lower Lakes, it occurs sporadically in Lake Milanovac, while in Lake Kaluderovac it completely cowers the bottom in the southern, shallower part of the lake, up to the bottom at 5.2 m, where it forms the community Charetum delicatulae Doll 1989 ex G˛abka et Owsianny 2010. Chara hispida is a quite rare species occurring only in the Lakes Proš´ce (Fig. 11) and Kozjak. It inhabits the shallow part of the littoral zone, up to 4 m deep water in Lake Proš´ce and up to 8 m deep water in Lake Kozjak. In both lakes, it is associated with other stonewort species, most frequently with Ch. contraria, as well as with some vascular plant species. Chara aspera is another rather rare stonewort in the Plitvice Lakes, reported only from two lakes. Blaženˇci´c and Blaženˇci´c (1994) have reported this species growing only in Lake Veliko jezero inside the community Myriophyllum verticillatum-Chara contraria. It was present in low abundance in 3–4 m deep water. During the more recent field studies, it was recorded again in Lake Kozjak, where it formed relatively small swards, mosaically scattered more or less around the lake in a shallow up to 4 m deep littoral belt. It grew alone or was intermixed with other stonewort species and scattered individuals of Myriophyllum spicatum and Stuckenia perfoliata. These stands can be considered as association Charetum asperae Corillion 1957.

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Fig. 11 Sward of Chara hispida L. during the low water level in late summer in Lake Proš´ce. Photo by Antun Alegro

The only member of the genus Nitella in the whole lake system of the Plitvice Lakes is N. opaca. Swards of this species form the deepest vegetation belt in the two largest lakes, Lake Proš´ce and Lake Kozjak, as well as in the Lakes Ciginovac and Okrugljak, the two lakes immediately downstream from the first lake of the system—Lake Proš´ce. This species forms monodominant community Nitelletum opacae Corillion 1957, which occupies quite deep zones: 9–11 m in Lake Proš´ce, 5–10.5 m (i.e. the bottom) in Lake Okrugljak and 11–20.5 m in Lake Kozjak. In Lake Ciginovac the species is restricted to a small area in its eastern part, growing in 7–8 m deep water. Blaženˇci´c and Blaženˇci´c (1994) noticed that for optimal development of N. opaca permanent introduction of cold water, usually from the underwater springs, is essential. In Lake Kozjak, habitat characteristics are especially suitable for N. opaca in its eastern part, where the species was very abundant in the deepest part of the lake. In general, stonewort vegetation of the Plitvice Lakes forms three main belts (Blaženˇci´c and Blaženˇci´c 1994). Charetum contariae is developed in shallower parts, which are well illuminated, exposed to the mechanical action of water, water level fluctuations and diurnal and seasonal fluctuations of temperature. Charetum globularis occupies an intermediate position characterized by reduced light intensity, increased hydrostatic pressure, presence of permanent underwater currents (in Lakes Kozjak and Proš´ce) and lower water temperature without significant variations. Nitteletum opacae forms the deepest belt in conditions of even lower light intensity, higher hydrostatic pressure and already mentioned inflow of cold water.

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Syntaxonomical scheme Class Potamogetonetea Klika in Klika et Novák 1941 Order Potamogetonetalia Koch 1926 Alliance Potamogetonion Libbert 1931 Ass. Myriophylletum verticillati Gaudet ex Šumberová in Chytrý 2011 Loose, spotted stands of Potamogeton natans and P. nodosus Class Charetea intermediae F. Fukarek 1961 Order Charetalia intermediae Sauer 1937 Alliance Charion intermediae Sauer 1937 Ass. Charetum contrariae Corillion 1957 Ass. Charetum globularis Zutshi ex Šumberová et al. 2011 Ass. Charetum asperae Corillion 1957 Ass. Charetum delicatulae Doll 1989 ex G˛abka et Owsianny 2010 Community Myriophyllum verticillatum-Chara contraria Alliance Charion vulgaris (W. Krause et Lang 1977) W. Krause 1981 Ass. Charetum vulgaris Corillion 1957 Order Nitelletalia W. Krause 1969 Alliance Nitellion flexilis W. Krause 1969 Ass. Nitelletum opacae Corillion 1957

4 Vegetation of Tufa Waterfalls and Barriers (Cratoneurion) Tufa barriers are one of the fundamental phenomena of the Plitvice Lakes and although its vegetation is perhaps the most noticeable biological component, certainly accounting for the greatest biomass, it was never systematically investigated until very recently (Alegro 2019a, b; Alegro et al. 2019). In the early twentieth century, Pevalek (1925, 1935) started the research on the role which photosynthetic organisms (mosses and algae) play in the tufa formation. The period of more intensive botanical research followed in the fifties and was led by Zlatko Pavleti´c. He mainly investigated algae and bryophytes, as well as their interactions with macrozoobenthos communities, a work he conducted in cooperation with a colleague zoologist Ivo Matoniˇckin (Matoniˇckin and Pavleti´c 1960, 1961a, b, 1962, 1963, 1965, 1967a, b; Pavleti´c 1957; Matoniˇckin et al. 1966).

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After Pavleti´c’s fundamental paper from 1957, Karl and Höfler (1961) published an overview of moss communities on tufa barriers in the Plitvice Lakes, giving a review of all previous research made by Pevalek and Pavleti´c. Since then, no further systematic research of vegetation of the waterfalls or barriers has been conducted up until 2016 (Alegro 2019a, b). The following overview is based on this more recent research, which included both bryophytes and vascular plants. Bryophytes constitute the most dominated part of the tufa barriers vegetation. On the other hand, vascular plant species make its less significant part, at least regarding their cover and abundance within this vegetation. Furthermore, this predominantly bryophyte vegetation is characterised by a very constant floristic composition with a relatively small number of species, however, some of them being represented with higher abundances (Fig. 12). The species Palustriella commutata is by far the most widely distributed bryophyte in the whole barrage system and has the highest abundance when compared to all other bryophytes (Fig. 13). Thus, the bryophyte vegetation of the Plitvice Lakes can be placed within the alliance Cratoneurion commutati Koch 1928, which is defined as the vegetation of moss-rich calcareous water springs in the montane and subalpine belts of Europe and Greenland (Mucina et al. 2016). In the vegetation of the waterfalls and barriers of the Plitvice Lakes, 44 bryophyte species were recorded so far (out of which 26 are constantly present within the community), along with 52 vascular plants, 18 of which are tree and shrub species (Alegro 2019a, b; Alegro et al. 2019).

Fig. 12 Distribution of main species on a waterfall. Drownig by Nikola Koleti´c

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Fig. 13 Waterfall at Lake Galovac densely covered by moss Palustriella commutata (Hedw.) Ochyra. Photo by Antun Alegro

Water velocity and light are recognized as the most important environmental factors affecting the species richness in this type of vegetation in the Plitvice Lakes. With the increase of the mechanic action of water and decrease in shading (mostly by trees), there is an evident decrease in the number of bryophyte species, which is ultimately reduced to four main species - Palustriella commutata, Eucladium verticillatum, Hymenostylium recurvirostrum and Apopellia endiviifolia, as well as somewhat less common Ptychostomum pseudotriquetrum. The strongest water force encountered below very high waterfalls, such as Veliki and Mali Prštavac, only the species Hymenostylium recurvirostrum can endure. Although the waterfall vegetation has constant composition regarding the dominant species, three main vegetation types can be recognized differing in the species richness and the abundance of dominant species. The most widespread vegetation type occurs in the barrage system of all of the Lower Lakes, while in the Upper Lakes, it is restricted to the exposed, not shaded waterfall tops (Fig. 14), vertical waterfall walls and its foothills, which are under the strong mechanical action of water (Fig. 15). This vegetation type is the species poorest, with a very constant species composition and relatively low species number. Besides the already mentioned five main bryophyte species, only three species of vascular plants occur constantly in this vegetation—Petasites kablikianus, Eupatorium cannabinum and Agrostis stolonifera (Fig. 16). Other species are only occasionally present or occur in low abundances. This vegetation type slightly differs in the Lower Lakes when compared to its localities in the Upper Lakes. In the Lower Lakes, the species Palustriella commutata is not a dominant bryophyte species but occurs in codominance with Hymenostylium recurvirostrum, which is, to a greater or lesser extent, intermixed with morphologically and ecologically similar Eucladium

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verticillatum. Furthermore, on the most sun-exposed waterfalls (Slap Milke Trnine Waterfall, Veliki slap Waterfall) rather small populations of Didymodon tophaceus occur. Interestingly, this species was much more common in the Plitvice Lakes in the middle of the twentieth century than it is nowadays (Pavleti´c 1957), which is a result of the increased shade caused by denser populations of vascular plants, especially Salix purpurea scrub. Some bryophyte species, which occur more or less regularly in this vegetation type in the Upper Lakes, such as Plagiomnium rostratum and Fissidens gracilifolius, are absent from the Lower Lakes, while others, such as Brachythecium rivulare are very rare in the Lower Lakes. The second vegetation type is widespread on the shaded waterfall tops and especially on the cascading barrages within the forest (Fig. 17). Here, the mechanical action of water is weaker, while shadiness and influence of the forest vegetation are stronger, also there is a greater amount of humus, compared to the localities inhabited by the previously described vegetation type. The tree species that most often overshadow these waterfalls are Acer pseudoplatanus and Fraxinus excelsior, which are characteristic of the alliance Fraxino excelsioris-Acerion pseudoplatani P. Fukarek 1969, the vegetation of Submediterranean mesophilous broad-leaved ash-maple scree and ravine forests of the Balkan Region (Mucina et al. 2016). These stands come within the belt of Fagus sylvatica forest, which covers the largest part of the National Park. Therefore, along with maple and ash, the beech trees are also present here, as well as Ostrya carpinifolia coming from thermophilic vegetation developed on steep slopes on dolomite bedrock. Other tree species are only sporadic. Sambucus nigra is the only constant species in the shrub layer, while Salix cinerea and S. purpurea occur infrequently. The five main bryophyte species mentioned before (Palustriella commutata, Eucladium verticillatum, Hymenostylium recurvirostrum, Apopellia endiviifolia

Fig. 14 The most widespread vegetation type occurring in fast running water in sun-exposed habitats on the top of the waterfalls. Photo by Antun Alegro

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Fig. 15 The most widespread vegetation type occurring on waterefall foothills, which are under the strong mechanical action of water. Photo by Antun Alegro

Fig. 16 Three main vascular plant species in the vegetation of the waterfalls. From the left to the right: Petasites kablikianus Bercht., Agrostis stolonifera L. (grass) and Eupatorium cannabinum L. Photo by Antun Alegro

and Ptychostomum pseudotriquetrum) are the dominant bryophytes of this vegetation type as well, with P. pseudotriquetrum being much more vigorously developed here than in the other two vegetation types. Other aquatic mosses present here are Brachythecium rivulare and Cratoneuron filicinum, which are mostly absent from the

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first vegetation type. Some additional bryophyte species characteristic of wet habitats such as Fissidens adianthoides and Plagiomnium rostratum are also widespread here. Furthermore, this vegetation type has the highest diversity of vascular plants. The three main vascular species present in the first type are dominant here as well. Moreover, several other, mainly forest species, such as Angelica sylvestris, Lactuca muralis, Calamagrostis varia, Brachypodium sylvaticum and Hedera helix are quite common within this type. These are mostly the species of moist and nutrient-rich forest habitats, while tufa barriers represent their secondary habitat. The third vegetation type is characteristic of wet rocks, rock shelters and temporary waterfalls (Fig. 18). These habitats are not exposed to the strong mechanic action of water, however, they are still permanently wet and shaded. Such ecological conditions allow greater bryophyte diversity compared to the previous two vegetation types. Besides all already mentioned bryophyte species, a great number of bryophytes characteristic of wet rocks, such as Orthothecium rufescens, Hydrogonium croceum, Gymnostomum calcareum, Fissidens gracilifolius, Conocephalum salebrosum and Jungermannia atrovirens are frequently encountered here. In addition, Didymodon spadiceus, Mesoptychia collaris and several others are common but occurring with a lower frequency. Vascular flora consists mainly of the species belonging to the vegetation of shaded carbonate rocks and forest vegetation, with ferns Asplenium scolopendrium, A. trichomanes, A. viride and A. ruta-muraria being the most abundant and obvious representatives. With its omnipresence, high abundance and biomass, bryophyte Palustriella commutata (Fig. 19), is the main attribute of the vegetation of the Plitvice Lakes barrage system. It is the dominant bryophyte species in the whole system with some exceptions in the Lower Lakes, where it is codominant with Hymenostylium recurvirostrum. As an ecologically plastic species, it equally thrives in fast water and on

Fig. 17 Vegetation of shaded waterfalls. Photo by Antun Alegro

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Fig. 18 Vegetation of wet rocks and rock shelters. Photo by Antun Alegro

wet rocks. Furthermore, it grows successfully in sun-exposed, as well as in shaded habitats. It forms turfs and curtains over the waterfalls giving them a characteristic appearance. Growing in great quantity, with its densely pinnate stems covered with numerous leaves, it significantly increases the overall surface available for tufa deposition. For this reason, it is recognized as the most important plant component involved in barrier formation. Several other species (Eucladium verticillatum, Cratoneuron filicinum, Hymenostylium recurvirostrum, Ptychostomum pseudotriquetrum, Brachythecium rivulare (Fig. 20) and some others) are involved in tufa deposition as well (Fig. 18), but due to their significantly lower abundances, their contribution to the tufa formation in the Plitvice Lakes is not as notable as that of Palustriella commutata. Nevertheless, some of these species, as well as the liverwort Apopellia endiviifolia and the vascular plant Petasites kablikianus deserve to be emphasized for they surely make a particularly significant part of the Plitvice Lakes vegetation. Eucladium verticillatum forms very dense, low tufts, which successfully resit to the water force, enabling it to grow on the foothills of high waterfalls and in rapid water flow (Fig. 21). Ecologically and morphologically very similar is Hymenostylium recurvirostrum (Fig. 22), although not as common, especially not in the Upper Lakes. Here, it has its optimum in the marginal parts of the waterfalls and in areas with the strongest mechanical water action, which other species cannot withstand. By contrast, as a heliophilous species, it is codominant or even locally dominant in the Lower Lakes, forming large carpets on the bottom of shallow water exposed to the sun. As already mentioned, another constant and relatively abundant species in the vegetation of the waterfalls is Ptychostomum pseudotriquetrum (Fig. 23). It forms loose tufts which can reach almost 10 cm in height. It is especially luxurious on

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Fig. 19 Palustriella commutata (Hedw.) Ochyra, the most abundant moss species in the vegetation of the waterfalls. Photo by Antun Alegro

Fig. 20 Brachythecium rivulare Schimp. with tufa deposits. Photo by Antun Alegro

forest waterfalls and the margins of high waterfalls, avoiding direct exposure to the water flow. The species Apopellia endiviifolia (Fig. 24) is a liverwort occurring in the Plitvice Lakes on submerged microlocalities such as bottoms of small pools on the foothills of the waterfalls, in puddles between rocks, on the bottom of the channels, on wet rocks and soil in the zone of water level fluctuation and other similar places, where it

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Fig. 21 Tufts of moss Eucladium verticillatum (With.) Bruch and Schimp. Photo by Antun Alegro

Fig. 22 Hymenostylium recurvirostrum (Hedw.) Dixon with capsules. Photo by Antun Alegro

can form dense carpets. Although a prominent part of the Plitvice Lakes vegetation, this species is not involved in tufa deposition. By far the most widespread vascular plant species in the vegetation of the waterfalls is Petasites kablikianus (Fig. 25), giving this vegetation a phytogeographical uniqueness. Namely, this is Carpathian species with only a few scattered localities across the Balkans (Dudáš et al. 2019), while the Plitvice Lakes are the only locality

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Fig. 23 Ptychostomum pseudotriquetrum (Hedw.) J. R.S pence and H. P. Ramsay ex Holyoak and N. Pedersen, a common moss, especially in the vegetation of shaded waterfalls. Photo by Antun Alegro

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Fig. 24 Liverwort species Apopellia endiviifolia (Dicks.) Nebel and D. Quandt is common in the vegetation of waterfalls but it is not involved in tufa deposition. Photo by Antun Alegro

of this species in Croatia. Such fragmented distribution is most probably a consequence of the Ice Ages, which gives relict character to the vegetation of the Plitvice Lakes waterfalls. Petasites kablikianus occurs in all three vegetation types, except for the places with strong mechanic water action.

Fig. 25 Petasites kablikianus Bercht. is a Carpathian element, in Croatia known only from the Plitvice Lakes. Photo by Antun Alegro

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With its species composition, the vegetation of the tufa waterfalls of the Plitvice Lakes is a unique and the most diverse tufa forming vegetation in Croatia. Spreading along 8 km of the barrage system, the Plitvice Lakes are the largest area harbouring this distinctive moss-rich vegetation of calcareous and oligotrophic waters belonging to the alliance Cratoneurion commutati Koch 1928, not only in Croatia but in the whole of Europe (Alegro et al. 2019). As such, the Plitvice Lakes are crucial both for the understanding of the tufa formation process and for the protection of this unique vegetation.

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Horvat I, Glavaˇc V, Ellenberg H (1974) Vegetation Südosteuropas. Gustav Fischer Verlag, Stuttgart, Geobotanica SelectaBd. lV Matoniˇckin I, Pavleti´c Z (1960) Biološke karakteristike sedrenih slapova u našim krškim rijekama. Geografski Glasnik 22:43–56 Matoniˇckin I, Pavleti´c Z (1961a) Biljni i životinjski svijet na sedrenim slapovima jugoslavenskih krških voda. Biološki Glasnik 14:105–128 Matoniˇckin I, Pavleti´c Z (1961b) Contributo alla conoscenza dell’ ecologia delle biocenosi sulle cascade travertinose nella regione Carstica Jugoslava. Hydrobiologia 18:225–244 Matoniˇckin I, Pavleti´c Z (1962) Entwicklung der Lebensgemeinschaften und ihre Bedeutung für die Bildung und Erhaltung der Kalktuff-Wasserfälle. Arch Hydrobiol 58:467–473 Matoniˇckin I, Pavleti´c Z (1963) Prethodna ekološko – biocenološka istraživanja opskrbnih voda Plitviˇckih jezera. Acta Bot Croat 22:141–174 Matoniˇckin I, Pavleti´c Z (1965) Op´ce karakteristike biocenoza opskrbnih voda Plitviˇckih jezera. Plitviˇcki Bilten 1:33–38 Matoniˇckin I, Pavleti´c Z (1967a) Hidrologija potoˇcnog Sistema Plitviˇckih jezera i njegove ekološkobiocenološke znaˇcajke. Krš Jugoslavije 5:83–126 Matoniˇckin I, Pavleti´c Z (1967b) Tipovi vrela jugoslavenskih krških rijeka i njihove biocenološke karakteristike. Krš Jugoslavije 5:127–137 Matoniˇckin I, Pavleti´c Z, Tavˇcar V (1966) Brzina vode kao ekološki factor u krškim vodama teku´cicama. Biološki Glasnik 19:51–63 Mertz P (2000) Pflanzengesellschaften Mitteleuropas und der Alpen. EcoMed, Landsberg/Lech Mucina L, Bültmann H, Dierßen K et al (2016) Vegetation of Europe: Hierarchical floristic classification system of plant, lichen, and algal communities. Appl Veg Sci 19:3–264 Pavleti´c Z (1957) Ekološki odnosi briofitske vegetacije na slapovima Plitviˇckih jezera. Acta Bot Croat 16:63–88 Pavlus N, Novosel A (2006) Eksperimentalno uklanjanje makrovegetacije na pokusnim plohama Plitviˇckih jezera. Plitviˇcki Bilten 6:93–114 Pevalek I (1925) Oblici fitogenih inkrustacija i sedre na Plitviˇckim jezerima i njihovo geološko znamenovanje. Glasnik Hrv Prir Društva 38(39):101–110 Pevalek I (1935) Der Travertin und die Plitvicer Seen. Verh- Int Ver Limnol 7:165–181 Pott R, Remy D (2000) Gewässer des Binnenlandes. Verlag Eugen Ulmer, Stuttgart (Hohenheim) ˇ Sáldo J (2011) Cladietum marisci Allorge 1921. In: Chytrý M (ed) Vegetace Ceské republiky 3. Vodní a mokˇradní vegetace. Academia, Praha, p 546–552 Stanˇci´c Z (2007) Marshland vegetation of the class Phragmito-Magnocaricetea in Croatia. Biologia, Bratislava 62:297–314 Stanˇci´c Z, Žganec K, Gottstein S (2010) Marshland vegetation of Plitvice Lakes National Park (Croatia). Candollea 65:147–167 Stilinovi´c B (1998) The Plitvice Lakes - a natural phenomenon in the middle of the Dinaric Karst in Croatia. Eur Water Manag 1:15–24 Stilinovi´c B, Futaˇc N (1989) Prilog poznavanju sanitarne vrijednosti nekih opskrbnih voda i jezera na podruˇcju Nacionalnog parka Plitvice od 1977. do 1986. godine. Plitviˇcki Bilten 2:7–15 Šegulja N (2005) Vegetacija travnjaka, cretišta i moˇcvarnih staništa Nacionalnog parka Plitviˇcka jezera. Nat Croat 14(Suppl. 2):1–194 Šumberová K (2011) Vegetace vodních roslin zakoˇrenˇených ve dnˇe (Potametea). In: Chytrý M (ed) ˇ Vegetace Ceské republiky 3. Vodní a mokˇradní vegetace. Academia, Praha, p 100–247 Wilmanns O (1998) Ökologische Pflanzensoziologie. Quelle & Meyer Verlag, Wiesbaden

Plankton Communities Ivanˇcica Ternjej, Maria Špoljar, Igor Stankovi´c, Marija Gligora Udoviˇc, and Petar Žutini´c

Abstract The spatial and temporal distributions of phytoplankton and zooplankton assemblages in the different limnological gradients of two Plitvice lakes (Kozjak and Proš´ce) are presented. The lakes are dimictic and oligo-mesotrophic. The phytoplankton community consists of 128 taxa belonging to the groups of Bacillariophyta, Charophyta, Chlorophyta Ochrophyta, Cyanobacteria, Cryptophyta and Miozoa, while the zooplankton community consists of 43 open water zone species: 35 Rotatoria, four Cladocera and four Copepoda. The main environmental factors affecting the phytoplankton community are nutrients (TP, TN) and conductivity. Chlorophyll a concentration is three times lower and total phytoplankton biomass is two times lower in Lake Kozjak than in Lake Proš´ce. Diatoms are the dominant component of phytoplankton in both lakes, while co-dominance of Ochhrophyta and Chlorophyta can be observed in Lake Proš´ce. A total of 18 Reynolds FGs are represented in Plitvice Lakes, with FGs A, B, C, D and P being the most represented in Lake Kozjak and B, C, D, E and F in Lake Proš´ce. The main environmental and spatial factors affecting zooplankton composition were compared to quantify shifts in their relative importance over time and to identify any factors affecting temporal changes. I. Ternjej (B) · M. Špoljar Division of Zoology, Faculty of Science, Department of Biology, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] M. Špoljar e-mail: [email protected] M. G. Udoviˇc · P. Žutini´c Division of Botany, Faculty of Science, Department of Biology, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] P. Žutini´c e-mail: [email protected] I. Stankovi´c Stankovi´c Central Water Management Laboratory, Hrvatske Vode, Ulica Grada Vukovara 220, 10000 Zagreb, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_10

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Keywords Phytoplankton · Chlorophyta · Zooplankton · Rotatoria · Cladocera · Copepoda · Dimictic lakes

1 Study Area with Brief Description of Light Availability, Thermal Stratification and Oxygen Depth Profile of Lakes Kozjak and Proš´ce The Plitvice Lakes are composed of 16 cascade lakes divided by tufa barriers. Phytoplankton and zooplankton are traditionally investigated in two largest lakes, Kozjak and Proš´ce (Fig. 1). Lakes Kozjak and Proš´ce undergo a period of thermal stratification lasting for seven and four months, respectively (Fig. 2), followed by a complete vertical mixing in winter, and often a formation of a thick ice cover in January and February, depending on the season. The mixing depth (Zmix ) for Lake Kozjak ranges from around 15–20 m during the summer stratification to 40 m during winter overturn (December). In Lake Proš´ce, Zmix ranges between 10–15 m during the summer stratification, except in winter months when the deepest Zmix was noted at 30 m. The maximum euphotic depth (Zeu ) in Lakes Kozjak and Proš´ce ranges between 17– 25 m and 9–25 m, respectively. During the maximum thermal stratification, the limit of the euphotic layer in Lake Kozjak is located at around 25 m, whilst in Lake Proš´ce it reaches up to 11 m. In general, Zeu in both Lakes is similar or greater than the mixing depth (Zeu /Zmix ≥ 1) from May to September–October. Both lakes are productive through the entire water column during the winter period. During summer stratification, most chlorophyll a production is in the upper

Fig. 1 Simplified map of Plitvice Lakes showing position and maximum depth of plankton sampling sites in lakes Kozjak and Proš´ce

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Fig. 2 Temperature, oxygen, and chlorophyll a depth profiles of lakes Kozjak and Proš´ce. Months are indicated on x-axis with the first letter from March to February

layer with the highest concentration around the thermocline (Fig. 2). The concentration in Lake Kozjak can reach up to 4.5 µg L−1 , while in Lake Proš´ce the concentration of chlorophyll a can be tripled. Lake Kozjak has an oligotrophic character, while Lake Proš´ce has a mesotrophic character with summer hypoxia in the hypolimnion.

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2 Phytoplankton Phytoplankton comprises an extremely diverse group of organisms that respond to resource availability, predators, and interactions within and between species. This high phytoplankton diversity is responsible for primary production within freshwater ecosystems, and thus its understanding in space and time is highly relevant. Changes in the species concept (Salmaso et al. 2014) have led to difficulties in identifying phytoplankton species and resulted in the development of a new functional approach to phytoplankton, which aims to group species with similar morphological and functional characteristics to reveal the optimal ecological strategies for specific habitat conditions (Caroni et al. 2012; Naselli-Flores and Barone 2012). The functional approach makes environmental studies comparable and allows for easier assessment of environmental responses to changing conditions (Kruk et al. 2002; Naselli-Flores et al. 2003; Weithoff 2003). The traditional taxonomic classification seemed to be of limited use for this purpose, especially because higher taxa consist of species with very different structural and functional properties (Salmaso et al. 2012). Functional groups approach provides a better understanding of ecosystem functioning and allows a more accurate prediction of its dynamics. The studies on phytoplankton of the Plitvice Lakes began in the last century with surveys of Brunnthaler (1900), Car (1906), Krmpoti´c (1913), and Pevalek (1919). After the proclamation of the national park, the studies intensified through the works of Matoniˇckin and Pavleti´c (1963, 1964, 1965, 1967), Maloseja (1985, 1987, 1989), Plenkovi´c-Moraj (1981), Maloseja and Plenkovi´c-Moraj (1986) and ventured into the twenty-first century. The last comprehensive survey of phytoplankton in Lakes Kozjak and Proš´ce conducted during the 2014, 2016 and 2017, is presented here.

2.1 Phytoplankton Species Taxonomical Composition A total of 128 taxa of phytoplankton are identified, with 68 taxa being present in both lakes. Selected taxa are shown on Figs. 3, 4 and 5. The taxonomically most diverse group is Chlorophyta, followed by Bacillariophyta, Ochrophyta, Cyanobacteria, Cryptophyta, Miozoa and Charophyta. According to the survey in 2014, 2016 and 2017, the phytoplankton biomass in Lake Kozjak was low, with little variations (monthly average from 0.25 to 2.04 mg L−1 ) (Fig. 6). The algal biomass in Lake Proš´ce is somewhat higher, with monthly average from 0.42 to 4.65 mg L−1 . In Lake Kozjak a total of 105 taxa were identified, distributed in six taxonomic divisions as follows: Chlorophyta (34%), Bacillariophyta (21%), Ochrophyta (18%), Cyanobacteria (13%), Cryptophyta (7%) and Miozoa (6%). In terms of total phytoplankton biomass, the most representative group was Bacillariophyta (average 0.45 mg L−1 ), with Miozoa and Cryptophyta as subdominant groups (average 0.05 mg L−1 ). Species composition of Lake Proš´ce revealed 89 taxa distributed in six taxonomic divisions: Bacillariophyta (27%), Ochrophyta (24%), Chlorophyta (19%),

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Cyanobacteria (11%), Cryptophyta (10%), Miozoa (6%), Charophyta (3%). Total phytoplankton biomass was dominated by Bacillariophyta (average 0.94 mg L−1 ), while Ochrophyta (average 0.36 mg L−1 ) and Chlorophyta (average 0.27 mg L−1 ) were subdominant. The phytoplankton assemblage of Lake Kozjak is in temporal scale characterized by a strong dominance of diatoms, initially represented by several pennate (Ulnaria acus (Kützing) M.Aboal, Fragilaria crotonensis Kitton, Ulnaria delicatissima (W.Smith) Aboal and P. C. Silva, Fragilaria tenera var. nanana (Lange-Bertalot) Lange-Bertalot and S. Ulrich, Asterionella formosa Hassall) and afterwards by centric species (Pantocsekiella costei (J. C. Druart and F. Straub) K. T. Kiss and E. Ács, Cyclotella distinguenda Hustedt, Cyclotella plitvicensis Hustedt). All of the listed genera are typically common in spring and summer diatom blooms (Sabater

Fig. 3 SEM micrographs of selected phytoplankton taxa: 1. Stephanodiscus medius, 2. Stephanodiscus minutulus, 3. Cyclotella plitvicensis, 4. Cyclotella distinguenda. Scale bar: 5 µm. Photos by M. Gligora Udoviˇc and I. Špoljari´c

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Fig. 4 LM micrographs of selected taxa: 1. Dinobryon cylindricum, 2. Gyrodinium helveticum, 3. Dinobryon sociale, 4. Peridinium willei, 5. Sphaerocystis schroeterii, 6. Ceratium hirundinella, 7. Mallomonas sp., 8. Cyclotella distinguenda. Scale bar: 20 µm. Photos by. I. Stankovi´c

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Fig. 5 LM micrographs of selected taxa: 1. Bitrichia danubialis, 2. Chrysocapsella planctonica, 3. Chroococcus minutus, 4. Crucigeniella irregularis, 5. Oocystis marssonii, 6. Nephrocytium aghradianum, 7. Asterionella formosa, 8. Fragilaria crotonensis. Scale bar: 20 µm. Photos by I. Stankovi´c

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Fig. 6 Relative abundance of functional groups, biomass of taxonomic groups of phytoplankton, silicates, total phosphorus and total nitrogen in Lakes Kozjak and Proš´ce in 2014, 2016 and 2017. Months are abbreviated with the first capital letter, starting with April and ending with September in each year. Number of taxa in each taxonomic group are expressed in brackets

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2009). The referred pennates are defined as acclimating R-strategist species, that are tolerant of mixing conditions and can maintain their growth at low light levels (Reynolds 1997). The morphological features, which include elongate, linear to lanceolate valves, and the star-shaped colonies formed by Asterionella, help them to resist sinking and to exploit the lower underwater irradiance available for growth during and immediately after mixing of the entire water column (Antoniou et al. 2014) during spring overturn. The steady increase of temperature and solar radiation stabilizes the water column and forms a stable summer stratification. This event promotes a replacement of the R-strategy pennales to C-strategists, represented by the centric diatoms often recorded in oligotrophic environments with lots of available light and low P-nutrient concentration (Reynolds 1997, 1998). C-species are smallcelled, invasive, fast-growing planktonic competitors that grow well at low temperatures and are characterized by a short lifespan (Reynolds 1997). In Lake Kozjak, Cyclotella species show seasonal dominance throughout the summer stratification and a gradual decline during the autumn overturn. Development of a continuous thermocline with the density gradient in the metalimnion enables the small centric diatoms to remain dominant in the euphotic layer throughout the stratification period (Padisák et al. 2003). The strong thermal stratification, together with a relatively short retention time and a relatively deep mixing depth of the lake favours these species over other diatoms (Tolotti et al. 2007), thus allowing them to effectively exploit habitats with stabilized light and temperature (Winder et al. 2009) and preventing them from sinking to the hypolimnion. Although less numerous in terms of species number, as well as in cell size, cryptophytes seize a significant share in the overall phytoplankton biomass of the late summer phytoplankton assemblage of Lake Kozjak, particularly the species Plagioselmis nannoplanctica (H. Skuja) G. Novarino, I. A. N. Lucas and S. Morrall and Cryptomonas marssonii Skuja. Both are frequently found in the pelagial of large freshwater lakes (Javornický 2003; Novarino 2012) and tend to proliferate during late summer phytoplankton bloom when the normally low TP levels are even more depleted from the water column. Furthermore, the mixotrophic lifestyle makes them efficient users of nutrients from bacterial cells (Urabe et al. 2000) under of lownutrient conditions prevalent in karstic lakes. Through their small cell size and high surface to volume ratio, these species attain high rates of growth and respiration are able to survive over long periods in the stratified water column (Reynolds 1997; Dos Santos and Calijuri 1998). Even though being the most diverse taxa group in Lake Kozjak, chlorophytes generally do not occupy a large biomass proportion of the plankton. While most species display only sporadic occurrence, notable members of the group include species of the genera Chlamydomonas, Oocystis and Sphaerocystis. The latter two are represented by the slow-growing, large unicellular species or small-celled colonies capable of regulating their position in the water column and resource-conserving. They are characterized as S-strategy species, or stress survivors, and are favoured by low resources but high energy conditions (Reynolds 1997). On the other hand, Chlamydomonas is a typical unicellular motile C-strategist which is able to maintain in a thermally stratified oligo- and mesotrophic systems (Fonseca and Bicudo 2010). In addition to cryptophytes,

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Ochrophyta and Miozoa are groups of phytoplankton largely consisting of species with well documented mixotrophy, acting as important players in the flow of carbon and other nutrients in the planktonic food web of aquatic ecosystems (Mitra et al. 2014). These groups also comprise an important part of phytoplankton biomass in oligotrophic karstic Lake Kozjak. The most prominent members of the group Ochrophyta include nonmotile colonial species Dinobryon divergens O.E.Imhof and motile unicellular Chromulina sp., whereas genus Gyrodinium comprising euplanktic species that are unicellular, unarmoured and motile (Takano and Horiguchi 2004) and armoured flagellate species Peridinium willei Huitfeldt-Kaas represent the most important dinoflagellates. The optimum growth conditions for Chromulina and Dinobryon include conditions of increased light availability and low concentrations of carbon, phosphorus and other nutrients (Watson et al. 2001), both of which are attained in Lake Kozjak throughout the vegetation period. Moreover, the phagotrophy of these species provides important advantages in the nutrient-limiting oligotrophic conditions (Caron et al. 1993; Gaedke 1998; Rottberger 2013). Correspondingly, typical habitats of Gyrodinium and Peridinium are deep stratified lakes, where they can actively swim between the nutrient-poor light-saturated epilimnion and nutrientrich light-reduced hypolimnion (Tardio et al. 2003; Niesel et al. 2007). Both species are temperature-tolerant, stress-tolerant strategists able to grow under nutrient depletion (Reynolds 1997; Niesel et al. 2007), with a pronounced capacity for luxury consumption (Pollingher 1988). The cyanobacterial species generally account for a negligible percentage of phytoplankton biomass in Lake Kozjak. On sporadic occasions, during the summer peak the non-vacuolate buoyant representatives including small-celled, colonial Anathece smithii (Komárková-Legnerová and Cronberg) Komárek, Kastovsky and Jezberová and unicellular or few-celled Chroococcus minutus (Kützing) Nägeli appear in a relatively low biomass in the epilimnion of a highly stratified water column (Reynolds 2006; Komárek et al. 2011; Bresciani et al. 2018). The spring assemblage of Lake Proš´ce is predominantly defined by pennate diatoms represented by U. acus and A. formosa. These R-strategy species have low requirements for nutrients and are morphologically adapted to thrive in conditions of lower light availability during a gradual formation of stratification (Reynolds 1997). Low phosphorus levels also favour miozoan dinoflagellate P. willei, an S-strategist (sensu Reynolds) which retains a subdominant position during the diatom bloom. The species has several adaptive strategies including vertical migration through the whole water column in search for nutrients, and the capacity of luxury consumption of phosphorus (Pollingher 1988). After the spring pulse of pennales the total phytoplankton biomass usually shows a slight decrease, due to development of thermal stratification which acts as a barrier restraining mixing of the water column and further limits the availability of resources (Fantin-Cruz et al. 2015). In such conditions mixotrophic, large-colony forming chrysophyte D. divergens capable of both photosynthesis and phagotrophy (Kamjunke et al. 2007; Unrein et al. 2010) becomes particularly dominant. This S-type species, tolerant to relatively low nutrient availability (Reynolds 1993) and strong stratification (Madgwick et al. 2006), often produces large summer blooms with well over 40% in the phytoplankton biomass lasting

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for a relatively short period, usually in June or July. Further stratification thickening in Lake Proš´ce promotes a replacement of Dinobryon either with small-celled, fast-growing planktonic C-strategists, represented by the centric diatoms P. costei, C. distinguenda and C. plitvicensis, or with a microplanktic, coenobial palmelloid chlorophyte Sphaerocystis schroeteri Chodat. The latter species is common in the summer periods of oligotrophic systems and is adapted to survival in the pelagic under relatively low phosphorus conditions associated with clear water and high transparency (Reynolds 2006). Comparably to Lake Kozjak, cryptophytes play a role as important contributors to the diversity of phytoplankton assemblage of Lake Proš´ce. The cryptomonads are common in oligo- and mesotrophic lakes throughout the vegetative period (Berman et al. 1992; Salmaso et al. 2003). However, P. nannoplanctica and C. marssonii are the principal species accounting for the majority of biomass attributed to the group, usually appearing in the Plitvice Lakes karstic system during late spring or even more pronouncedly in the late summer. Such prevalence is possibly controlled through the carbon supply (Sommer 1989), since the mixotrophic trait renders these species unconstrained by nutrients (Reynolds 2006). These small, unequally biflagellated C-strategists show tolerance to stratification and sensitivity to mixing (Reynolds 2006), and tend to follow after the centric diatom biomass pulse (Munawar and Munawar 2013). Other species, such as charophytes and cyanobacterial representatives, constitute only a very minor portion of phytoplankton biomass in Lake Proš´ce occurring sporadically during the summer stratification.

2.2 Reynolds Functional Classification of Phytoplankton Reynolds classification assorts phytoplankton species with similar morphological, physiological and ecological traits into ecological associations, the so-called functional groups (FGs) or coda (Reynolds et al. 2002). A total of 18 Reynolds functional groups characterize Lake Kozjak: A, B, C, D, E, F, J, K, LO , MP, N, P, S1, T, X1, X2, X3, and Y. The most important are diatom represented groups: codon D characterized by Ulnaria ulna, centric diatoms from group B represented by Cyclotella distinguenda and Cyclotella plitvicensis with association A representative Pantocsekiella costei, and to some extent codon C typified by the pennate colonial diatom Asterionella formosa and group P member Fragilaria crotonensis (Fig. 6). Codon X2 and Y, represented by cryptophyte species Plagioselmis nannoplanctica and Cryptomonas marssonii, show a relatively low but steady presence during the vegetation period, with a slight increase in biomass towards the late summer. Functional group LO , which comprises Miozoan dinoflagellates Peridinium willei and Gyrodinium helveticum (Penard) Y. Takano and T. Horiguchi together with cyanobacterium Chroococcus minutus, commonly persists in Lake Kozjak during the summer stratification period. Whereas codon K representative Anathece smithii appears sporadically during the summer peak, group E members from the genus Dinobryon (mainly D. divergens and Dinobryon sociale (Ehrenberg) Ehrenberg) are usually noted by

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a slight biomass increase after the first diatom spring bloom. Representatives from group F (Oocystis and Sphaerocystis) occur during the highly stratified conditions in the summer. Lake Proš´ce is characterized by 18 FGs as follows: A, B, C, D, E, F, J, K, LO , MP, N, P, S1, T, X1, X2, X3, and Y. The spring period in the Lake is distinguished by complete domination of functional groups C and D due to vernal bloom of Asterionella formosa and Ulnaria acus, respectively (Fig. 6). Group E representative Dinobryon divergens becomes dominant in the spring–summer transitional period before the stratification is complete. Though present during the complete vegetation period, codon LO (Peridinium willei and Gyrodinium helveticum) is also associated to the transitional periods before and after the complete stratification. At the beginning of summer, functional group E is usually replaced either by centric diatoms from codon B (Cyclotella distinguenda, Cyclotella plitvicensis and Lindavia radiosa (Grunow) De Toni and Forti) and codon A (Pantocsekiella costei), which interchange in dominance, usually with over 50% of phytoplankton biomass. Fragilaria crotonensis, a member of group P also appears subdominantly with the aforementioned centrics. However, during some years (i.e. 2017) association F (Sphaerocystis schroeteri) can become a dominant member of the summer phytoplankton, reaching up to 72% of the total biomass. Cryptophytes from the functional groups X2 (Plagioselmis nannoplanctica) and Y (genus Cryptomonas), together with codon X3 (Ochromonas sp.), constitute an important part of assemblage during the complete vegetation period, occasionally occupying a large portion of the phytoplankton biomass second only to diatoms.

2.3 Phytoplankton and Their Relation to Environmental Parameters The environmental variables of the lakes are listed in Table 1. Somewhat higher fluctuations in water temperature were demonstrated for Lake Kozjak (mean values ranging from 8.2 to 22.1 °C), as compared to Lake Proš´ce (mean values varying from 9.4 to 19.0 °C). The combination of limestone and dolomite karst bedrock causes high input of dissolved inorganic carbon into the water, thus conditioning relatively high mean values of pH (between 7.9 and 8.4) and electrical conductivity (between 366 and 443 µS cm–1 ). The lowest dissolved oxygen concentrations in Lake Kozjak were measured in September 2014 (10.0 mg L−1 ), and the highest during summer stratification in July 2017 (12.6 mg L−1 ). Both highest (12.9 mg L−1 ) and lowest (8.5 mg L−1 ) concentrations of dissolved oxygen in Lake Proš´ce were determined during summer stratification in 2017 (June and September, respectively). The values of total phosphorus in Lake Kozjak ranged from the lowest in April 2016 (7 µg L−1 ) to the highest in May 2014 (29 µg L−1 ). The measured total phosphorus for Lake Proš´ce reached minimum concentration in April 2017 (12 µg L−1 ), whereas

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Table 1 The environmental variables of the lakes

Lake Kozjak

Lake Proš´ce

Temperature [°C]

8.2–22.1

9.4–19.0

pH

7.9–8.4

8.0–8.3

Conductivity [µS cm−1 ]

366–409

408–443

Oxygen [mg L−1 ]

10.0–12.6

8.5–12.9

7–29

12–38

Total phosphorus [µg P L−1 ] Total nitrogen [µg N

L−1 ]

Silicates [µg SiO2 L−1 ]

510–990

600–940

0.90–3.41

0.35–4.96

the maximum was present in August 2016 (38 µg L−1 ). The lakes are characterized by high total nitrogen concentrations (mean values ranging from 510 to 990 µg L−1 ), while mean values for SiO2 range between 0.35 and 4.96 µg L−1 . The relation between the environmental variables of the water and functional groups of phytoplankton was analysed by canonical correspondence analysis (CCA). CCA performed for the 7 most important environmental variables and 18 coda (Table 1 and Fig. 7) indicated eigenvalues for the first two axes of 0.037 and 0.016, respectively, explaining 90.3% of total variance on the first 2 axes. According to Monte Carlo permutation test, ordination of all canonical axes was statistically significant (p = 0.002). Canonical coefficients and intra-set correlations carried out on the functional groups showed that TN and conductivity were the most important variables for the ordination axis 1. Regarding axis 2, TP was the variables that weighted most for ordination. Functional groups K, S1 and N were associated with SiO2 , and groups T, F and X3 with temperature. Group C was associated positively with TN, whilst groups X1, B and E were interrelated with TP. Groups P and X2 correlated with pH, and coda Y and J correlated positively with TP and temperature. The phytoplankton of Lakes Kozjak and Proš´ce is pronouncedly characterized by the diatom dominated assemblages. Such structure is typified by a vernal bloom of pennates from the associations C, D and accompanying codon P. Associatons C and D, which are usually typical for more enriched turbid conditions (Padisák et al. 2009), were dominant during the spring period when the onset of thermocline formation). Conversely, thermal stratification with highly insolated water column during the summer stratification period supports a high biomass of centric diatoms from groups A and B (Tolotti et al. 2007; Gligora Udoviˇc et al. 2015), acclimatized for conditions of a relatively short retention time and deep mixing (Žutini´c et al. 2014). This corresponds to the typology and trophic status of both lakes, as Cyclotellacomplex species are widely characteristic for oligo- to mesotrophic lakes (Wunsam et al. 1995). The recorded species, such as Lindavia radiosa, C. distinguenda and C. plitvicensis, share similar morphological characters and even partly overlap in the ecological and trophic spectrum with no strict delimitation between associations A and B (Reynolds et al. 2002).

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Fig. 7 Triplot of the CCA analysis based on the biomass of Reynolds functional groups with environmental variables and samples

Development of a strong thermal stratification with slightly elevated nutrient concentrations and stabilized light properties facilitate this dominance of the small celled Cyclotella species in the summer epilimnion of both lakes (Rühland et al. 2010). Besides diatoms, other main descriptors of the lakes reflect the optimal, bestfitting ecological conditions in which they seasonally or sporadically occur. Codon E, which is typical for oligotrophic, slightly alkaline, nutrient-poor lakes (Reynolds 1997), and includes species associated with low P-nutrients (Reynolds 1986) represented mainly by Dinobryon divergens, was dominant during spring–summer transition, decreasing in biomass with the ongoing summer stratification. Codon LO , which encompasses mostly eurytopic species capable of living in a wide range of trophic conditions, tolerant to segregated nutrients and sensitive to prolonged or deep mixing (Reynolds et al. 2002), consisted almost exclusively of dinoflagellates (Peridinium willei, Gyrodinium helveticum) appearing at the end of stratification. Codon F, which was noted during the summer in both lakes, are representative for clear epilimnetic waters with species from the genus Oocystis and Sphaerocystis schroeteri. Coda X2 (Plagioselmis nannoplanctica, Cryptomonas marssonii) and Y (Ochromonas sp.) are commonly found throughout the vegetative period in the oligotrophic to mesotrophic lakes (Reynolds 2006).

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3 Zooplankton 3.1 Rotifers in Zooplankton Freshwater Rotifera are micro-sized (50−200/500 µm) metazoans, comprising of around 2000 species (Bdelloidea—370 species, Monogononta—1450 species), dwelling in plankton, periphyton and benthos. Their main features are corona and trophi, also important by identification, and life cycle including parthenogenesis. Rotifers are often ubiquitous, or widespread, with pivotal role in carbon flux and food webs, between primary producers, microbial loop and higher trophic levels, and are recognized as most important primary consumers (Wallace et al. 2006). Based on their structural (diversity, abundance, biomass) and functional (feeding guilds, habitat selection, body size) traits they are sentinel organisms for environmental alterations and water quality assessment (Špoljar 2013; Stamou et al. 2019). Among zooplankton, they are present in high diversity and abundance (Shumka et al. 2018; Kuczy´nska-Kippen 2020), nevertheless in biomass are often transcended by their counterparts, planktonic crustaceans, cladocerans and copepods. In zooplankton studies on Plitvice Lakes rotifers are included in research conducted between 1951 and 1953, and are recorded in monography by Emili (1958), unfortunately without valid diversity survey. Almost 30 years later (1985/86) rotifers were intensively studied, mainly in two largest and deepest lakes, Proš´ce and Kozjak, by Erben (1991) and Habdija et al. (1993), as well as in periphyton (Erben 1987). At the beginning of XXI century (sampling conducted during 2000) rotifers from plankton, littoral and benthos were considered within longitudinal transport of seston in research by Špoljar (2003) and Špoljar et al. (2007a, b). Rotifers in bryophytes and tufa sediment (sampling in 2009) were included in meiofauna records (Dražina 2012; Dražina et al. 2013, 2017). Last extensive study related to plankton, with assessment of rotifers was conducted in 2009, only in Lake Kozjak (Dujmovi´c 2011; Mihaljevi´c et al. 2011). In this chapter, focus will be on planktonic rotifer forms, while semiplanktonic and benthic forms will consider in the Chap. 6. Beside diversity, rotifer´s survey (Table 1) is presented through the functional traits based on: (1) feeding types discerned according to their food-collecting mechanism and the size of food particles (D—detrivores, A—algivores, O—omnivores, P—predators); and (2) ecological groups according to their habitat selection (L— littoral organisms, B—benthic organisms, P—open water zone organisms (Špoljar et al. 2011; Kuczy´nska-Kippen et al. 2020). Rotifer´s functional feeding groups categories were modified, however based on the feeding types categories by Karabin (1985), often referred in the previous literature: microfilter-feeders (detritivores), are partly or exclusively consumers on bacteria-detritus suspension and nanophytoplankton (particles < 20 µm), for instance specimens of genera: Anuraeopsis, Filinia, Kellicotia, Keratella; macrofilter-feeders (algivores, omnivores) feed on nanophytoplankton too, but mainly on net algae (filamentous algae, dinoflagellates), and sometimes on animal (i.e. protozoans) food as well (particles 20–50 µm) and present:

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species of the genera Ascomorpha, Gastropus, Polyarthra, Synchaeta, Trichocerca; and predator species, which actively catch their prey, presented by Asplanchna, Ploesoma.

3.2 Diversity Composition and Temporal Patterns in Rotifer Assemblages The data on the rotifers diversity are mainly from the 1980s of the twentieth century. Rotifers of the Plitvice Lakes are taxonomically divided into 100 species, 87 species are determined from the subclass Monogonota and 15 species from the subclass Bdelloidea (Fig. 8 and Table 2). All bdelloid rotifers inhabit sediment, littoral lake zone, bryophytes or lotic habitats, while among monogonont rotifers 35 species inhabit lentic habitats and are perfectly adapted to pelagic environmental conditions (Kellicotia, Keratella, Synchaeta, Polyarthra, Trichocerca). The majority of monogonont rotifers (50 species) are semiplanktonic, with morphological adaptation to benthic habitats, inhabiting the littoral of lakes or the sediment of the lotic stretches between lakes (Lecane, Lepadella, Colurella). The most diverse genus among planktonic rotifers was Trichocerca (8 species), and among littoral rotifers Lecane (9 species).

Fig. 8 Dominant planktonic detritivor rotifer species a Keratella cochlearis, b Keratella quadrata, c Kellicotica longispina with morphological adaptation on pelagial, in comparison to littoral semiplanktonic species with foot and toes as adaptation for contact with sediment/surface, Lecane closterocerca often in lotic habitats on Plitvice Lakes. Photos by M. Špoljar

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Table 2 Rotifers taxonomy and functional traits according to habitat selection (P—open water zone organisms feeding types, L—littoral organisms, B—benthic organisms), and feeding type (A—algivores, D—detrivores, O—omnivores, P—predators) in the Plitvice Lakes Species/Taxa

P/L/B

FFG

Species/Taxa

ROTIFERA

ROTIFERA

Eurotatoria-Monogononta

Eurotatoria-Monogononta

P/L/B

FFG

Anuraeopsis fissa (Gosse, 1851)

P

D

Platyias quadricornis (Ehrenberg, 1832)

L

D

Ascomorpha ecaudis Perty, 1850

P

A

Ploesoma hudsoni (Imhof, 1891)

P

P

Ascomorpha ovalis (Bergendahl, 1982)

P

A

Polyarthra dolichoptera Idelson, 1925

P

A

Ascomorpha saltans Bartsch, 1870

P

A

Polyarthra euryptera Wierzejski, 1891

P

A

Asplanchna giodi-brightwelli complex

P

P

Polyarthra remata Skorikov, 1896

P

A

Asplanchna priodonta Gosse, 1850

P

P

Polyarthra vulgaris Carlin, 1943

P

A

Brachionus angularis Gosse, 1851

P

D

Pompholyx sp.

P

D

Brachionus calyciflorus complex

P

D

Proales sp.

L

D

Cephalodella forficata (Ehrenberg, 1832)

L

O

Ptygura sp.

P

D

Cephalodella gibba (Ehrenberg, 1832)

L

O

Scaridium longicauda (Müller, 1786)

L

D

Cephalodella megalocephala (Glascott, 1893)

L

O

Squatinella mutica (Ehrenberg, 1832)

L

D

Cephalodella spp.

L

Synchaeta oblonga Ehrenberg, 1831

P

A

Collotheca mutabilis (Hudson, 1885)

P

D

Synchaeta pectinata Ehrenberg, P 1832

A

Colurella obtusa (Glascott, 1893)

L

D

Synchaeta tremula (Müller, 1786)

P

A

Colurella sinistra Carlin, 1939 L

D

Taphrocampa sp.

L

D

Colurella uncinata (O.F. Müller, 1773)

L

D

Testudinella parva (Ternetz, 1892)

L

D

Dicranophorus forcipatus (Müller, 1786)

L

P

Testudinella patina (Hermann, 1783)

L

D

Dicranophorus grandis (Ehrenberg, 1832)

L

P

Trichocerca birostris (Minkiewicz, 1900)

P

O

Dicranophorus hercules Wiszniewski, 1932

L

P

Trichocerca capucina P (Wierzejski & Zacharias, 1893)

O (continued)

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Table 2 (continued) Species/Taxa

P/L/B

FFG

Species/Taxa

P/L/B

FFG

Dicranophorus sp.

L

P

Trichocerca cylindrica (Imhof, 1891)

P

O

Encentrum mustela (Milne, 1885)

L

A

Trichocerca elongata (Gosse, 1886)

L

A

Encentrum putorius Wulfert, 1936

L

A

Trichocerca inermis (Linder, 1904)

L

A

Encentrum sp.

L

A

Trichocerca pusilla (Jennings, 1903)

P

A

Epiphanes macroura (Barrois and Daday, 1894)

P

D

Trichocerca similis (Wierzejski, P 1893)

A

Euchlanis dilatata Ehrenberg, L 1832

D

Trichocerca stylata (Gosse, 1851)

P

O

Euchlanis triquetra Ehrenberg, 1838

L

D

Trichocerca spp.

P

Filinia terminalis (Plate, 1886)

P

A

Trichotria pocillum (O.F. Müller, 1776)

L

O

Gastropus stylifer Imhof, 1891

P

A

Trichocerca stylata (Gosse, 1851)

P

O

Hexarthra sp.

P

D

Trichotria tetractis (Ehrenberg, 1830)

L

O

Kellicottia longispina (Kellicott, 1879)

P

D

Wierzejskiella velox (Wiszniewski, 1932)

L

A

Keratella cohlearis (Gosse, 1851)

P

D

Wigrella depressa Wiszniewski, L 1932

A

Keratella quadrata (O.F. Müller, 1786)

P

D

Eurotatoria-Bdelloidea

Lecane agilis (Bryce, 1892)

L

D

Dissotrocha aculeata (Ehrenberg 1832)

B

D

Lecane bulla (Gosse, 1851)

L

D

Habrotrocha bidens (Gosse 1851)

B

D

Lecane closterocerca (Schmarda, 1859)

L

D

Habrotrocha cf. pulchra

B

D

Lecane elongata Harring and Myers, 1926

L

D

Habrotrocha collaris (Ehrenberg 1832)

B

D

Lecane flexilis (Gosse, 1886)

L

D

Habrotrocha constricta (Dujardin 1841)

B

D

Lecane furcata (Murray, 1913)

L

D

Habrotrocha elusa elusa Milne 1916

B

D

Lecane ludwigi (Eckstein, 1883)

L

D

Habrotrocha gracilis Montet 1915

B

D (continued)

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Table 2 (continued) Species/Taxa

P/L/B

FFG

Species/Taxa

P/L/B

FFG

Lecane luna (Müller, 1776)

L

D

Macrotrachela quadricornifera quadricornifera Milne

B

D

Lecane lunaris (Ehrenberg, 1832)

L

D

Macrotrachela quadricornifera scutellata Schulte 195

B

D

Lecane spp.

L

D

Macrotrachela sp.

B

D

Lepadella patella (O.F. Müller, 1786)

L

D

Mniobia sp.

B

D

Lepadella rhomboides (Gosse, 1886)

L

D

Philodina acuticornis Murray 1902

B

D

Lepadella triptera (Ehrenberg, 1830)

L

D

Philodina roseola Ehrenberg 1832

B

D

Lepadella spp.

L

D

Philodina sp.

B

D

Macrochaetus subquadratus Perty, 1850

L

D

Pleuretra brycei (Weber 1898)

B

D

Microcodon clavus Ehrenberg, 1830

L

D

Monommata longiseta (Müller, 1786)

L

D

Mytilina mucronata (O.F.Müller, 1773)

L

D

Mytilina ventralis (Ehrenberg, L 1832)

D

Notholca acuminata (Ehrenberg, 1832)

P

D

Notholca foliacea (Ehrenberg, L 1838)

O

Paradicranophorus hudsoni Glascott, 1893)

O

L

Monogont planktonic rotifers are constant and dominant in the zooplankton of lakes (Erben 1991; Špoljar 2003), while littoral and benthic species are rare presented occasionally and accidentally in plankton samples. According to the last survey in 2009 in Lake Kozjak: (i) constant species were: Collotheca mutabilis (Hudson, 1885), Kellicottia longispina (Kellicott, 1879), Keratella cohlearis (Gosse, 1851), Keratella quadrata (O. F. Müller, 1786) and Gastropus stylifer Imhof, 1891; and (ii) dominant species were: Ascomorpha saltans Bartsch, 1870, Collotheca mutabilis (Hudson, 1885), Kellicottia longispina (Kellicott, 1879), Keratella cohlearis (Gosse, 1851), Keratella quadrata (O. F. Müller, 1786), Polyarthra vulgaris Carlin, 1943, Synchaeta tremula (Müller, 1786), Trichocerca similis (Wierzejski, 1893) (according to previous taxonomy it was Trichocerca birostris (Minkiewicz, 1900)), Trichocerca pusilla (Jennings, 1903) (Dujmovi´c 2011).

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From the half of the twentieth century, the abundance of rotifers in summer zooplankton increased from 86 to 92% in Lake Proš´ce, or from 52 to 92% in Lake Kozjak (Emili 1958; Erben 1991). In the vertical distribution, the maximum of rotifer abundance occurred between meta- and hypolimnion (Erben 1991; Habdija et al. 1993; Dujmovi´c 2011). This shift was probably enhanced by increased algal biomass, concurrent with the boundary between trophogenic and tropholytic water layers, in the thermocline zone. During summer, Keratella cochlearis dominated in both large lakes, with metalimnion maxima of 150 and 50 ind. L−1 , in Proš´ce and Kozjak, respectively. In autumn, the abundance of Asplanchna priodonta Gosse, 1850 increased in the epilimnion, while K. cochlearis remained dominant with a maximum of 6 ind L−1 in Lake Proš´ce. In Lake Kozjak, A. priodonta took over domination in the epilimnion (5 ind L−1 ; Erben 1991). For the 1985 study period, trophic structure was constructed using zooplankton biomass in Lake Kozjak (Habdija et al. 1993). Microfilter feeders (detritivores), K. cochlearis and Filinia terminalis (Plate, 1886), consumers of bacterial detritus suspensions, reached their biomass peak in the lower metalimnion in early June (40.7%). Consumers of bacterial detritus and nanophytoplankton, K. quadrata and K. longispina, reached the highest biomassin summer consistently exceeding 50% of the total biomass of non-predatory zooplankton. Among macrofilter feeders (algivores), Synchaeta species occurred in significant biomass proportions in spring, and in autumn the dominant algivores were Polyarthra spp. (in September), Trichocerca spp. (in October) and Gastropus stylifer Imhof, 1891 (in November). Rotifers A. priodonta and Ploesoma hudsoni (Imhof, 1891) are predators and mainly contributed less than 5 µg L−1 to the predatory zooplankton. A comprehensive study at Plitvice Lakes in 2009 (Primc-Habdija et al. 2011) showed more precise spatiotemporal patterns of rotifer populations and generally agreed with previous results (Erben 1991; Habdija et al. 1993). Rotifer populations increase from May to September (mean in the water column 50 to 96 ind. L−1 ) and reached their maximum in June (253 ind. L−1 ), at the metallimnion/hypolimnion boundary. In the vertical distribution, the abundances of rotifers increased in the water layer from 15 to 20 m and 20 to 25 m, 70 and 52 ind. L−1 (annual mean), respectively. Detritivores reached their maximum in June (in the water column 85 ind. L−1 ), with the main contribution of K. cochlearis (43% of the total rotifer abundance), and decreased from August and the rest of the year to 20 ind. L−1 . Algal predators from the genera Trichocerca, Synchaeta, Polyarthra and Ascomorpha reached their maximum (20 ind. L−1 ) at 15–20 m in September. The results indicated rotifer populations as the main grazers over nano- and microphytoplnkton (Dujmovi´c 2011). Competition with crustaceans as well as top-down control by predators (predatory crustaceans, fish) likely have a significant influence on the spatio-temporal patterns of rotifer populations, as evidenced by the decline in rotifer populations in summer (Tasevska et al. 2017; Špoljar et al. 2018; Shumka et al. 2018).

Plankton Communities Table 3 Ecological quality assessment based on physico-chemical and biological indicators HRL Type 2 Mountain, deep, small lakes, Proš´ce Lake and Kozjak Lake in epilimnion layer (0 up to 10 m depth) (modified from Primc-Habdija et al. 2011)

263 Kozjak Lake

Proš´ce Lake

species / sample

16.0

16.3

Rotatoria (ind. /L)

26.8

99.7

Cladocera (ind. /L)

3.2

3.1

Copepoda (ind. L)

3.2

18.1

Shannon’s H’

4213

2766

Simpson E 1/D

0.304

0.108

Saprobic indeks

1.60

1.62

3.3 Rotifers as Indicator of Environmental Conditions and Water Quality Higher abundances of rotifers, especially the detritivore K. cochlearis, and a lower Shannon diversity index, among other parameters according to Water Framework Directive (2000), indicate higher ecological quality in Lake Kozjak compared to Lake Proš´ce (Table 3, Primc-Habdija et al. 2011). Higher abundances and biomass of detritivores generally indicate the presence of a higher amount of organic material and higher productivity of the lake. However, at Lake Kozjak, during the first detection of species of the genus Brachionus was noted during sampling in 2009, indices of higher saprobity (Primc-Habdija et al. 2011). This could be explained by possible leakage and impact of municipal water from the pump in the vicinity of Kozjak Lake. Increased abundance of rotifers was observed in both lakes in the temporal scale of 50 years interval (1958–2009). Alkaline environment of the Plitvice Lakes hydrosystem is a suitable habitat for alkaline tolerant species, for example K. longispina and Synchaeta pectinata Ehrenberg, 1832. Phytoplankton abundant growth is probably suppressed by large-sized algivorous crustaceans, during the summer, and thus algivorous rotifers are outcompeted. In spring and autumn, the abundance of algivorous rotifers increases and they significantly graze the phytoplankton (Dujmovi´c 2011). The study of rotifers in any aquatic system, especially in protected areas, based on their structural and functional characteristics along with other biological elements, quickly reveals changes in environmental conditions and ecosystem function.

3.4 Microcrustacean in Zooplankton of Plitvice Lakes The composition of zooplankton and phytoplankton of the Plitvice Lakes has been studied since the end of the nineteenth century (Šoštari´c 1889; Car 1906; Krmpoti´c 1913). These investigations of plankton were limited only to the plankton qualitative composition and only of the surface water layer. The first detailed and extensive research in the Plitvice Lakes was carried out in the period from 1950 to 1960. Petrik

264 Table 4 Microcrustacean taxonomy and functional traits: feeding type (A—algivores, D—detrivores, O—omnivores, P—predators)

I. Ternjej et al. Species/Taxa

FFG

CLADOCERA Daphnia hyaline Leydig1860

A

Ceriodaphnia pulchella G. O. Sars 1862

D

Bosmina longirostris (O. F. Müller 1785)

D

Polyphemus pediculus (Linnaeus 1761)

P

COPEPODA Thermocyclops crassus (Fischer 1853)

O

Macrocyclops albidus (Jurine 1820)

O

Megacyclops viridis (Jurine 1820)

O

Cyclops sp.

O

(1958) described a vertical stratification of ecological parameters, while biocenological studies of the plankton community was published by Emili (1958). His research included vertical stratification of zooplankton and phytoplankton and the seasonal changes in their qualitative and quantitative composition. The fauna of Copepoda and Cladocera as an integral part of the macrozooplankton were treated only as groups without detailed species determination. Despite all this research, the fauna and zooplankton community dynamics of the Plitvice Lakes remained poorly known until almost the end of the twentieth century. Data from the late 1980s are available for Lake Kozjak (Habdija et al. 1989). Last study related to plankton was conducted in 2009, only in Lake Kozjak (Dujmovi´c 2011; Mihaljevi´c et al. 2011). A total of eight species of plankton microcrustaceans (macrozooplankton) have been identified in Lakes Kozjak and Proš´ce (Table 4). Four species belong to the Cladocera group: Daphnia hyaline Leydig, 1860, Ceriodaphnia pulchella G. O. Sars, 1862, Bosmina longirostris (O. F. Müller, 1785) and Polyphemus pediculus (Linnaeus, 1761); the remaining three species belong to the Copepoda group: Thermocyclops crassus (Fischer, 1853), Macrocyclops albidus (Jurine, 1820), Megacyclops viridis (Jurine, 1820) and Cyclops sp. (Fig. 9).

3.5 Spatial and Temporal Patterns of Plankton Microcrustaceans Total abundance of plankton microcrustaceans in Lake Kozjak does not exceed 35 ind L−1 . Copepods are more numerous than cladocerans throughout the annual cycle. At the beginning of spring, in April, the copepod species T. crassus is found almost exclusively in the macrozooplankton. Cladocerans are represented by a small number of individuals, mainly of the species B. longirostris: the largest population of the species is found at depths below 20 m. Most of the microcrustaceans are found in epi- and metalimnia at depths from 5 to 20 m. The first maximum is in

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Fig. 9 a Megacyclops viridis and b Daphnia hyaline. Photos by Igor Stankovi´c

June, when two species of Cladocera appear in the zooplankton, D. hyalina and C. pulchella. Both species dominate the Cladocera group until the end of the year, and develop particularly dense populations in September. They are very abundant in the thermoclinal layer at depths of 10 to 20 m. During the same period, the predator species P. pediculus appears in the zooplankton. The second maximum of the macrozooplankton is in September, mainly due to the high population density of Cladocera. The numerical dominance of individual species changes during the annual cycle, both temporally and spatially (Fig. 10). Nevertheless, we can identify some general trends. Among copepods, the dominant species is T. crassus, while the other two species occur only sporadically and with small numbers of individuals. The Cladocera group is dominated by B. longirostris during spring (April–June). During summer it is suppressed by D. hialyna and C. pulchella. Their populations do not overlap spatially, with Daphnia occurring in large numbers at depths of 5–15 m and Ceriodaphnia at depths of 15–20 m. Total macrozooplankton biomass in Lake Kozjak ranges from up to 304.5 µg L−1 . During the summer, but also during the autumn maximum, Cladocera have a higher share in the total biomass of macrozooplankton. The total abundance of plankton microcrustaceans in Lake Proš´ce is up to 137 ind. L−1 . The annual cycle of macrozooplankton begins in spring and has a large maximum, usually recorded in August. Copepods are numerically predominant, with the exception of May and September. The species M. viridis and Cyclops sp. occur only sporadically while T. crassus is present through the year and is the most abundant species of copepods. Cladocera are less abundant. In spring, during April and May, the species B. longirostris occurs, and is especially numerous in the meta- and hypolimnic layers. The species D. hyalina and C. pulchela are present

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Fig. 10 Spatial and temporal population density of Cladocera and Copepoda in the Lakes Kozjak and Proš´ce

throughout whole year, but their populations are particularly dense in summer. C. pulchella dominates in the meta- and hypolimnia from 15 to 35 m depth, while D. hyalina dominates in the epi- and metalimnia at depths between 5 and 20 m. The predatory species of Cladocera P. pediculus is present in summer, but with a low population density. From September to December, the number of all groups and species gradually decreases. Winter dormancy occurs and the surface layer of the lake is covered with ice. The total biomass of macrozooplankton in Lake Proš´ce ranges up to 1696 µg L−1 . Maximum of biomass is in September. During summer, the biomass is dominated by Cladocera, while during the rest of the year Copepoda account for a larger proportion of the total microzooplankton biomass. Correlation of plankton population density with environmental parameters was analyzed during research of the Plitvice Lakes in 2011 (Primc-Habdija et al. 2011). A significant correlation was found between total macrozooplankton abundance and concentration of total phosphorus, nitrate, dissolved oxygen, and water temperature in Lake Kozjak. In Lake Proš´ce, a significant correlation was found between the total macrozooplankton abundance and ortho-phosphate and water temperature. The

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number of cladocerans is correlated with total phosphate and ortho-phosphate, while the number of copepods is correlated with ortho-phosphate and temperature. In general, temperature and nutrients can significantly determine the macrozooplankton community. Temperature affects growth, determines the duration of each phase in the life cycle, and the occurrence of individual species throughout the year. Therefore, the total abundance of macrozooplankton in both lakes studied correlates positively with temperature. The life cycle and especially the reproduction and development of cladocerans is affected by temperature changes. During summer, growth and development can be very intense and rapid, lasting only a few days (depending on the species), while in winter most cladocerans produce permanent eggs. In contrast to Cladocera, most copepods are active year-round and even in winter, especially the cyclopoid group. In the spatial distribution of thermal stratification of the Plitvice Lakes, there are significant differences between Lakes Kozjak and Proš´ce due to the morphological and hydrological position in the cascade system of the Plitvice Lakes. Lake Proš´ce is characterized by lower temperature values compared to Lake Kozjak. Therefore, it is not surprising that the “cold water” copepods correlate with the temperature in Lake Proš´ce. The concentration of nutrients, especially nitrogen and phosphorus, also determines the dynamics and time sequence of macrozooplankton populations. This is due to differences in the carbon (C), nitrogen (N), phosphorus (P) content in different macrozooplankton species (Sterner and Elser 2002). These differences affect competition between populations and the concentration and regeneration of nutrients in the lake ecosystem (Olsen et al. 1986; Elser et al. 2007). For example, some taxa are sensitive to P deficiency because of the high phosphorus content in their cells. This is particularly true for Cladocera (Anderson and Hesen 2005; Iwabuchi and Urabe 2010) and is most pronounced in species of the genus Bosmina (Urabe and Watanabe 1992). A statistically significant dependence of Cladocera and phosphorus was found in both lakes, supporting the above facts. On the other hand, copepods have higher N:P cellular ratio (Anderson and Hesen 2005) and are generally more sensitive to nitrogen deficiency (Van Nieuwerburgh et al. 2004). In Lake Kozjak, a positive correlation between abundance and biomass of copepods with nitrates was found, while this is not the case in Lake Proš´ce. However, we must take into account that in Lake Proš´ce the concentration of ortho-phosphate and total phosphorus was very low, significantly lower than in Lake Kozjak.

3.6 Trophic Assemblages of Zooplankton Non-metric multidimensional scaling (MDS) techniques combined with Euclidean distance of species abundance data were applied to the species matrix to determine similarities between zooplankton assemblages in Lakes Proš´ce and Kozjak. The contribution of different taxa to the observed groupings was investigated

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Fig. 11 MDS ordination of major zooplankton groups in Lakes Kozjak (K) and Proš´ce (P)

using SIMPER, a “similarity percentages” procedure. Zooplankton data were log-transformed to stabilize variance prior to multivariate analyzes. MDS ordination analyzes (Fig. 11) separated samples from two lakes at a threshold distance of 5. The first group consists of samples from Lake Kozjak, the second from Lake Proš´ce. The differences between the Rotifera group in Lakes Kozjak and Proš´ce are the most obvious. The taxa identified by SIMPER as most important in distinguishing two lakes are shown in Table 5. Only species contributing 3% or more to the difference between samples were used in the analyses. Lake Kozjak corresponds to the dominance of cyclopoid copepodites and Keratella cochlearis (more than 20%), other species such as Ceriodaphnia pulchela, Kellicottia longispina, Ascomorpha ecaudis and Daphnia hyalina have an average similarity of less than 10%. Lake Proš´ce is dominated by Keratella cochlearis (over 20%), cyclopoid copepodites and Ceriodaphnia pulchela. The difference between the lakes with an average dissimilarity is 61.31% (Table 4), and the species Keratella cochlearis contributes to this difference with average dissimilarity of 13.39%. The differences between lakes revealed by the MDS and SIMPER analyzes can also be explained by trophic guilds (Fig. 12). Small grazers (microfilter feeders) are the most abundant trophic group. Among misrocrustaceans, these are Bosmina, Ceriodaphnia and, among rotifers, Keratella cochlearis. In fact, we find the species identified by the SIMPER analysis as characteristic of Lake Proš´ce. The higher abundance of microfilter feeders also corresponds with diatom-dominated phytoplankton. The occurrence of species such as Daphnia and Ceriodaphnia correlates with high abundance of diatoms and small green algae. Macrofilter feeders are more numerous during periods of larger algae in phytoplankton community; usually spring and fall. In both lakes, predators can be found regardless of the season and their population density is low.

Plankton Communities Table 5 SIMPER results showing species contribution to similarities within, and dissimilarities between zooplankton of lakes Kozjak and Proš´ce

269 Species

Average similarity (%)

Average disimilarity (%)

37.8

38.37

61.31

Kozjak

Proš´ce

Kozjak-Proš´ce Contrib%

Contrib%

Contrib%

copepodites

29.29

19.86

4.59

Keratella cochlearis

20.26

23.25

13.39

Ceriodaphnia pulchela

7.82

16.16

4.87

Kellicottia longispina

7.67

8.5

9.04

7.25

5.45

4.34

2.77

Ascomorpha ecaudis Daphnia hyalina

6.39

nauplii

5.13

3.58

Collotheca mutabilis

4.53

4.24

Synchaeta tremula

3.01

3.99

Trichocerca birostris

3.01

5.08

Gastropus stylifer

3.44

Polyarthra major

Fig. 12 Trophic guilds of zooplankton in Lakes Kozjak and Proš´ce

4.88 6.1

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Aquatic Insects of Plitvice Lakes Marija Ivkovi´c, Viktor Baranov, Valentina Dori´c, Vlatka Miˇceti´c Stankovi´c, Ana Previši´c, and Marina Vilenica

Abstract Aquatic insects are the most species-rich group that inhabit freshwaters. They are connected to water by at least one life stage, usually that of the larvae, and some spend their entire life in freshwater habitats. The majority of aquatic insects’ larvae develop in water; while adults emerge and spend their lives primarily in terrestrial environments where they mate, disperse and in some cases feed. Aquatic insects are sensitive to environmental conditions, which is why they are wiedly used in biomonitoring. Mayflies, stoneflies and caddisflies are among the most commonly used indicators, but other taxa also show high potential. In the Plitvice Lakes, there are confirmed records of 352 species of aquatic insects. The most abundant order of aquatic insects is Diptera, with 165 recorded species, followed by Trichoptera with 91 species, and Plecoptera with 31 species. All other aquatic insect orders are present with lower species richness, with 25 taxa recorded in Coleoptera, 18 taxa in Ephemeroptera and 14 in Odonata. Megaloptera and Neuroptera each have four species. The aquatic insects reflect the uniqueness and peculiarities of the barrage system of the Plitvice Lakes in both, their composition and ecology. Although some M. Ivkovi´c (B) · V. Dori´c · A. Previši´c Divison of Zoology, Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] V. Dori´c e-mail: [email protected] A. Previši´c e-mail: [email protected] V. Baranov Department of Biology II, LMU Munich Biocenter, 82152 Planegg-Martinsried, Germany e-mail: [email protected] V. Miˇceti´c Stankovi´c Croatian Natural History Museum, Deželi´ceva 30, 10000 Zagreb, Croatia e-mail: [email protected] M. Vilenica Faculty of Teacher Education, University of Zagreb, Trg Matice Hrvatske 12, 44250 Petrinja, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_11

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groups of aquatic insects have been well studied throughout the years, there are still many unknowns and many species that need to be recorded. Keywords Aquatic insects · Ephemeroptera · Odonata · Plecoptera · Megaloptera · Neuroptera · Coleoptera · Diptera · Trichoptera · Species richness · Emergence traps

1 Introduction Aquatic insects are among the most numerous and ecologically important groups of animals in freshwaters. Many insect ingroups (orders) contain aquatic or semiaquatic species. They occur in most freshwater habitats and are probably the best studied group of freshwater invertebrates (Dodds and Whiles 2010). Insect ingroups that have almost exclusively aquatic immatures are Ephemeroptera, Odonata, Plecoptera, Trichoptera, and Megaloptera while Coleoptera, Diptera and Hemiptera (Heteroptera) have many representatives in both freshwater and terrestrial habitats. The orders Lepidoptera, Neuroptera, Hymenoptera (all aquatic species exclusively parasitic), Orthoptera (aquatic species mostly present in the southern hemisphere), and Mecoptera (only in the southern hemisphere) occur in freshwater habitats, but not abundantly (Lancaster and Downes 2013). In most aquatic insect taxa, larval development takes place in water, while adults emerge from aquatic environments into terrestrial habitats primarily to mate and disperse. The exception to this are majority of aquatic Coleoptera and Hemiptera (Heteroptera) with full aquatic life cycles (Nillson 1996; Dodds and Whiles 2010). Aquatic insects have been widely used as indicator taxa in biomonitoring programmes worldwide (Bauernfeind and Moog 2000). They are strongly affected by anthropogenic changes in freshwater environments, and respond to habitat modification (Poikane et al. 2016; Vilenica et al. 2020) and to various levels of nutrient enrichment (Dori´c et al. 2021). Therefore, aquatic insects are considered as one of the most reliable ecological indicators of environmental quality in freshwater (Pilotto et al. 2015; Poikane et al. 2016). Since the Plitvice Lakes National Park is under anthropogenic pressure due to touristic activities, that in turn may change the ecological balance of the ecosystem(s) it is highly important to continue monitoring aquatic insect communities as they are one of the best monitoring tools. Consequently, to include effective conservation and management measures when planning sustainable development of tourism in the area when changes are detected.

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2 Data and Methods At the Plitvice Lakes, aquatic insects have been in detail or sporadically studied at 25 sites (Table 1 and Fig. 1). Adults were mostly collected using emergence traps (Fig. 2d). Pyramid-type emergence traps were placed in such way to ensure representative sampling of emergence from all present microhabitats. Each trap was a 50 cm tall, four-sided pyramid with a base of 45 × 45 cm, fastened to the streambed to allow free movement of larvae in and out of the sampling area. The side-frames of the traps were covered with 1 mm mesh netting. On some sites which were characterised by deep water, floating traps were placed. Each trap was 80 cm tall, four-sided pyramid with a base 90 × 90 cm, attached to the bottom by an anchor. At the top of each trap were collection containers filled with preservative (2% formaldehyde with detergent). The containers were emptied monthly and samples preserved in 80% ethanol (Ivkovi´c et al. 2012). Other methods of collecting adult insects included sweep nets, yellow pan traps, aspirators, hand collecting and UV-light traps, whereas aquatic insects larvae and adult Coleoptera were collected using Surber sampler (25 × 25 cm, 500 µm mesh size) and kick-net sampler (25 × 25 cm, 500 µm mesh size) at all present microhabitats. Specimens were preserved in 80% or 96% ethanol.

3 Results At the Plitvice Lakes there are confirmed records of 352 species of aquatic insects. The most abundant order of aquatic insects is Diptera, with 165 recorded species (47% of all aquatic insect species recorded), followed by Trichoptera with 91 species (26%) and then Plecoptera and Coleoptera with 31 (9%) and with 25 taxa (7%) recorded, respectively (Fig. 3). All other aquatic insect orders had less than 25 recorded species together (Fig. 3).

3.1 Mayflies (Ephemeroptera) There are little over 3 000 described species of mayflies known worldwide (BarberJames et al. 2008), of which about 370 are known from Europe (Bauernfeind and Soldán 2012), with about 150 taxa recorded from the Balkans (Bauernfeind 2018 and references herein). Mayflies differ from all other extant insects by having two winged-adult stages (subimago and imago) (Brittain and Sartori 2003). As they are often amongst the most abundant groups of benthic macroinvertebrates, contributing with approximately 25% of the total benthic biomass (Williams 1980), they play an important role in secondary production and represent an important food source for various aquatic and terrestrial predators (Brittain and Sartori 2003).

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Table 1 Sampling sites of aquatic insects in the Plitvice Lakes Site Name

Site ID Latitude

Spring of the Bijela rijeka Stream

1

44°49' 58'' N 15°33' 25'' E 720

Longitude

Elevation (m)

Upper reach of the Bijela rijeka Stream

2

44°50' 04'' N 15°33' 33'' E 715

Plitviˇcki Ljeskovac Village

3

44°50' 27'' N 15°35' 40'' E 668

Spring of the Crna rijeka Stream

4

44°50' 14'' N 15°36' 28'' E 680

Upper reach of the Crna rijeka Stream

5

44°50' 10'' N 15°36' 30'' E 670

Crna rijeka Stream by the bridge

6

44°50' 22'' N 15°35' 59'' E 665

Lake Proš´ce

7

44°51' 33'' N 15°36' 09'' E 635

Labudovac tufa barrier

8

44°52' 17'' N 15°35' 59'' E 630

Lake Okrugljak

9

44°52' 23'' N 15°35' 56'' E 626

Lake Batinovac

10

44°52' 16'' N 15°36' 11'' E 624

Lake Crno-Malo Lake-Lake Vir tufa barrier 11

44°52' 25'' N 15°36' 10'' E 603

Lake Batinovac tufa barrier

12

44°52' 21'' N 15°36' 15'' E 605

Lake Galovac tufa barrier

13

44°52' 32'' N 15°36' 29'' E 594

Lake Gradinsko

14

44°52' 39'' N 15°36' 37'' E 565

Lake Burget tufa barrier

15

44°52' 47'' N 15°36' 53'' E 547

Rijeˇcica Stream

16

44°52' 27'' N 15°36' 47'' E 555

Lake Kozjak

17

44°52' 40'' N 15°37' 07'' E 535

Kozjaˇcki mostovi tufa barrier

18

44°53' 39'' N 15°36' 32'' E 545

Slap Milke Trnine Waterfall

19

44°53' 53'' N 15°36' 39'' E 540

Lake Gavanovac tufa barrier Lake Kaluderovac

20

44°53' 58'' N 15°36' 39'' E 537

21

44°54' 02'' N 15°36' 40'' E 535

Lake Kaluderovac tufa barrier

22

44°54' 08'' N 15°36' 38'' E 505

Sartuk Stream

23

44°55' 57'' N 15°33' 10'' E 765

Plitvica Stream

24

44°54' 08'' N 15°36' 27'' E 555

Korana River in Korana Village

25

44°55' 33'' N 15°37' 09'' E 390

In Croatia, systematic studies of mayflies intensified during the last decade, including the area of Plitvice Lakes. Mayflies can be considered as one of the wellstudied aquatic insects in the Plitvice Lakes system (Vilenica et al. 2014, 2017a, b, 2018a; Vilenica and Ivkovi´c 2021). Due to the specific environmental conditions in the Plitvice Lakes, such as water temperature, high alkalinity and low nutrient availability, mayfly diversity and abundance are rather low with a total of 17 species and one taxa (Table 2) (Vilenica et al. 2014, 2017a, b). Although mayflies inhabit different aquatic habitats, majority of species are confined to lotic ones. Accordingly, their lower diversity in lentic habitats (Bauernfeind and Soldán 2012) is obvious with a total of seven species occurring in the lakes of the Plitvice Lakes (Vilenica et al. 2014) (Table 2). The occurrence of mayflies in lotic habitats is mainly influenced by water temperature, followed by microhabitat

Aquatic Insects of Plitvice Lakes

279

Fig. 1 Sampling sites in the Plitvice Lakes (see Table 1 for site IDs)

composition and food availability (Vilenica et al. 2017a, b, 2018a). As springs are characterized by constant water temperatures, they tend to have low mayfly diversity (e.g., Bauernfeind and Moog 2000; Bauernfeind and Soldán 2012). Only four taxa occur in the springs of the Plitvice Lakes (Table 2). Two species of particular note are Rhithrogena braaschi Jacob, 1974 (Fig. 4a) and Baetis cf. nubecularis Eaton, 1898. Rhithrogena braaschi is endemic to Balkan region, and its biology and ecology are still poorly known. Additionaly, taxonomically unresolved species of Baetis from the group alpinus (sensu Müller-Liebenau, 1969) is known only to occur in the area of the Plitvice Lakes and nowhere else. Lower lotic area and tufa barriers of Plitvice Lakes have the most diverse mayfly assemblages (Table 2). This can be explained by the diversity of the rhithral habitats there, which are characterized by more suitable environmental conditions for mayflies. Among these favorable factors are higher water temperature accompanied with a variety of available microhabitats and food resources (Miliša et al. 2006). Although the physico-chemical properties play the most important role in mayfly distribution in the Plitvice Lakes system, microhabitat heterogeneity is also important in shaping mayfly assemblages (Vilenica et al. 2018a). Organic substrates (moss and macrophytes) and coarse inorganic sediments (pebbles and tufa pieces) associated with higher water velocity support high mayfly species richness (Plenkovi´c-Moraj et al. 2002; Miliša et al. 2006; Bauernfeind and Soldán 2012). Each mayfly species emerges at its own characteristic time of the year primarily driven by photoperiod and water temperature (Brittain and Sartori 2003; Vilenica

280

M. Ivkovi´c et al.

Aquatic Insects of Plitvice Lakes

281

◄Fig. 2 Sampling methods of aquatic insects and their habitats at the Plitvice Lakes: a Spring of the Bijela rijeka Stream, b Sweeping, hand collecting and photographing at the Upper reach of the Bijela rijeka Stream, c Plitvica Stream, d Collecting container of a pyramid-type emergence trap, e Collecting adult insects using a UV-light trap, f Korana River in Korana village, g Kozjaˇcki mostovi tufa barrier and collecting with Surber sampler, h collecting with an aspirator at the Labudovac tufa barrier. Photos by A. Previši´c, Z. Mihaljevi´c and M. Ivkovi´c

Fig. 3 Percentage of aquatic insect orders represented in the Plitvice Lakes

and Ivkovi´c 2021). From the springs of the Plitvice Lakes system, mayflies were recorded to emerge between March and November. Emergence from tufa barriers is also seasonal and related to changes in water temperature, i.e., the increase of the water temperature during the spring triggers the beginning of emergence, whereas, as when the temperature decreases towards the autumn, fewer individuals emerge (Vilenica et al. 2017b; Vilenica and Ivkovi´c 2021) (Fig. 5). Vilenica and Ivkovi´c (2021) investigated a long-term mayfly emergence from the spring of the Bijela rijeka Stream, Labudovac tufa barrier, and Kozjaˇcki mostovi tufa barrier. They reported a higher abundance of mayflies emerging from the spring in years with a higher discharge, while the opposite was showed for tufa barriers (Fig. 5). This can be

282

M. Ivkovi´c et al.

Table 2 Mayfly species recorded in freshwater habitats in the Plitvice Lakes (modified from Vilenica et al. 2014) Species/Habitat type

Spring

Stream and River

Tufa barrier

Lake

Baetidae Alainites muticus (Linnaeus, 1758)

●

●

●

Baetis cf. nubecularis (Eaton, 1898)

●

●

●

Baetis rhodani (Pictet, 1843)

●

●

●

Centroptilum luteolum (Muller, 1776)

●

●

Procloeon pennulatum (Eaton, 1870)

●

●

●

Caenidae ●

Caenis horaria (Linnaeus, 1758)

●

Ephemerellidae Seratella ignita (Poda, 1761)

●

●

Torleya major (Klapalek, 1905)

●

●

●

●

Ephemeridae Ephemera danica (Müller, 1764)

● ●

Ephemera vulgata (Linnaeus, 1758) Heptageniidae ●

Ecdyonurus submontanus (Landa, 1969)

●

Electrogena lateralis (Curtis, 1834) Rhithrogena braaschi (Jacob, 1974)

●

●

Leptophlebiidae ●

Habrophlebia lauta (Eaton, 1884)

● ●

Leptophlebia vespertina (Linnaeus, 1758) ●

Paraleptophlebia submarginata (Stephens, 1835)

● ●

Paraleptophlebia werneri (Ulmer, 1920) Siphlonuridae Siphlonurus croaticus (Ulmer, 1920) Number of species

4

●

●

13

12

7

related to drift of the organisms and food availability. During high water discharge, many mayflies enter the drift to emerge downstream. Some species exhibit emergence period in accordance with the typical Central European emergence patterns, such as the semivoltine Ephemera danica Muller, 1764 and univoltine Paraleptophlebia submarginata (Stephens, 1835), where emergence from tufa barriers and lower lotic habitats occurs mostly during the spring and early summer (Vilenica and Ivkovi´c 2021). Mayflies are extremely sensitive to anthropogenic disturbance in their habitats and are often among the first to disappear when faced with any pollution (Vilenica et al. 2019, 2020).

Aquatic Insects of Plitvice Lakes

283

Fig. 4 Two species of mayflies occurring in the Plitvice Lakes: a Rhithrogena braaschi, b Ephemera vulgata (Linnaeus, 1758). Photos by A. Previši´c

Fig. 5 Long-term mayfly emergence from two types of karst lotic habitats in the Plitvice lakes system: a Spring of the Bijela rijeka Stream; b Labudovac tufa barrier; c Kozjaˇcki mostovi tufa barrier (modified from Vilenica and Ivkovi´c 2021)

284

M. Ivkovi´c et al.

3.2 Dragonflies and Damselflies (Odonata) There are about 6300 described extant species of Odonata worldwide (Schorr and Paulson 2017). Of those 143 inhabit European freshwaters (Kalkman et al. 2008; Boudot and Kalkman 2015). Dragonflies and damselflies spend their larval life in aquatic environment, while after emergence, as adults they use a wide range of terrestrial habitats (Clausnitzer et al. 2009). Vilenica (2017) analyzed ecological traits of Odonata in the Plitvice Lakes lotic habitats and reported eight species. Within the national monitoring programme, six additional species were found in the lentic habitats, i.e. lakes. A total of 14 species are recorded in the Plitvice lakes system up to now (Table 3). Prior to this, various limnological studies on aquatic macroinvertebrates of the Plitvice Lakes reported dragonfly occurence, but these data were sporadic or included only data related to the genus or even order level (e.g. Matoniˇckin 1959; Habdija et al. 2004; Serti´c Peri´c et al. 2015). Since Vilenica (2017) studied only the larval stage of Odonata many species are not listed, as it is essential to sample larvae, exuviae and adults for a complete picture of the fauna (Samways et al. 2009; Raebel et al. 2010). Odonata assemblages in the Plivice Lakes are primarily influenced by hydrology and water temperature. One of the pecularities of the Plitvice Lakes is a relatively low water temperature all year round. Vilenica (2017) showed that this is one of the most important factors for the low diversity and abundance of Odonata in this system. Since most species prefer higher water temperatures (Kalkman et al. 2008), their absence in karst springs and rareness in upper lotic habitats, is not surprising (Table 3). Well-oxygenated tufa barriers, with the highest water temperatures in the Plitvice Lakes, support high Odonata species richness, with eight species occuring there, with predominance of stream species (e.g. Calopteryx virgo (Linnaeus, 1758) (Fig. 6a)) (Vilenica 2017) (Table 3). The highest number of species is present in lakes (nine species) as both lentic species (e.g. Libellula depressa (Linnaeus, 1758)) and species inhabiting wide range of habitats occur there (e.g. Ischnura elegans (Vander Linden, 1820)) (Table 3). The most common species in the lotic habitats of the Plitvice Lakes is Onychogomphus forcipatus (Linnaeus, 1758) (Fig. 6b), a species that prefers rivers and stony streams, but can also be found in well-oxygenated lakes with clear water (Askew 2004).

3.3 Stoneflies (Plecoptera) More than 3500 species are described worldwide (Fochetti and Tierno de Figueroa 2008) of which more than 425 species occur in Europe (Fochetti and Tierno de Figueroa 2004). Larvae live in the aquatic environment. Normally, they are very sensitive to any and all external stressors, which makes them excellent bioindicators.

Aquatic Insects of Plitvice Lakes

285

Table 3 Dragonfly and Damselflies species recorded in freshwater habitats in the Plitvice Lakes Species/Habitat type

Stream and River

Tufa barrier

Lake

●

● ●

●

●

●

●

Anisoptera Cordulegastridae Cordulegaster bidentata Selys, 1843 Gomphidae Gomphus vulgatissimus (Linnaeus, 1758) Onychogompus forcipatus (Linnaeus, 1758) Libellulidae ●

Crocothemis erythraea (Brullé, 1832)

●

Libellula depressa (Linnaeus, 1758) ●

Orthetrum coerulescens (Fabricius, 1798) Cordulia aenea (Linnaeus, 1758)

●

Aeshna cyanea (Müller, 1764)

●

Zygoptera Calopterygidae ●

Calopteryx virgo (Linnaeus, 1758) Coenagrionidae

●

Coenagrion puella (Linnaeus, 1758) Enallagma cyathigerum (Charpentier, 1840)

●

Ischnura elegans (Vander Linden, 1820)

●

Lestidae ●

Chalcolestes viridis (Vander Linden, 1825) Platycnemididae Platycnemis pennipes (Pallas, 1771)

●

●

●

Number of species

3

8

9

Fig. 6 Common species of Odonata in the Plitvice Lakes: a Calopteryx virgo, b Onychogomphus forcipatus. Photos by A. Previši´c and V. Inshyna

286

M. Ivkovi´c et al.

In addition, terrestrial adults are poor fliers, which is why stoneflies have a higher percentage of endemism (Fochetti and Tierno de Figueroa 2008). Several studies investigated stonefly fauna (Popijaˇc and Sivec 2009, 2011) and ecology (i.e., habitat and microhabitat preferences, and emergence periods) in the Plitvice Lakes (Ridl et al. 2018). So far, 31 species were recorded in lotic habitats of the lake system (Table 4). The occurrence of stoneflies within the system is mainly influenced by water temperature, and food availability, which are closely related to hydrology (Ridl et al. 2018). As most stonefly species prefer the headwaters of lotic habitats, characterized by low water temperature (Graf et al. 2009, 2021a, b) (Fig. 7), their species richness decreases downstream the system due to the water temperature increase. The cold-water thermal conditions at springs cause a relatively high number of cold-stenotherm species inhabit springs and upper lotic habitats (Ridl et al. 2018). Stoneflies are exhibiting high microhabitat specificity (Ridl et al. 2018), driven by the availability of the specific food sources (Graf et al. 2009, 2021a). Ridl et al. (2018) reported significantly higher abundances of several species at microhabitats with mosses, because stonefly larvae can use mosses as a refuge from predators and as a food source (Graf et al. 2009). Their emergence is well-studied in the Plitvice Lakes (Ridl et al. 2018). It mainly occurs between February and November and follows the typical Central European patterns (Zwick 2011; Graf et al. 2021a). Most of the species recorded are univoltine and emerge during the spring. Upper lotic habitats and springs are characterized by low annual temperature variability. Hence, the beginning of the stonefly emergence from these habitats is triggered by the photoperiod (Ridl et al. 2018). Emergence is more seasonal at downstream sites, as they are characterized by higher annual temperature variability, which is crucial for start and termination of stoneflies emergence (Ridl et al. 2018). Stoneflies are commonly used as bioindicators of freshwater system’s quality. Increasing pollution and habitat alterations resulted in reduction of populations or extinction of many stonefly species, which is why the stoneflies are among the most endangered groups of insects (Fochetti and Tierno de Figueroa 2004). The ecological consequences of such high stonefly species loss are not yet completely understood, but it is possible that it could disturb the normal flow of nutrients and energy to downstream areas (Baranov et al. 2020).

3.4 Alderflies (Megaloptera) Two extant families of alderflies, the Corydalidae (dobsonflies and fishflies) and the Sialidae (alderflies), include 350 species worldwide (Cover and Bogan 2015). Ten species of alderflies occur in Europe. They all belong to genus Sialis (Fig. 8a) (Vshivkova 1985; Monserrat 2014). Alderfly taxonomy, biology and ecology are relatively well researched in Europe, yet the data regarding Balkans are still rather limited (Cover and Resh 2008). With an aim to fill that gap in understanding alderfly habitat and microhabitat preferences in the Balkan freshwaters, Vilenica et al. (2018b) conducted a systematic study in

Aquatic Insects of Plitvice Lakes

287

Table 4 Stonefly species recorded in lotic freshwater habitats in the Plitvice Lakes Species/Habitat type

Spring

Stream and River

Tufa barrier

●

●

●

Leuctridae Leuctra albida (Kempny, 1899) Leuctra cingulata (Kempny, 1899)

●

Leuctra fusca (Linnaeus 1758)

●

Leuctra handlirschi (Kampny, 1898)

● ●

Leuctra hippopus (Kampny, 1899) ●

●

Leuctra nigra (Olivier, 1811)

●

●

Leuctra prima (Kempny, 1899)

●

●

Leuctra cf. pusilla Krno, 1985

●

●

Leuctra inermis (Kempny, 1899)

●

●

Leuctra major (Brinck, 1949)

●

Nemouridae Amphinemura triangularis (Ris, 1902)

●

●

Nemoura avicularis (Morton, 1894)

●

●

Nemoura cinerea (Retzius, 1783)

●

●

Nemoura flexuosa (Aubert, 1949)

●

Nemoura marginata (Pictet, 1835)

● ●

Nemoura minima (Aubert, 1946) Nemurella pictetii (Klapálek, 1900)

●

●

Protonemura auberti (Illies, 1954)

●

● ●

Protonemura intricata (Ris, 1902) Protonemura nitida (Pictet, 1836)

●

●

● ●

Protonemura praecox (Morton, 1894)

●

●

Perlodidae ●

Besdolus imhoffi (Pictet, 1841) Isoperla inermis (Ka´canski and Zwick, 1970)

●

●

Isoperla cf. lugens (Klapálek, 1923)

●

● ●

Isoperla oxylepis (Despax, 1936)

●

Isoperla rivulorum (Pictet, 1841) Perlodes cf. intricatus (Pictet, 1841)

●

●

●

Taeniopterygidae ●

Brachyptera monilicornis (Pictet, 1841) Brachyptera risi (Morton, 1896)

●

●

Brachyptera tristis (Klapálek, 1901)

●

●

Taeniopteryx hubaulti Aubert, 1946

●

●

Number of species

14

29

12

288

M. Ivkovi´c et al.

Fig. 7 Adult stoneflies from Plitvice Lakes: a Nemouridae at the spring of the Bijela rijeka Stream in copulation, b Perlodes sp. at the Spring of the Crna rijeka Stream. Photos by A. Previši´c

Fig. 8 a Megaloptera, Sialis sp., b Neuroptera, Osmylus fulvicephalus. Photos by: A. Previši´c

lotic habitats of the Plitvice Lakes. Like many other aquatic insects in the same system, alderflies showed to be influenced by the pecularities of karst freshwater habitats, such as high alkalinity, low water temperature and low food availability. Consequently, they showed to be rare but important insect predators in the lotic and lentic habitats of the Plitvice Lakes system, with a total of four species recorded (Matoniˇckin et al. 1971; Matoniˇckin 1987; Vilenica et al. 2018b). Sialis fuliginosa Pictet, 1836 was the most widespread with larvae inhabiting all habitats (Table 5). Alderfly larvae live in a wide range of lotic and lentic habitats, most commonly in soft sediments, where they feed on various small invertebrates (Cover and Bogan 2015). In the Plitvice Lakes they are the most diverse and abundant at sites with low water current and substrates with silt and leaf litter, while they seem to avoid sites dominated by cobbles and mosses, as well as sites with intermittent water flow (Vilenica et al. 2018b).

Aquatic Insects of Plitvice Lakes

289

Table 5 Alderfly species recorded in freshwater habitats in the Plitvice Lakes (modified from Vilenica et al. 2018b) Species/Habitat type

Spring

Stream and River

Tufa barrier

●

●

Lake

Sialidae Sialis fuliginosa (Pictet, 1836)

●

Sialis morio (Klingstedt, 1932)

●

Sialis sordida (Klingstedt, 1932)

●

Number of species

●

●

Sialis lutaria (Linnaeus, 1758)

1

● ●

4

1

3

3.5 Lacewings (Neuroptera) While most families and species are terrestrial, the families Nevrorthidae and Sisyridae have exclusively aquatic larvae, as do some representatives of the family Osmylidae (Aspöck et al. 2001). Most Neuroptera, including species with aquatic larvae, are predators, both as larvae and adults (Hölzel and Weißmair 2002). There are less than 20 species of aquatic Neuroptera present in Europe (Aspöck et al. 2001; Rausch and Weißmair 2007) and the ecology and biology of most species is well known (Hölzel and Weißmair 2002). The larvae of the genus Sisyra (Sisyridae) are aquatic predators/parasites on sponges and bryozoans (Weißmair 2005). Larvae of the genus Nevrorthus (Nevrorthidae) and of the genus Osmylus (Osmylidae), with the most widespread species Osmylus fulvicephalus (Scopoli, 1763) (Fig. 8b), feed on various different aquatic insect larvae (Hölzel and Weißmair 2002). So far, four species of aquatic Neuroptera are recorded in the Plitvice Lakes (Table 6) (Ivkovi´c and Weißmair 2011). Table 6 Lacewing species recorded in lotic freshwater habitats in the Plitvice Lakes Spring

Stream and River

Tufa barrier

●

●

●

Sisyra bureschi (Rausch and Weißmair, 2007)

●

●

Sisyra nigra (Retzius, 1783)

●

Sisyra terminalis Curtis, 1854

●

Species/Habitat type Osmylidae Osmylus fulvicephalus (Scopoli, 1763) Sisyridae

Number of species

1

4

2

290

M. Ivkovi´c et al.

3.6 Water Beetles (Coleoptera) Water beetles are an ecological guild within the order Coleoptera with 13,000 described species (Bilton et al. 2019), of which about 25 families are represented in Europe (Jäch and Balke 2008). They are all characterized by the connection with water in at least one stage of their life cycle (Jäch 1998). The ancestors of water beetles date back to Late Triassic or the Early Jurassic with approximatly ten repeated macroecological shifts between aquatic and terrestrial habitats during the geological history (McKenna et al. 2015). This has led to the remarkable diversity of water beetles today inhabiting most of aquatic habitats (Crowson 1981; McKenna et al. 2015; Bilton et al. 2019). Although water beetles are widely used in biomonitoring (e.g. Elliott 2008), they remain poorly studied in Southeastern Europe (Miˇceti´c Stankovi´c et al. 2015, 2018a, b, 2019). In the Plitvice Lakes, Miˇceti´c Stankovi´c et al. (2019) conducted one of the first studies on water beetles in this part of Europe. The study included the analysis of taxonomy, population aspects and ecological traits of benthic water beetles in lotic habitats in one-year cycle with many results reported for the first time. A total of seven families, 13 genera and 23 species and 2 taxa were recorded (Table 7). Riffle beetles (Elmidae) and the genus Elmis dominate throughout the Plitvice Lakes. Elmis bosnica Zaitzev, 1908 (Fig. 9a, b) dominates in springs and streams Bijela rijeka and Crna rijeka. Recent study revealed that it prefers bryophyte community of Cinclidotus aquaticus (Hedw.) Bruch and Schimp. and Platyhypnidium riparioides (Hedw.) Dix. Species richness of water beetles increases downstream. Species with a narrow distribution range colonize spring areas, whilst widespread species, especially of the European chorotype, occur at tufa barriers, the Plitvica Stream, and the Korana River. The distribution of water beetles is defined by a longitudinal gradient in the Plitvice Lakes, and the genus Elmis proved to be a good indicator of stream zonation. For example, E. bosnica is recorded only in springs and streams Bijela rijeka and Crna rijeka, while E. aenea (Müller, 1806) and E. rioloides (Kuwert, 1890) prefer habitats far from spring areas. Adults showed a preference for bryophytes over angiosperms and were most abundant in spring and summer. Larvae dominated during the fall and winter months and showed no preference for any substrate type (Miˇceti´c Stankovi´c et al. 2019). Scirtidae of the the Plitvice Lakes are showing some interesting features in terms of its phenological patterns depending on habitat type. The family is present in tufa barriers, the Plitvica Stream, and the Korana River throughout the year, while in organically depleted springs it occurs only in winter. In addition, an indicative ecomorphological pattern in the larval distribution is detected, as larvae with broadly flattened bodies (e.g. Elmis bosnica and Elodes sp.) preferred habitats in springs with abundant moss cover on a solid riverbed. In contrast, larvae with cylindrical bodies (e.g. Hydrocyphon deflexicollis group and the genus Riolus) preferred tufa barriers (Miˇceti´c Stankovi´c et al. 2019). The tree canopy coverage altered the life cycle and initiated differences in phenological patterns of crenal Elmis bosnica between two springs studied. The spring

Aquatic Insects of Plitvice Lakes

291

Table 7 Water beetle taxa recorded in lotic freshwater habitats in the Plitvice Lakes Taxa/Habitat type

Spring

Stream and River

Tufa barrier

Gyrinidae ●

Orectochilus villosus (Müller, 1776) Dytiscidae

●

Hydroglyphus geminus (Fabricius, 1792) ●

Platambus maculatus (Linnaeus, 1758) Helophoridae Helophorus brevipalpis (Bedel, 1881)

●

Helophorus sp. ~

● ●

Hydrophilidae ●

Anacaena globulus (Paykull, 1798) ●

Laccobius striatulus (Fabricius, 1801) Hydraenidae

●

Hydraena gracilis (Germar, 1824) Hydraena melas (Dalla Torre, 1877)

●

Hydraena minutissima (Stephens, 1829) Hydraena riparia (Kugelann, 1794)

●

●

●

●

●

●

Hydraena subintegra (Ganglbauer, 1901)

●

Ochthebius metallescens (Rosenhauer, 1847)

●

Scirtidae Elodes sp.

●

Hydrocyphon deflexicollis (Müller, 1821)

●

●

●

● ●

Hydrocyphon novaki Nyholm, 1967 Elmidae Elmis aenea (Müller, 1806) Elmis riolodes (Kuwert, 1890)

●

●

●

●

Elmis bosnica Zaitzev, 1908

●

●

Esolus parallelepipedus (Müller, 1806)

●

●

●

Limnius volckmari (Panzer, 1793)

●

Oulimnius tuberculatus (Müller, 1806)

●

Riolus cupreus (Müller, 1806)

●

●

Riolus nitens (Müller, 1817)

●

●

Riolus subviolaceus (Müller, 1817)

●

●

21

14

Number of taxa

6

292

M. Ivkovi´c et al.

Fig. 9 Dominant water beetle Elmis bosnica in spring areas of the Plitvice Lakes: a on a fingertip and b on a bryophyte. Photos by V. Miˇceti´c Stankovi´c

with an open canopy proved to be a more favourable habitat for adults than the shaded spring (Fig. 10) (Miˇceti´c Stankovi´c et al. 2019). In the Plitvice Lakes, the spatial structure of vegetation proved to be a more important factor for water beetles’ distribution, than its composition. Angiosperms clearly increase species richness of aquatic beetles by developing higher complexity of available habitat (Miˇceti´c Stankovi´c et al. 2018b).

Fig. 10 Canopy alters phenology and life history of Elmis bosnica in the Bijela rijeka Stream and the Crna rijeka Stream. Abbreviations: S2: microhabitat with angiosperms; S3: microhabitat with bryophytes; SBR: Spring of the Bijela rijeka Stream, URBR: Upper reach of the Bijela rijeka Stream, SCR: Spring of the Crna rijeka Stream, MRCR: Upper reach of the Crna rijeka Stream, LRCR: The Crna rijeka Stream by the bridge (modified from Miˇceti´c Stankovi´c et al. 2019)

Aquatic Insects of Plitvice Lakes

293

Tufa barriers yielded the highest abundance of water beetles, suggesting that a rich phytobenthos layer on the surface of bryophytes provides favorable conditions for scraper and collector-gatherer Riolus species. Limnius volckmari (Panzer, 1793) preferres vegetation on coarse sediments to vegetation on solid stones and its distribution is determined by conductivity. Rhitral species Elmis aenea and E. rioloides showed a preference for shallower water depths. The endemic species of the Balkans, Hydraena subintegra Ganglbauer, 1901 (Jäch and Skale 2015) prefers deeper water with slower current and higher conductivity, while Scirtidae prefer high flow and deeper, alkaline water (Miˇceti´c Stankovi´c et al. 2019). The Elmidae and Elmis bosnica showed remarkable sensitivity to environmental conditions, so they should definitely be included in future conservation and protection measures. Further studies should also include lentic freshwater habitats, as a completely different species composition of water beetles can be expected there.

3.7 True Flies (Diptera) Diptera, or true flies, are the most varied group of insects, in ecology and morphology. People, in general, dislike Diptera as they know them only as annoyance or as disease carrying blood-sucking insects, but true flies are as well main actors in the organic material reprocessing, from the sewage to the leaf litter of the forest ecosystems. Diptera are also responsible for other overall ecosystem services as they are one of the main pollinating groups of insects and used in pest control (Pape 2009; Marshall 2012; Adler and Courtney 2019). In freshwater habitats Diptera are the dominating macroorganisams, and they are the group of insects with the biggest number of described species in freshwaters. Almost one-third of all described fly species, approximately 46 000 species, are somehow connected to an aquatic environment during their development (Adler and Courtney 2019). Their numerosity, omnipresence, and diversity in ecological and morphological way, positions them as main provisioners of ecosystem services (Hölker et al. 2015). Aquatic larvae of flies are regularly major constructors and keystone species that change the abiotic and biotic environments through various activities such as digging, scraping, suspension feeding, and predation (Wotton et al. 1998; Adler and Courtney 2019). The high abundance in aquatic fly populations sometimes reached can provide the single or foremost dietary element for other organisms, and as both predators and herbivores, they can serve as biological control agents (Collins and Blackwell 2000; Werner and Pont 2003; Adler and Courtney 2019). They can be indicators of historical and future ecological and climate change, while at the same time they are a crucial as indicators of water quality from the first years of bioassessment (Walker 1987; Mihaljevi´c et al. 1998; Larocque et al. 2001; Adler and Courtney 2019). As insects that undergo holometabolous development, all aquatic Diptera have a life cycle that includes a series of different stages or instars. A typical fly life cycle consists of a short-term egg stage (usually a few days or weeks, only sometimes much longer), three or four larval instars (rarely more), a pupal

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M. Ivkovi´c et al.

stage of variable length, and an adult stage lasting from fewer than two hours (family Deuterophlebiidae) to several weeks or months (Courtney et al. 2017; Lackmann and Butler 2018; Adler and Courtney 2019). Utmost of aquatic Diptera are free-living insects that need a wet environment in at least one part of their life cycle (Adler and Courtney 2019) or, in more strict sense, aquatic Diptera are considered as those linked with water bodies (Courtney et al. 2017). Until now, we recognize 158 dipteran families worldwide, but only 41 families have aquatic representatives (Adler and Courtney 2019), and in Europe there are only 130 families of Diptera, from which about 25 of them are connected to aquatic habitats (Oosterbroek 2006). In the Plitvice Lakes, there are 165 confirmed species of aquatic Diptera, in 13 families, with highest number of species ocuring in springs (75 species) (Table 8). So far, three species of aquatic Diptera have been described from the Plitvice Lakes, Chironomidae Micropsectra uva Giłka, Zakrzewska, Baranov and Dominiak, 2013 and Psychodidae Berdeniella keroveci Kvifte, Ivkovi´c and Klari´c, 2013, and Pericoma miljenkoi Kvifte and Ivkovi´c, 2018. Families Empididae, Simuliidae, Dixidae as well as the genus Limnophora from the family Muscidae have been studied in detail, and their ecological and moreover phenological preferences have been well documented (Ivkovi´c et al. 2012, 2014; Ivkovi´c and Pont 2016; Ivankovi´c et al. 2019).

3.7.1

Non-biting Midges (Chironomidae)

Chironomids or non-biting midges are the most widely distributed and often the most abundant insects in freshwater habitats (Armitage et al. 1995; Ferrington et al. 2008). Chironomid larve inhabit both lotic and lentic habitats as well as all kinds of substrates in freshwater, they can be found in moss, among stones, in the sand as well as silt and organic matter. The type of food that larval chironomids consume is species-specific and it can vary from algae and macrophytes to detritus and other invertebrates. Adult chironomids usually do not feed since they have reduced mouth parts and their only function is to reproduce (Armitage et al. 1995, Vallenduuk and Moller Pilot 2007). Due to their abundance in both aquatic and terrestrial stages, Chironomidae are a very important part of both freshwater and terrestrial food webs and nutrient cycling (Armitage et al. 1995; Baranov et al. 2016). In the Plitvice Lakes system chironomids observe the greatest species richness of all recorded dipteran families with a total of 69 recorded species so far (Table 8). In Lakes Proš´ce and Kozjak 33 species were recorded and 32 species are known from springs (Ivkovi´c et al. 2020). So far, 50 species are recorded at the Kozjaˇcki mostovi tufa barrier (Dori´c, personal observation) (Fig. 11). The most abundant taxa found in the samples from Lake Kozjak were representatives of the genus Cladotanytarsus and, in Lake Proš´ce, the carnivorous taxa Conchapelopia. These autecological data were used in the construction of a new metric that can be used to assess organic enrichment in lakes (Dori´c et al. 2021).

Aquatic Insects of Plitvice Lakes

295

Table 8 Aquatic Diptera recorded in freshwater habitats in the Plitvice Lakes (taken from Ivkovi´c et al. 2020) Species/Habitat type

Spring Stream Tufa Lake and barrier River

Athericidae ●

Ibisia marginata (Fabricius, 1781)

●

Chironomidae Ablabesmyia (Ablabesmyia) monilis (Linnaeus, 1758)

●

Acricotopus lucens (Zetterstedt, 1850)

●

Apsectrotanypus trifascipennis (Zetterstedt, 1838)

●

Brillia bifida (Kieffer, 1909)

● ●

Brillia longifurca Kieffer, 1921 Chaetocladius dentiforceps (Edwards, 1929)

●

Chaetocladius melaleucus (Meigen, 1818)

●

Corynoneura lobata (Edwards, 1924)

● ●

Cricotopus (Cricotopus) bicinctus (Meigen, 1818) ●

Cricotopus (Cricotopus) fuscus (Kieffer, 1909)

●

Cryptochironomus (Cryptochironomus) albofasciatus (Staeger, 1839) Diamesa thomasi Serra-Tosio, 1970

●

Diamesa tonsa (Haliday in Walker, 1856)

●

Dicrotendipes nervosus (Staeger, 1839)

●

Einfeldia dissidens (Walker, 1856)

●

Endochironomus cf. dispar sensu Moller Pillot, 2009

●

Epoicocladius ephemerae (Kieffer, 1924)

●

Eukiefferiella devonica (Edwards, 1929)

●

Eukiefferiella ilkleyensis (Edwards, 1929)

●

Eukiefferiella minor (Edwards, 1929)

●

Eukiefferiella gracei (Edwards, 1929)

● ●

Harinischia fuscimanus (Kieffer, 1921)

●

Heterotrissocladius marcidus (Walker, 1856) Krenopelopia binotata (Wiedemann, 1817)

● ●

Labrundinia longipalpis (Goetghebuer, 1921) Limnophyes cf. minimus sensu Langton and Pinder, 2007

●

Limnophyes gurgicola (Edwards, 1929)

● ●

Macropelopia cf. fehlmanni sensu Kieffer, 1912 Metriocnemus cf. albolineatus sensu Langton and Pinder, 2007 ● Metriocnemus eurynothus (Holmgren, 1883)

● (continued)

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M. Ivkovi´c et al.

Table 8 (continued) Species/Habitat type

Spring Stream Tufa Lake and barrier River

Metriocnemus intergerivus (Sæther, 1995)

●

Micropsectra notescens (Walker, 1856)

●

Micropsectra uva Giłka, Zakrzewska, Baranov and Dominiak, 2013

●

Microtendipes pedellus (De Geer, 1776)

●

Microtendipes tarsalis (Walker, 1856)

●

Monodiamesa bathyphila (Kieffer, 1918)

● ●

Nilothauma brayi (Goetghebuer, 1921) Orthocladius (Mesorthocladius) frigidus (Zetterstedt 1838)

● ●

Paracladius conversus (Walker, 1856)

●

Paracladopelma camptolabis (Kieffer, 1913) Parametriocnemus cf. stylatus sensu Moller Pillot, 2013

● ●

Parametriocnemus stylatus (Spaerck, 1923) Paraphaenocladius cf. exagitans sensu Moller Pillot, 2013

●

Paraphaenocladius impensus (Walker, 1856)

●

Paraphaenocladius cf. irritus sensu Moller Pillot, 2013

●

●

Paratanytarsus lauterborni (Kieffer, 1909)

●

Paratendipes albimanus (Meigen, 1818)

●

Paratrichocladius skirwithensis (Edwards, 1929)

●

Phaenopsectra flavipes (Meigen 1818)

●

●

Polypedilum (Pentapedilum) exsectum (Kieffer, 1916)

●

Polypedilum (Polypedilum) nubeculosum (Meigen, 1804)

●

Polypedilum (Tripodura) scalaenum (Schrank, 1803)

●

Potthastia longimanus Kieffer, 1922

● ●

Procladius (Holotanypus) choreus (Meigen, 1804) Prodiamesa olivacea (Meigen, 1818)

●

● ●

Psectrocladius (Psectrocladius) barbimanus (Edwards, 1929) ●

Rheotanytarsus nigricauda Fittkau, 1960

● ●

Rheotanytarsus pentapoda (Kieffer, 1909) ●

Stempellina bausei (Kieffer, 1911) Tanytarsus brundini Lindeberg, 1963

●

●

Psectrocladius (Psectrocladius) psilopterus (Kieffer, 1906) Rheocricotopus effusus (Walker, 1856)

Synorthocladius semivirens (Kieffer, 1909)

●

●

●

● ● (continued)

Aquatic Insects of Plitvice Lakes

297

Table 8 (continued) Species/Habitat type

Spring Stream Tufa Lake and barrier River ●

Tanytarsus heusdensis Goetghebuer, 1923 Thienemannia gracilis Kieffer, 1909

● ●

Thienemannimyia carnea (Fabricius, 1805) Tvetenia veralli (Edwards, 1929)

●

Zavrelia pentatoma Kieffer and Bause, 1913

●

Zavreliella marmorata (van der Wulp, 1859)

●

Dixidae Dixa dilatata Strobl, 1900

●

Dixa maculata Meigen, 1818

●

●

Dixa nebulosa Meigen, 1830

●

●

●

●

Dixa nubilipennis Curtis, 1832 Dixa puberula Loew, 1849

●

●

●

Dixa submaculata Edwards, 1920

●

●

●

●

Dixella aestivalis (Meigen, 1818)

● ●

Dixella amphibia (De Geer, 1776)

●

Dixella autumnalis (Meigen, 1838) Empididae ●

Chelifera concinnicauda Collin, 1927 Chelifera flavella (Zetterstedt, 1838)

●

Chelifera precabunda Collin, 1961

●

●

Chelifera precatoria (Fallén, 1816)

●

● ●

Chelifera pyrenaica Vaillant, 1981 Chelifera siveci Wagner, 1984

●

Chelifera trapezina (Zetterstedt, 1838)

●

● ●

Clinocera stagnalis (Haliday, 1833)

●

Clinocera wesmaeli (Macquart, 1835)

●

Dolichocephala guttata (Haliday, 1833)

●

Dolichocephala ocellata (Costa, 1854)

●

●

● ●

Hemerodromia laudatoria Collin, 1927

●

Hemerodromia melangyna Collin, 1927

●

Hemerodromia oratoria (Fallén, 1816)

●

● ● ● ●

Hemerodromia raptoria Meigen, 1830 Hemerodromia unilineata Zetterstedt, 1842

●

● ●

Chelifera stigmatica (Schiner, 1862)

●

●

●

●

● (continued)

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M. Ivkovi´c et al.

Table 8 (continued) Species/Habitat type

Spring Stream Tufa Lake and barrier River

Kowarzia barbatula Mik, 1880

●

●

●

●

Kowarzia bipunctata (Haliday, 1833) Wiedemannia aquilex (Loew, 1869)

●

●

Wiedemannia lamellata (Loew, 1869)

●

●

●

Wiedemannia zetterstedti (Fallén, 1826)

● ●

●

Limoniidae Antocha (Antocha) vitripennis (Meigen, 1830) Dicranomyia (Dicranomyia) chorea (Meigen, 1818)

●

●

●

Dicranomyia (Dicranomyia) didyma (Meigen, 1804)

●

●

●

Dicranomyia (Dicranomyia) imbecilla Lackschewitz, 1941

●

● ●

Dicranomyia (Dicranomyia) mitis (Meigen, 1830) complex

●

Ellipteroides (Ellipteroides) lateralis (Macquart, 1835) ●

Eloeophila apicata (Loew, 1871)

●

Eloeophila maculata (Meigen, 1804) Epiphragma (Epiphragma) ocellare (Linnaeus, 1760)

●

Gonomyia (Gonomyia) tenella (Meigen, 1818)

●

●

●

●

●

Hexatoma (Eriocera) chirothecata (Scopoli, 1763) Limonia hercegovinae (Strobl, 1898)

●

Lipsothrix nobilis Loew, 1873 Molophilus (Molophilus) bifidus Goetghebuer, 1920

●

● ●

Molophilus (Molophilus) repentinus Starý, 1971 Ormosia (Oreophila) bergrothi (Strobl, 1895)

●

Paradelphomyia (Oxyrhiza) senilis (Haliday, 1833)

●

● ●

Rhabdomastix (Rhabdomastix) edwardsi Tjeder, 1967 Rhypholophus phryganopterus Kolenati, 1860

●

●

Muscidae Limnophora croatica Pont and Ivkovi´c, 2013

●

Limnophora olympiae Lyneborg, 1965

●

●

● ●

●

●

Limnophora riparia (Fallén, 1824)

●

●

●

Limnophora setinerva Schnabl, 1911

●

●

●

Limnophora pulchriceps (Loew, 1860)

Limnophora tigrina (Am Stein, 1860)

●

Limnophora triangula (Fallén, 1825)

●

Lispe tentaculata (De Geer, 1776)

●

● (continued)

Aquatic Insects of Plitvice Lakes

299

Table 8 (continued) Species/Habitat type

Spring Stream Tufa Lake and barrier River ●

Lispocephala brachialis (Rondani, 1877) Lispocephala spuria (Zetterstedt, 1838)

●

Pediciidae Dicranota (Dicranota) bimaculata (Schummel, 1829)

● ●

Dicranota (Paradicranota) pavida (Haliday, 1833) Pedicia (Amalopis) occulta (Meigen, 1830)

●

●

Tricyphona (Tricyphona) immaculata (Meigen, 1804)

●

●

●

Psychodidae Sycorax feuerborni Jung, 1954

●

Sycorax tonnoiri Jung, 1953

●

Berdeniella keroveci Kvifte, Ivkovi´c and Klari´c, 2013

●

Pericoma blandula Eaton, 1893

●

Pericoma miljenkoi Kvifte and Ivkovi´c, 2018

● ●

Pericoma pseudocalcilega Krek, 1972 Psychoda (Logima) albipennis Zetterstedt, 1850 complex

●

Psychoda (Psychodocha) gemina (Eaton, 1904)

●

Jungiella valachia (Vaillant, 1963)

●

Scathophagidae Acanthocnema latipennis Becker, 1894

●

●

Simuliidae Prosimulium tomosvaryi (Enderlein, 1921)

●

Simulium (Eusimulium) angustipes Edwards, 1915

●

●

Simulium (Eusimulium) rubzovianum (Sherban, 1961)

●

●

Simulium (Nevermannia) angustitarse (Lundström, 1911)

●

Simulium (Nevermannia) costatum Friederichs, 1920

●

●

●

Simulium (Simulium) monticola Friederichs, 1920

●

●

Simulium (Simulium) ornatum Meigen, 1818 complex

●

● ●

Simulium (Simulium) trifasciatum Curtis, 1839 Simulium (Simulium) tuberosum (Lundström, 1911)

●

●

Simulium (Simulium) variegatum Meigen, 1818

●

●

Simulium (Trichodagmia) auricoma Meigen, 1818

●

● ●

Simulium (Wilhelmia) pseudequinum Séguy, 1921 Stratiomyidae Oxycera pardalina Meigen, 1822

●

●

● (continued)

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Table 8 (continued) Species/Habitat type

Spring Stream Tufa Lake and barrier River

Oxycera limbata Loew, 1862

●

Oxycera turcica Ustuner and Hasbenli, 2004

●

Nemotelus pantherinus (Linnaeus, 1758)

●

●

●

Oplodontha viridula (Fabricius, 1775) Tabanidae Chrysops caecutiens (Linnaeus, 1758)

●

Chrysops viduatus (Fabricius, 1794)

●

Tipulidae Tipula (Savtshenkia) rufina rufina Meigen, 1818

●

Number of species

75

71

53

38

Fig. 11 Chironomidae in the Plitvice Lakes: a Chironomidae sample from an emergence trap at the Kozjaˇcki mostovi tufa barrier and b Tanytarsini. Photos by V. Dori´c and V. Baranov

The dominant species in the Spring of the Bijela rijeka Stream was Chaetocladius melaleucus (Meigen, 1818) which is primarily detritus feeder and this site has high levels of organic particles (Ivkovi´c et al. 2015). The identification efforts of chironomids from the Plitvice Lakes are still ongoing since we believe that there are many more species of this family inhabiting this unique ecosystem.

3.7.2

Meniscus Midges (Dixidae)

Dixidae or meniscus midges (Diptera) are found in lentic and lotic aquatic habitats. Only two genera, Dixa and Dixella, ocupay Europe (Pape and Beuk 2012). Larvae are filtrators, feeding on microorganisms and organic particles suspended in the water or

Aquatic Insects of Plitvice Lakes

301

on the water surface. After emergence, adults do not feed and are short-lived inhabiting terrestrial habitats in close proximity to water (Wagner 1997a; Disney 1999). At Plitvice Lakes, there are 6 species of the genus Dixa and 3 species of the genus Dixella (Table 8). Only Dixa species were studied in detail and they are primarily lotic species. Dixa submaculata Edwards, 1920 is the most abundant species at the spring sites and sites close to springs. Dixa nebulosa Meigen, 1830 is most numerous at the Kozjaˇcki mostovi tufa barrier. Dixa puberula Loew, 1849 is the dominant species at most of the sites, especially tufa barriers and streams. Dixa puberula and Dixa submaculata are univoltine, bivoltine or trivoltine depending on the site, while Dixa nebulosa is univoltine (Ivankovi´c et al. 2019). Water temperature is the main ecological factor that influences the phenology of Dixidae at the tufa barriers, Plitvica Stream, and Korana River, while discharge drives fluctuations of the abundance of Dixa through the years (Ivankovi´c et al. 2019).

3.7.3

Aquatic Danceflies (Empididae)

Aquatic Empididae (subfamilies Clinocerinae and Hemerodromiinae) have aquatic immature stages and terrestrial adults. They are predators, both as larvae and as adults, and therefore important as secondary consumers in stream food webs. They mainly feed on Simuliidae larvae (Werner and Pont 2003), Chironomidae larvae and adults (Harkrider 2000; Sinclair and Harkrider 2004) and Psychodidae (Ivkovi´c and Plant 2015). The preferred habitat for larvae and pupae is below the surface of underwater stones, in moss mats in streams, or in the sediments and in the hygropetric zone (Wagner and Gathmann 1996). Data on microhabitat preferences are very scarce, but there is evidence, that larvae prefer moss and gravel (Wagner and Gathmann 1996; Ivkovi´c et al. 2007, 2012). The two subfamilies have different behavioural strategies, and they utilise different microhabitats as adults. Adult Hemerodromiinae are relatively poor fliers (Wagner 1997b), mainly living and hunting in riparian vegetation. In contrast, adult Clinocerinae are good and active fliers found only in very close proximity to water and are observed walking over the surface of wet stones or moss mats (Ivkovi´c et al. 2007). Clinocerinae are generally confined to clean and cool running water (Sinclair 2008) and Hemerodromiinae are also present in such habitats, but they inhabit warmer parts of streams and rivers as well (Ivkovi´c et al. 2013a). At Plitvice Lakes studies have been done on microhabitat preference, phenology patterns and influence of physical and chemical factors on community composition of aquatic Empididae (Ivkovi´c et al. 2012). The dominant genus is Chelifera, while the most abundant species is Hemerodromia unilineata Zetterstedt, 1842. In Table 9 we see that majority of aquatic Empididae have univoltine cycle, one generation per year. Considerable differences in composition and structure of aquatic dance flies’ assemblages were recorded along a longitudinal gradient of the studied sites, primarily related to differences in physical and chemical parameters of the water. Water temperature is the main factor influencing the timing of emergence. In years with higher water temperatures, the start of emergence is earlier (Ivkovi´c et al. 2012). For Hemerodromia unilineata it was established that emergence

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Table 9 The community of aquatic dance flies and flight period in the Plitvice Lakes Species/Month

J

F

M

A

M

J

J

A

S

O

N

●

●

●

●

D

Subfamily Hemerodromiinae Chelifera concinnicauda (Collin, 1927) Chelifera flavella (Zetterstedt, 1838) Chelifera precabunda (Collin, 1961) Chelifera pyrenaica (Vaillant, 1981)

●

Chelifera siveci (Wagner, 1984)

●

●

●

●

●

●

● ●

●

●

●

●

●

●

●

●

● ●

●

●

●

●

●

●

●

●

●

●

●

Hemerodromia melangyna (Collin, 1927)

●

●

●

●

Hemerodromia oratoria (Fallén, 1816)

●

●

●

●

Hemerodromia raptoria (Meigen, 1830)

●

●

●

Hemerodromia unilineata (Zetterstedt, 1842)

●

●

●

●

Chelifera stigmatica (Schiner, 1962) Chelifera trapezina (Zetterstedt, 1838) Hemerodromia laudatoria (Collin, 1927)

●

●

●

Subfamily Clinocerinae ●

Dolichocephala guttata (Haliday, 1833)

● ●

Dolichocephala ocellata (Costa, 1854) ●

Clinocera stagnalis (Haliday, 1833)

●

Kowarzia barbatula (Mik, 1880)

●

● ●

● ●

Wiedemannia (Philolutra) aquilex (Loew, 1869)

●

●

●

●

Wiedemannia (Pseudowiedemannia) lamellata (Loew, 1869)

●

●

●

●

●

●

starts when water temperature reaches 15 °C (Ivkovi´c et al. 2013b). The highest abundance of aquatic dance flies is recorded in lotic habitats with fast water current over substrates of moss, gravel and tufa (Ivkovi´c et al. 2012).

3.7.4

Blackflies (Simuliidae)

Blackflies are insects of high veterinary and medical importance as the females of most species are hematophagous (Crosskey 1990; Adler et al. 2004). They have

Aquatic Insects of Plitvice Lakes

303

aquatic larvae and pupae, whereas the adults emerge from the water and are active in terrestrial habitats, usually close to breeding sites. The larvae feed mostly by filtering, but also by scraping organic material and preying on small macroinvertebrates (Crosskey 1990). The adults feed on carbohydrates from flower nectar, floem juice and honeydew. In addition, the females usually need to have a blood meal from mammals or birds for egg production (Adler et al. 2004). So far 10 hematophagous species are known from the Plitvice Lakes. The dominant species is Simulium angustipes Edwards, 1915, a vector transmitting avian trypanosomes, specifically associated with tufa barriers. Water temperature, alkalinity, conductivity, and habitat type are factors affecting blackfly species composition the most, in the oligotrophic karstic hydrosystem. Emergence patterns of blackflies have been studied in Plitvice Lakes, with multiple generations per year detected in S. angustipes, whereas in Simulium costatum Friederichs, 1920, the number of generations differed between sites with constant and those with variable water temperature (Fig. 12) (Ivkovi´c et al. 2014). The studies of emergence patterns might significantly contribute to understanding of the ecology of blackflies and the associated limnology and might help in predicting mass occurrences of potentially dangerous species.

3.8 Trichoptera With the exception of a few species that are only terrestrial, caddisflies have aquatic eggs, larvae and pupae, and terrestrial adults. With more than 16 266 extant species worldwide, belonging to 618 genera and 51 families, caddisflies are the most species rich of all primarily aquatic insect orders (Morse et al. 2019). Currently, more than 1700 species are known from Europe (Neu et al. 2018), with Dinaric western Balkan region recognised as one of diversity hotspots for Trichoptera (Schmidt-Kloiber and Hering 2015; Graf et al. 2021b). Caddisflies inhabit all aquatic habitat types, lotic and lentic, thus representing one of the most abundant segments of the aquatic fauna (Morse et al. 2019). Furthermore, caddisflies feature a remarkable ecological diversity, and diversity in their functional traits. All of this makes caddisflies crucial food webs and transport of energy and matter within aquatic ecosystems, as well as across aquatic-terrestrial ecosystem boundaries (Morse et al. 2019). Larvae of the caddisflies are often construct portable cases and stationary tubes and retreats, as well as pupal shelters, using silk. This remarkable variability in construction behaviour has had an important evolutionary significance for caddisflies, enabling development of the whole array of larval feeding behaviour (Wiggins 2007). They are considered “the underwater architects” as they are extensively “engineering” their environments (Wiggins 2007). Moreover, lotic species show clear preferences for particular zones, mainly due to water temperature variability (i.e. longitudinal zonation preferences), and many species are sensitive to organic pollution (Graf et al. 2021b). Thus, caddisflies represent ideal sentinel

304

M. Ivkovi´c et al.

Fig. 12 Blackfly emergence patterns at three representative study sites during 2007 and 2008. BL: Labudovac tufa barrier; BNB: Lake Kaluderovac tufa barrier; KS: Korana River in Korana Village (modified from Ivkovi´c et al. 2014)

organisms and knowledge on their communities has been incorporated in routine freshwater biomonitoring (e.g. Birk et al. 2012). The Plitvice Lakes is one of the regions with the longest record of extensive research on caddisflies in Croatia. Various aspects of faunistics, ecology and distribution of caddisflies within the Plitvice Lakes have been studied (Marinkovi´cGospodneti´c 1971, 1979), however, intensive systematic research started in the 1990’s (Kuˇcini´c and Malicky 2002; Kuˇcini´c et al. 2017) and continues until the present. So far, two new caddisfly taxa were described from this area: Drusus croaticus Marinkovi´c-Gospodneti´c, 1971 and Rhyacophila dorsalis plitvicensis Malicky and Kuˇcini´c, 2002. A comprehensive study using emergence traps has been carried

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Fig. 13 Different life stages of caddisflies in the Plitvice Lakes; a egg masses with eggs and first instar larvae of Drusus croaticus in the spring of the Bijela rijeka Stream, b fifth instar larva of Drusus croaticus in the spring of the Bijela rijeka Stream c Tinodes dives (Pictet, 1834) and d Potamophylax sp. Photos by A. Previši´c

out since 2000 revealing emergence patterns (Previši´c et al. 2007; Šemniˇcki et al. 2011), composition and community structure (Šemniˇcki et al. 2012), and diversity of caddisflies (Pozojevi´c et al. 2021). Currently, a total of 91 caddisfly species, from 47 genera and 16 families, have been recorded from the Plitvice Lakes (Table 10) (Previši´c et al. 2010, 2013; Šemniˇcki et al. 2012; Kuˇcini´c et al. 2017; Pozojevi´c et al. 2021). In comparison to the other well-studied areas in the region (e.g. Krka River, Cetina River, Dobra River), the Plitvice Lakes system harbour highly diverse caddisfly community (Kuˇcini´c et al. 2017). The four major habitat types encompassed in the detailed surveys of caddisfly fauna, i.e. springs, streams and rivers, lakes and tufa barriers, all show relatively similar species richness, ranging from 45 to 58 species per habitat type, respectively (Table 10). The composition, structure and diversity of caddisfly communities in the various habitats in the Plitvice Lakes is reflecting differences in the main functional traits of the species composing the respective communities (Šemniˇcki et al. 2012; Pozojevi´c et al. 2021). Water temperature was recognised as the main environmental driver

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Table 10 Species richness of caddisflies in different habitat types in the Plitvice Lakes Species/Habitat type

Spring Stream and River Tufa barrier Lake

Beraeidae Beraea pullata (Curtis, 1834)

●

Beraeamyia schmidi (Botosaneanu, 1960)

●

●

●

●

Beraeodes minutus (Linnaeus 1761) Ernodes articularis (Pictet, 1834)

●

Ernodes vicinus (McLachlan, 1879)

●

Ecnomidae ●

Ecnomus tenellus (Rambur, 1842) Goeridae

●

Goera pilosa (Fabricius, 1775) Litax niger (Hagen, 1859)

●

Silo pallipes (Fabricius, 1781)

●

●

Glossosomatidae Glossosoma bifidum (McLachlan, 1879)

●

Glossosoma discophorum (Klapalek, 1902)

●

●

Synagapetus krawanyi (Ulmer, 1938)

●

●

Hydropsychidae Hydropsyche incognita (Pitsch, 1993)

●

●

Hydropsyche instabilis (Curtis, 1834)

●

●

●

Hydropsyche saxonica (McLachlan, 1884)

●

●

●

Hydroptilidae ●

Hydroptila cognata (Mosely, 1930)

●

Hydroptila occulta (Eaton, 1873) Hydroptila rheni (Ris, 1896)

●

Hydroptila tineoides (Dalman, 1819)

●

Lepidostomatidae Crunoecia cf. irrorata (Curtis, 1834)

●

Crunoecia kempnyi (Morton, 1901)

●

Lepidostoma hirtum (Fabricius, 1775)

●

●

●

●

●

●

Leptoceridae Adicella filicornis (Pictet, 1834)

●

Adicella syriaca (Ulmer, 1907) Athripsodes aterrimus (Stephens, 1836)

●

Athripsodes bilineatus (Linnaeus, 1758) Athriposdes cinereus (Curtis 1834) Ceraclea annulicornis (Stephens, 1836)

●

●

●

●

●

●

●

●

●

● (continued)

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Table 10 (continued) Species/Habitat type

Spring Stream and River Tufa barrier Lake

Ceraclea dissimilis (Stephens, 1836)

●

Mystacides azurea (Linnaeus, 1761)

●

●

●

●

●

●

Mystacides nigra (Linnaeus, 1758)

●

Oecetis lacustris (Pictet, 1834)

●

Oecetis testacea (Curtis, 1834)

●

●

●

Limnephilidae Allogamus uncatus (Brauer, 1857)

●

Apatania muliebris (McLachlan, 1866)

●

Drusus croaticus Marinkovi´c-Gospodneti´c, 1971

●

● ● ●

Chaetopteryx fusca (Brauer, 1857) Chaetopteryx gonospina (Marinkovi´c-Gospodneti´c, 1966)

●

Glyphotaelius pellucidus (Retzius, 1783)

●

●

Grammotaulius nigropunctatus (Retzius, 1783)

●

● ●

Halesus digitatus (Schrank, 1781) Halesus tesselatus (Rambur, 1842)

●

●

●

●

●

●

●

●

●

Hydatophylax infumatus (McLachlan, 1865) Limnephilus affinis (Curtis, 1834)

●

Limnephilus auricula (Curtis, 1834)

●

● ●

Limnephilus extricatus (McLachlan, 1865) Limnephilus flavicornis (Fabricius, 1787)

●

Limnephitus hirsutus (Pictet, 1834)

●

●

Limnephilus ignavus (McLachlan, 1865)

●

●

Limnephilus lunatus (Curtis, 1834)

● ● ●

●

●

●

Limnephilus rhombicus (Linnaeus, 1758)

●

●

●

●

Limnephilus sparsus (Curtis, 1834)

●

●

●

●

Micropterna lateralis (Stephens, 1834)

●

Micropterna nycterobia (McLachlan, 1875)

●

Micropterna sequax (McLachlan, 1875)

●

●

●

●

●

●

Stenophylax permistus (McLachlan, 1895)

●

Stenophylax vibex (Curtis, 1834)

● ●

Potamophylax latipennis (Curtis, 1834)

● ●

Potamophylax luctuosus (Piller and Mitterpacher 1783) Potamophylax nigricornis (Pictet, 1834)

●

●

● (continued)

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Table 10 (continued) Species/Habitat type

Spring Stream and River Tufa barrier Lake

Potamophylax pallidus (Klapalek, 1899)

●

●

●

●

Potamophylax rotundipennis (Brauer, 1857)

●

●

●

Rhadicoleptus alpestris (Kolenati, 1848)

●

●

●

Odontoceridae ●

Odontocerum albicorne (Scopoli, 1763) Philopotamidae Philopotamus montanus (Donovan, 1813)

●

●

●

●

●

●

●

●

●

●

●

●

●

Philopotamus variegatus (Scopoli, 1763) Wormaldia occipitalis (Pictet, 1834) Wormaldia subnigra (McLachlan, 1865)

●

Phryganeidae Agrypnia varia (Fabricius, 1793)

●

Phryganea bipunctata (Retzius, 1783) ●

Phryganea grandis (Linnaeus, 1758)

●

Polycentropodidae Cyrnus trimaculatus (Curtis, 1834)

●

●

● ●

Neureclipsis bimaculata (Linnaeus, 1758) Plectrocnemia brevis (McLachlan, 1871)

●

Plectrocnemia conspersa (Curtis, 1834)

●

●

●

●

●

●

●

●

●

●

●

●

Polycentropus excisus (Klapalek, 1894) Polycentropus flavomaculalus (Pictet, 1834)

●

Polycentropus schmidi (Novak and Botosaneanu, 1965) Psychomyiidae

●

Lype phaeopa (Stephens, 1836) ●

●

●

●

●

●

●

Lype reducta (Hagen, 1868) ●

Psychomyia klapaleki (Malicky, 1995) Tinodes dives (Pictet, 1834)

●

Tinodes unicolor (Pictet, 1834) Tinodes waeneri (Linnaeus, 1758)

●

●

Rhyacophilidae Rhyacophila aurata (Brauer, 1857)

●

●

●

Rhyacophila dorsalis plitvicensis (Malicky and Kuˇcini´c, 2002)

●

●

●

●

●

Rhyacophila fasciata delici, (Kuˇcini´c and Valladolid, 2020)

●

(continued)

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Table 10 (continued) Species/Habitat type

Spring Stream and River Tufa barrier Lake

Rhyacophila schmidinarica (Urbaniˇc, Krušnik ● and Malicky, 2000)

●

●

●

Sericostoma flavicorne (Schneider, 1845)

●

●

●

●

Number of species

45

58

47

50

Rhyacophila tristis (Pictet, 1834)

● ●

●

Sericostomatidae ●

Notidobia ciliaris (Linnaeus, 1761)

of caddisfly communities assembly, in line with differences in species composition regarding their preference for a particular zone (Previši´c et al. 2010; Pozojevi´c et al. 2021). Springs and streams have a narrow gradient of water temperature fluctuation that governs other environmental parameters linked to temperature (e.g. oxygen saturation). These communities have the highest proportion of crenal/rhithral inhabiting cold-water stenotherm species with higher climate change vulnerability potential (e.g. Apatania muliebris, Crunoecia ssp., Ernodes ssp.; Graf et al. 2021b). D. croaticus, for instance, a dominant species in springs and highly abundant in upper stream reaches, as a micro-endemic and cold-water stenotherm crenal inhabitant, fulfils the majority of the criteria listed as potentially highly vulnerable to climate change (Graf et al. 2021b). Tufa barriers stand out as habitats with highest caddisfly diversity in the Plitvice Lakes when comparable datasets are analysed (Previši´c et al. 2007; Pozojevi´c et al. 2021). They feature a mixture of different habitat types (streams and lake outlets), with high spatial heterogeneity and diversity of food resources, thus supporting a community composed of lentic and lotic species with a high proportion of filterers and collectors (Šemniˇcki et al. 2012). Moreover, caddisfly communities at tufa barriers change along the longitudinal gradient, from the upstream barriers dominated by Hydropsyche species to one dominated by Wormaldia species at the downstream barrier. The composition of caddisfly communities at tufa barriers is determined by specific conditions in these habitats, rather than a typical longitudinal distribution mainly influenced by water temperature gradients (Šemniˇcki et al. 2012). Furthermore, species typical for stagnant waterbodies were collected at lakes and tufa barriers (e.g. Neureclipsis bimaculata, Oecetis lacustris, Mystacides nigra; Table 10). None of the species was recorded at all sampling sites although some occurred at all habitat types (e.g. Rhyacophila tristis, Wormaldia subnigra, Tinodes dives, Table 10), mainly due to habitat heterogeneity and diversity along the Plitvice Lakes system. Emergence patterns of caddisflies were analysed in regards to variations across their range (e.g. for widely distributed species such as Rhyacophila species, W. subnigra, Limnephilus lunatus; Previši´c et al. 2007; Šemniˇcki et al. 2011) and valuable insights on ecology and life cycles of insufficiently investigated species were obtained (e.g. D. croaticus, R. dorsalis plitvicensis, R. schmidinarica; Previši´c et al. 2007; Šemniˇcki et al. 2011). On the other hand, a short study with an 8-h sampling

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interval investigating the complexity of relationships between aquatic insect emergence patterns and environmental drivers. It provided novel data on the different drivers triggering emergence in the most abundant caddisfly species in springs (e.g. D. croaticus—daily emergence driven by light intensity) and tufa barriers (e.g. nocturnal emergence of Hydropsyche sp., Ivkovi´c et al. 2013b). Moreover, a recent study evaluating the performance of different diversity measures i.e., commonly used simple diversity indices vs. novel complex measures incorporating ecological information of species was conducted using caddisfly communities in different habitats in the Plitvice Lakes (Pozojevi´c et al. 2021). Thus, further state-of-the-art biodiversity studies providing novel insights into fundamental ecology of caddisflies, but also into changes at population and community levels due to climate change will be conducted using these data in the near future.

3.9 Other Ingroups (Orders) Some orders of aquatic insects or their representatives in aquatic habitats are not covered in this chapter due to inadequate studies of those groups in the Plitvice Lakes. Those orders are aquatic Hemiptera (Heteroptera), aquatic Hymenoptera (although the presence of the ectoparasite in aquatic insect larvae and pupae Agriotypus armatus Curtis, 1832 was confirmed in the Plitvice Lakes), and aquatic Lepidoptera. Acknowledgements We would like to thank Zlatko Mihaljevi´c, Marko Miliša, Miljenko Ivkovi´c, Ivana Pozojevi´c, Natalija Vuˇckovi´c, Sanja Žalac, Igor Stankovi´c, Mladen Kuˇcini´c, Petar Kruži´c and numerous students of the Faculty of Science, University of Zagreb for their help in the fieldwork. We would also like to thank Mladen Kerovec and Zlatko Mihaljevi´c for their financial support during many years of study. Furthermore, we are greatful to Gunnar Mikalsen Kvifte, Adrian C. Pont, Levente-Péter Kolcsár and Jana Nerudova for their help with identification of some specimens. In addition, we thank Valentyna Inshyna for providing a photograph of Onychogomphus forcipatus.

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The Fish of the Plitvice Lakes—A Wealth of Simplicity ´ Ivana Buj, Marko Caleta, Zoran Marˇci´c, Davor Zanella, and Perica Mustafi´c

Abstract The Plitvice Lakes system, although hydrographically belongs to the Korana River Basin and the Black Sea Watershed, actually represents unique ichthyological area. Despite the exceptional richness of aquatic habitats, the Plitvice Lakes fish community is naturally not rich in species. However, fish species inhabiting Plitvice Lakes watercourses are unique and represent an important biodiversity component. A keystone species is the Black Sea trout, perfectly adapted to the conditions both in streams and rivers, but also in lakes, where it reaches impressive sizes. The Italian spined loach and the Italian golden loach, inhabitants of the coastal parts of Lake Kozjak, are responsible for of one of the biggest surprises in ichthyology, as they represent the Mediterranean elements that colonized the Black Sea Basin. Minnows, small fish known from the Lika watercourses, are also probably native to the watercourses of the Plitvice Lakes, although data exist on their introductions. The uniqueness of the Plitvice Lakes ichthyofauna is a consequence of their location, peculiarities of the Dinaric karst, development of travertine barriers that caused the isolation of the Plitvice Lakes from other parts of the Black Sea basin, complex and largely unexplained geological history of the Plitvice srea; and the diversity of habitats. Keywords Freshwater fish · Ichthyofauna · Endemic species · Introduced species I. Buj (B) · Z. Marˇci´c · D. Zanella · P. Mustafi´c Divison of Zoology, Faculty of Science, Department of Biology, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] Z. Marˇci´c e-mail: [email protected] D. Zanella e-mail: [email protected] P. Mustafi´c e-mail: [email protected] ´ M. Caleta Faculty of Teacher Education, University of Zagreb, Savska cesta 77, Zagreb, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_12

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1 The Freshwater Habitats and Fish of the Lika and Alpine Regions of Croatia—The Contrast Between Endemic Fish and Introduced Species Croatia lies at the crossroads of three large biogeographic regions: Continental, Alpine and Mediterranean, and it is well known as a country with one of the most diverse freshwater ichthyofaunas in Europe, with more than 140 species present here ´ et al. 2015, 2019). In addition to the great species (Mrakovˇci´c et al. 2006; Caleta richness, the ichthyofauna is also characterised by a large number of endemic fish species, including 15 stenoendemic species that exclusively inhabit the waters of Croatia. However, Croatia’s freshwater ichthyofauna has not yet been fully explored, and it is reasonable to expect that the number of species will increase further in the near future. The reason for this great biodiversity lies in the geographic position of Croatia encompassing two large drainage basins, the Adriatic Basin and Black Sea Basin (also called the Danube Basin). Together with the highly complex geological past and isolation of the watercourses in the Adriatic Basin, this has resulted in an exceptional richness of freshwater fish species. From the ichthyological perspective, the Dinaric karst region is particularly interesting, with a large number of endemic species, some of which are found nowhere other than in Croatia (Bãnãrescu 2004; ´ et al. 2015). Smith and Darwall 2006; Oikonomou et al. 2014; Caleta The Lika region is situated inland of the Dinaric karst mountains, making up a substantial portion of Croatia’s Alpine region, and as such representing the place where the Continental and Mediterranean regions meet. Lika is a plateau surrounded by mountains: Velebit, Senjsko Bilo, Velika Kapela, Mala Kapela and Liˇcka Plješivica. The entire plateau is further broken up by smaller mountains and hills in higher elevation karst areas, the largest of which are the karst fields: Gacko Polje, Liˇcko Polje, Krbavsko Polje, Koreniˇcko Polje, southern Lika near Graˇcac, and especially the Liˇcko Pounje area around the Una River. Interesting, both of Croatia’s major drainage basins are present in the Lika region, with the waters of the Gacka River, the rivers in the Liˇcko Polje field and around Graˇcac as part of the Adriatic Basin, while the waters of the Stajniˇcko Polje, Krbavsko Polje and Liˇcko Pounje falling within the Black Sea Basin. The majority of the river systems of this closed plateau are sinking rivers, while only the Una River, flowing along the eastern border of Lika, and the Korana River, that receives the waters of the Plitvice Lakes, flow completely on the surface. The water regime of most of the watercourses in the Lika region is the rainy Mediterranean type. Due to the hydrological specificities, all the waters in the Lika region have exceptional hydrogeneration potential, and many have already been altered with the construction of dams and reservoirs, and the waters are used to turn the turbines of the hydropower plants (Bognar et al. 1975). The native ichthyofauna of the Lika River reflects the rich geological past and geographic position of Lika, given the large number of stenoendemic species found here, very few of which have a broad distribution. These are species in the genera Telestes, Delminichthys and Cobitis. The localities where these endemic species are still found today are typical karst watercourses and springs, which have abundant

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water during the colder part of the year and in the spring, while their water levels decline dramatically during the summer months, and some even partly or completely dry out. It is considered that these native genera have become perfectly adapted to these extreme conditions, and that they survive the hydrologically unfavourable parts of the year by taking refuge in parts of the watercourse that do not dry out, or by withdrawing underground. Of the native species, the rivers of the Lika plateau contains the Black Sea trout Salmo labrax Pallas, 1814; Italian minnow Phoxinus lumaireul (Schinz, 1840), and Weatherfish Misgurnus fossilis (Linnaeus, 1758), while the waters in the Liˇcko Pounje area also harbour populations of the European bullhead Cottus gobio Linnaeus, 1758, and Huchen Hucho hucho (Linnaeus, 1758). It is believed that the present day distribution of the native species is in fact the remnant of the ichthyofauna that once inhabited the entire Lika region. The Lika River has been most altered in terms of its fish community, as today these waters are inhabited exclusively by introduced, alien species. The only tributary to the Lika River that still contains endemic species is the Jadova River, which dries out in its middle section during the summer months. Freshwater systems are globally among the most threatened ecosystems due to a wide range of human activities. One of the most negative impacts on these ecosystems is the introduction and spread of alien species. However, very often, the impact of alien and invasive fish species on the native fauna has a joint, synergistic effect, together with habitat degradation and destruction, pollution, hydrological changes, overexploitation and climate change (Dudgeon et al. 2006; Moorhouse and Macdonald 2015). The Red Book of Freshwater Fauna of Croatia also lists alien species (non-indigenous, non-native) as a major cause of threat to the native ichthyofauna (Mrakovˇci´c et al. 2006). Alien fish have been introduced here from a number of continents, originating from various biogeographic areas. These species have a broader range of ecological tolerances and occupy a spectrum of ecological niches. Accordingly, not all alien species present an equal threat to the native ichthyofauna, nor is their influence equally present and strong in difference habitat types (ecological conditions). Certain species, following introduction, can have a small population size and may not necessarily have a negative impact on the native fauna. On the other hand, there are species that rapidly develop large populations, and they have an exceptionally large impact on the local ecosystems and native species, as they quickly and easily spread into new areas and new waters, making them invasive species. The influence of introduced, alien fish species on the native fauna can be direct (predation, hybridisation) or indirect, in the form of competition (for habitat or food resources) or changes to the ecosystem (impact on the habitat or the food web). Very often, the successful acclimatisation and spread of alien species is spurred by water pollution, habitat degradation and changes in the structure of the native fish communities. The consequences of these impacts can be seen in changes to the fish community composition, and reduced abundance or the complete disappearance of native fish species, including those that are rare, endemic and threatened. The introduction of alien fish in Croatia is a particular problem in karst waters and the watercourses of the Adriatic basin, which ´ are rich in endemic fish fauna (Caleta et al. 2015).

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Croatia’s freshwaters are inhabited by 19 alien fish species introduced from other countries, and that have since succeeded in establishing self-sustaining populations ´ et al. 2019). Four of these species are (Pofuk et al. 2017; Piria et al. 2018; Caleta found exclusively in the waters of the Black Sea Basin, two only in the Adriatic Basin, while 13 are found in both basins. Apart from alien species originating from various countries, a very big problem is represented by species native to certain watersheds in Croatia, but that were accidentally or intentionally transferred within Croatia and between different watercourses, watersheds and river basins. The largest number of such species are those that have been transferred from the Black Sea Basin to the Adriatic basin. Such species are very often referred as translocated species (Copp et al. 2005) although in conservation biology this term is used for species that are transferred for conservation and management reasons. However, such a form of translocation is called conservation translocation (IUCN/SSC 2013). Therefore, the number of alien species in the Adriatic Basin is significantly higher because a number of species were introduced from the Black Sea Basin. Although these species are native to Croatia, they are native only to the watercourses which they naturally inhabit, but they are equally exotic or invasive to the watercourses in which they are introduced, as well as species introduced from other countries and continents, causing equally negative or even worse consequences to the native ichthyofauna. To date, 17 species of fish from the Black Sea Basin have been recorded as introduced into other watercourses, either within the Black Sea Basin (where they ´ et al. were previously absent) or into the Adriatic Basin (Pofuk et al. 2017; Caleta 2019). These species often become quickly naturalised and acclimatised, and their negative impacts on the native ichthyofauna is often exceptionally harmful. As previously stated, the waters of the Lika region have high hydropower potential, and reservoirs have been created on several rivers: Kruš´cica on the Lika River, Gusi´c and Šviˇcko Lake on the Gacka River, Štikada on the Riˇcica River, and Sveti Rok on the Obsenica River. With the construction of these reservoirs, the original ecological conditions were strongly changed, and the new conditions have significantly altered the fish community, which is now dominated by alien species. In the Lika region, we today find the following species that have been introduced into Croatia: Rainbow trout Oncorhynchus mykiss (Walbaum, 1792), Prussian carp Carassius gibelio (Bloch, 1782), Grass carp Ctenopharyngodon idella (Valenciennes, 1844), Silver carp Hypophthalmichthys molitrix (Valenciennes, 1844), Bighead carp Hypophthalmichthys nobilis (Richardson, 1845), Eastern mosquitofish Gambusia hoolbroki Girard, 1859 and Pumpkinseed Lepomis gibbosus (Linnaeus, 1758). This region also contains species introduced from the waters of the Black Sea Basin: Common bleak Alburnus alburnus (Linnaeus, 1758), Crucian carp Carassius carassius (Linnaeus, 1758), Common carp Cyprinus carpio Linnaeus, 1758, Danube gudgeon Gobio obtusirostris Valenciennes, 1842, European bitterling Rhodeus amarus (Bloch, 1782), Common roach Rutilus rutilus (Linnaeus, 1758), Common rudd Scardinius erythrophthalmus (Linnaeus, 1758), Common chub Squalius cephalus (Linnaeus, 1758), Tench Tinca tinca (Linnaeus, 1758), European catfish Silurus glanis Linnaeus, 1758, Northern pike Esox lucius Linnaeus, 1758, European perch Perca fluviatilis Linnaeus, 1758 and Ruffe Gymnocephalus cernua (Linnaeus, 1758). The third group

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of alien species are those that have been introduced from the waters of the Adriatic Basin (from the Zrmanja River), as a consequence of pumping water for the pumpedstorage hydropower plant Velebit: Alborella Alburnus arborella (Bonaparte, 1841), Italian spined loach Cobitis bilineata Canestrini, 1865, Padanian goby Padogobius bonelli (Bonaparte, 1846), and Zrmanja chub Squalius zrmanjae Karaman, 1928. From the above, it can be concluded that the Lika reservoirs have since become regional centres of “diversity” of alien species, and the site of their spread further ´ into natural watercourses (Caleta et al. 2019). Lika is a region of contrasts in many ways, and perhaps the greatest contrast can be found in its ichthyofauna—from the exceptionally valuable endemic species, to the exceptionally high risks for their survival; from the perfect adaptations to suit the Lika watercourses, to species that thrive here thanks to altered habitats and unconscientious introductions by humans; from species whose only habitat in the world is found right here in Lika, to species with a very broad distribution range that do not belong in Lika at all. Unfortunately, for the freshwater fish, these contrasts cannot continue to coexist for long. Alien species are often the winners in this battle of contrasts, forcing out the unique, native species. Only our conviction to restore populations and to ensure the survival of the unique Lika fish fauna diversity will be able to prevent extinction of some of the most valuable components of biodiversity, not only at the Croatian scale, but also in European and global terms.

2 Position of the Plitvice Lakes on the Ichthyology Map of Croatia and Europe As stated above, the Lika region lies at the meeting place of two large drainage basins to which the waters in Croatia belong: the Black Sea Basin and the Adriatic Basin. In Lika, we find both drainage basins, with some rivers flowing into one and others into the other. At first glance, it is often unclear which basin the Lika rivers belong to, as they are mostly sinking rivers connected underground to the waters of another basin, without any surface connections. Though the Lika watercourses are typical karst rivers, situated in a relatively small area, their belonging to different drainage basins has created a number of differences between them. The rivers of the Black Sea Basin are usually long, with many tributaries, most of which are mutually interconnected. On the other hand, the rivers of the Adriatic Basin are usually short, have very few tributaries and are highly isolated from one another, at least their surface flows. Underground, the situation may be very different—this is a karst landscape where the underground hydrological systems are often much richer and more complex than the surface flows, and many watercourses have no surface connections at all, but are interconnected in the subterranean network. The differences in physico-chemical characters of water in the different drainage basins also creates different conditions for the development of fish communities, and the isolation of Adriatic watercourses has supported speciation. Furthermore, the waters belonging to different basins have

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been under the influence of different conditions and events during the geological past, which ultimately led to distinct fish communities in the water courses belonging to the Black Sea Basin, than those found in the waters of the Adriatic Basin. As already stated, most of the Plitvice Lakes area lies within the Lika region, and belongs to the Black Sea Basin (Fig. 1). The Korana River drains into the Kupa River, thus connecting the Plitvice Lakes with the remainder of the Black Sea Basin. However, the ichthyofauna of the Plitvice Lakes differs even from the ichthyofauna of the Korana River. This is due in part to the isolation of the Plitvice Lakes system, and partly due to their position and only partial understanding of the geological past of this area, and with it the evolutionary history of the freshwater fish of the lakes. Natural fragmentation of the water system, formed by the 16 tufa barriers that have creating the wonder of water in these lakes also poses constraints to fish community development, because few fish species can be adapted to such an environment. As a result, the waters of the Plitvice Lakes are inhabited by species that also inhabit other salmonid habitats of the Black Sea Basin, such as the Black Sea trout and Italian minnow. On the other hand, these habitats contain species that are characteristic for the Adriatic Basin, and are not found anywhere else in the Black Sea Basin except in the Plitvice Lakes system, such as the Italian spined loach and the Italian golden loach.

Fig. 1 Map of Croatia indicating the two large drainage basins, the Lika region, and the position of the Plitvice Lakes

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Hydrologically, the Plitvice Lakes system forms a part of the Black Sea Basin, like most of Central Europe, and the ichthyofauna composition of this system is quite different from all other watercourses in the Black Sea Basin and on the ichthyological map of Europe, making this area both unique and highly valuable. Unfortunately, the distinctiveness of this area is largely threatened by the introduction of alien species into the Plitvice Lakes system, and as a result has made the overall fish community to more closely resemble other waters in the Black Sea Basin. However, despite the fact that the waters of the Plitvice Lakes system now contain more species and more individuals of fish than 100 or 150 years ago, this is neither a natural nor desirable state, as this has brought the most distinctive features of the Plitvice Lakes to the brink of extinction. Parallel with the introduction of alien species, the Plitvice Lakes have also been exposed to a series of other anthropogenic pressures, also with negative consequences to the native ichthyofauna.

3 Diversity of Habitats of the Plitvice Lakes System as a Basis for Development of Fish Communities Aquatic ecosystems are crucial for maintenance of great biodiversity, but also numerous ecosystem services they provide. They recycle nutrients, purify water, attenuate floods, augment and maintain streamflow, recharge ground water, provide habitat for wildlife and recreation for people (National Research Council 1992). Besides services provided for humans, of which the supply of drinking water is the most important, around 126,000 currently recognized species rely on freshwater habitats worldwide (https://www.iucn.org). Fishes are the most diverse and particularly important indicator of the status of freshwater habitats (Moyle and Leidy 1992); meanwhile, freshwater systems have been identified as the most endangered ecosystems worldwide (https://www.iucn.org). Nevertheless, the importance of Plitvice Lakes habitats is even greater. Namely, Plitvice Lakes is one of the most important Natura 2000 sites for the 32A0 habitat type in Croatia. In addition to the lakes and tufa barriers that represent the basic phenomenon of this landscape and of the park, this area is also characterised by high species endemism, preserved specific subterranean habitats, and a range of other diverse habitats. Despite its multiple levels of protection, i.e., as a national park, as a UNESCO World Heritage site, and as part of the Natura 2000 network, most research in the past that led to this protection were based on the terrestrial habitats around the lakes. In fact, not a single freshwater species was listed in the designation of the Natura 2000 site. However, recent research has revealed that the most unique components of the biodiversity of this area are actually contained within the freshwater ecosystems. Though the lakes and waterfalls themselves are the most attractive and most visited part of the Plitvice Lakes (after which the national park was named), the watercourse system of the Plitvice Lakes includes a series of smaller rivers and streams. All of these come together into a mosaic of different habitats, from the headwaters, to the

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waterfalls and cascades, to the slower and calmer sections of flow, to the lakes; from habitats with a rock and gravel substrate, to the sandy and silty sediments in the lakes; from the shallow streams to the deep lakes. The diversity of habitats within the lakes, as well as in the rivers and streams of the complex Plitvice Lakes system, has resulted in different fish fauna within. The two largest lakes, Proš´cansko and Kozjak are also the deepest, each with depths of over 35 m. As such, these lakes offer a wider diversity of habitats to support different fish species, and have been readily colonised by introduced species. Moreover, sandy and muddy nearshore habitats of the Kozjak Lake provide perfect conditions for loach species. The remaining lakes are smaller and shallower, with a more impoverished fish fauna, most recording two to three species, where the upper lakes still retain the native species, Italian minnow (Phoxinus lumaireul) and Black Sea trout (Salmo labrax), though introduced species are also present. Fast watercourses with a pebbly substrate, cold water and high dissolved oxygen levels, found in the upstream sections of the lakes and in the source rivers in the headwaters section of the park, are optimal for trout species, particularly during the spawning season. Every native species has found its place among the diversity of habitats in the Plitvice Lakes, and a specific ecological niche in this unique, though vulnerable ecosystem. However, the alien species too have adapted well to the conditions in the waters of the Plitvice Lakes system, particularly in the lakes. Their competitive advantages and predation effects have in turn altered the ecosystem, and their metabolic products have also changed the ecological conditions of the habitats themselves.

4 How Did Events in the Geological Past and in Human History Affect the Fish Community in Plitvice Lakes National Park? There are numerous advanced methods now at our disposal to determine the age of an area, and to decipher the geological events that shaped it. Using such techniques, it has been established that the Plitvice Lakes received their present-day appearance in the later phases of the Pleistocene. According to the latest radiometric dating, older inactive barriers were precipitated between 250,000 and 300,000 years ago (MindelRiss) and a second phase occurred 90,000–130,000 (Riss-Würm) years ago. Active barriers have been produced within the last 8,000 years (Obeli´c 2011). However, the DNA of organisms living in an area often takes us even further back into the past, bearing witness to even earlier events that influences their evolution in that area. The DNA of the Black Sea trout studied in the waters of the Plitvice Lakes system has indicated that this species has been present and evolving in the Plitvice Lakes area for at least the entire Pleistocene, though the alternations between the glacial and interglacial periods impacted its intraspecies diversity (Buj et al. 2020). Some events that shaped the fish communities have not yet been elucidated, such as the colonisation paths of the loaches, that still remain a mystery.

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The natural uplifting of the tufa barriers led to rising water levels in Proš´cansko and Kozjak Lakes, and this was particularly evident until the early 1990s. That growth was suddenly halted, and downstream of the lake, the Korana River began to occasionally experience periods of dry river beds, as the water retracted underground (Bonacci 2013). The fish communities adapted to the lake habitats, though it appears that they will need to continue adapting to these changes in the system hydrology. In addition to natural adaptations to habitat change, humans have also played an important role in the alterations of the composition of the fish community. In the human desire to manage nature, and due to the attractiveness of the environment, the decision was made to allow sport fishing on the lakes as part of the tourism offer. For this purpose, the lakes were stocked with alien species throughout most of the twentieth century. Furthermore, there were numerous attempts to stock the lakes with salmonid fishes, in the attempt to build up fisheries with the breeding of salmonid species. During the Homeland War (1991–1995), the Serbian occupying forces even permitted dynamite fishing in the lakes, which further impacted the fish populations in the lake. Though human impacts on the fish communities of the Plitvice Lakes have been mostly negative over history, the situation has since changed dramatically for the better, with great efforts invested into renewing natural habitats and preserving the native fish communities.

5 The Ichthyofauna of the Plitvice Lakes The newest research and literature data pertaining to the waters within Plitvice Lakes National Park have shown that the current fish fauna consists of 10 species from 3 orders and four families. However, only three of this species are native to the Plitvice Lakes area: Black Sea trout Salmo labrax, Italian golden loach Sabanejewia larvata (De Filippi, 1859) and Italian spined loach Cobitis bilineata. The status of the Italian minnow Phoxinus lumaireul has not yet been fully clarified, and it is not known whether this species was introduced by humans, or whether the populations of this species were present earlier in the waters of the Plitvice Lakes system. It is possible that populations were present, but that additional individuals were also introduced. The remaining six species are alien species, all introduced over the past 100 years as a consequence of human activity. Below is a systematic overview of the freshwater fish present in the Plitvice Lakes system. ORDER: ESOCIFORMES FAMILY: ESOCIDAE Rafinesque, 1815 1. Esox lucius Linnaeus, 1758—Northern pike

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ORDER: CYPRINIFORMES FAMILY: COBITIDAE Swainson, 1838 2. Cobitis bilineata Canestrini, 1865—Italian spined loach 3. Sabanejewia larvata (De Filippi, 1859)—Italian golden loach FAMILY: LEUCISCIDAE Bonaparte, 1835 4. Phoxinus lumaireul (Schinz, 1840)—Italian minnow 5. Scardinius erythrophthalmus (Linnaeus, 1758)—Common rudd 6. Squalius cephalus (Linnaeus, 1758)—Common chub ORDER: SALMONIFORMES FAMILY: SALMONIDAE Schinz, 1822 7. 8. 9. 10.

Oncorhynchus mykiss (Walbaum, 1792)—Rainbow trout Salmo labrax Pallas, 1814—Black Sea trout Salmo trutta Linnaeus, 1758—Brown trout Salvelinus alpinus (Linnaeus, 1758)—Arctic charr.

It is also necessary to mention the native fish species recorded in the Korana River downstream of the lowest lake, and still within the boundaries of the Plitvice Lakes National Park. In that most upstream part of the Korana River, which at times dries out completely, five species of fish have been recorded that are native to the river: Danube spined loach C. elongatoides B˘acescu & Mayer, 1969, Italian minnow P. lumaireul, Danube barbel Barbus balcanicus Kotlík, Tsigenopoulos, Ráb & Berrebi 2002, Common chub S. cephalus and Spirlin Alburnoides bipunctatus (Bloch, 1782).

5.1 Trout—A Key Component of the Plitvice Lakes Aquatic Ecosystems Trout (Salmonidae) are among the most studied fish families in the world. Trout are medium large to large fish (lake forms can reach lengths of up to 1.5 m; Fig. 2) and are naturally distributed throughout the northern hemisphere. They are highly suitable fish for fisheries and aquaculture, and as such have been introduced to waters around the world. Rarely has a group of freshwater fish been so widely used in fisheries. However, despite the high interest of people for trout, the fundamental taxonomy of trout, and the recognition of species and categorisation of populations remains unresolved, and often this group is called the “disgrace of European ichthyology” (Kottelat and Freyhof 2007). The main reason for this lack of taxonomic resolution is the high morphological variability and plasticity, and therefore the morphological traits have little diagnostic value. Though body colouration can vary widely, the base

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colour is greyish to yellowish, with varying numbers of spots of different size. These spots can be red or black, occasionally with a white or black rim. Even individuals confirmed to belong to the same species can have high morphological variability, particularly if they live in different habitats. Despite the many habitats they occupy, trout have a narrow ecological valence and specific requirements for temperature and dissolved oxygen concentrations in water, so they require relatively cold, oxygenrich water. As stenovalent species, trout are not tolerant to pollution or significant environmental changes. The ecological conditions required by trout are typically found in the headwaters of rivers and in high elevation lakes. Though trout can live in lakes, all trout are considered migratory species, and they require a free path towards the headwaters for spawning. Accordingly, they are threatened by any barriers on the watercourse that interrupt longitudinal connectivity. Trout usually spawn in gravel, where they dig out spawning beds. All trout lay large eggs and therefore have a low fecundity, resulting in a low reproductive potential. They are active hunters, feeding on other fish and aquatic invertebrates, and can also eat small mammals if the opportunity arises. Though they are skilled predators in aquatic habitats, they are food for numerous terrestrial organisms, so trout play

Fig. 2 A male trout caught in Kozjak Lake weighing 13 kg

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a connecting role, transferring energy from aquatic into terrestrial systems. They are important also because they are well known and accessible to people, and can therefore serve as a flagship species, being a species that people can identify with and can more easily understand the reasons needed for their conservation. Trout are often considered a keystone species in their habitat because they are excellent predators that maintain the populations of other organisms, thereby affecting the entire ecosystem. Furthermore, trout never form dense populations (the way cyprinid species do), and so stable trout populations ensure a lower trophic level, retaining the favourable ecological state of the water. When the native trout community is replaced by a cyprinid community, changes in the habitat conditions are quickly evident. Invasive species tend to have explosive reproduction rates, which places great pressure on the environment due to the greater quantities of metabolic waste products and detritus, reduced oxygen concentrations, increased CO2 concentrations and ammonia compounds and increased microbial decomposition, and the consequence is a higher trophic level. These changes are exceptionally negative for the stenovalent trout, and the population decreases even further. Trout were known to inhabit both large lakes and other watercourses within the Plitvice Lakes system, and previously were the dominant fish (Langhoffer 1904; Frani´c 1910). As explained above, the notorious lack of knowledge on trout taxonomy resulted in incorrect nomenclature in the literature, with Salmo labrax referred to as S. trutta in most cases. Furthermore, early publications report the presence of two trout species in the Plitvice Lakes, where the lake form was considered another species or subspecies (Frani´c 1910). The first systematic and comprehensive study was conducted by Karaman (1932) who described how the trout from the Plitvice Lakes system have several morphological and anatomical traits that differ from those in the broader Dinaric region, such as a smaller head, smaller teeth, narrower supramandibular bones, shorter dorsal fin, smaller eyes, more red spots and fewer black spots on the skin, and a larger number of pyloric appendices. These differences led to a description of a new species, called Trutta likana (Karaman, 1932). The locus typicus of this newly described species was the Liˇcka Jesenica River, which lies outside the Plitvice Lakes system, though nearby. Later Karaman (1938) again mentioned the Lika trout as a subspecies Salmo taleri subsp. likana (Karaman 1938), though this quickly fell into oblivion. Trout were also mentioned as the dominant fish species in the Plitvice Lakes in later publications, though these publications were more focused on fisheries (Pažur 1970). Today, trout are again in the limelight, and a recent phylogenetic study (Buj et al. 2020) established that the Plitvice Lakes area in fact contains two evolutionary independent species of the genus Salmo, and they are often found jointly in mixed populations. The species of the genus Salmo have been recorded at 13 localities within Plitvice Lakes National Park, mostly in mixed populations of S. labrax and S. trutta. Though the introduced S. trutta is in direct competition with S. labrax, the Plitvica and Sartuk Streams still contain trout populations consisting only of the S. labrax individuals (Fig. 3). However, these “pure” populations have a small effective population size with a low level of intraspecies and intrapopulation diversity and restricted gene flow, which is likely the consequence of habitat fragmentation and

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Fig. 3 Plitvica Stream—one of the rare habitats of pure populations of the Black Sea trout in Europe, and one of the localities included in the conservation project

degradation. That study gave an assessment of the viability of populations of native S. labrax that clearly showed that this species is threatened with extinction, and the only way to ensure the survival of the native fish community of the Plitvice Lakes is to implement urgent conservation measures.

5.2 Loaches—Mysterious Residents of Kozjak Lake There are few groups of animals in Croatia and Europe so intensively analysed and described in detail as the loaches, small freshwater fishes belonging to the Cobitidae family. Dozens of scientific papers have been published on the loaches of Croatia, and the global scientific and expert literature on the members of this family could fill shelves and shelves in libraries. Even congresses are held solely on the topic of loach biology. Nonetheless, the Plitvice Lakes and loaches have prepared one of the greatest surprises in ichthyology, not only in Croatia but beyond. It has long since been known that loaches inhabit parts of Kozjak Lake (Frani´c 1910) (Fig. 4). This was also reported later by other authors conducting research in the Plitvice Lakes (e.g., Leiner and Pevalek-Kozlina 2005). However, the species of loaches living in Kozjak Lake are completely unexpected for this area. Several years ago, the ichthyologists of the Faculty of Science, University of Zagreb noticed that the loaches caught in the shore regions of Kozjak Lake were

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Fig. 4 Kozjak Lake, the only habitat of the Italian spined loach and Italian golden loach in the Black Sea Basin

not all the same, and that the individuals from shallower parts of the lake differed substantially from those caught in somewhat deeper waters with a leaf litter layer on the substrate (Fig. 5), and certainly were not members of the expected species, Cobitis elongatoides, or of the other loach species distributed in the watercourses of the Black Sea Basin in Croatia, the Balkan spined loach C. elongata Heckel & Kner, 1858. In order to determine the precise taxonomic status of the loaches from Kozjak Lake, their exterior appearance was examined in detail and molecular genetic analyses were conducted (Buj et al. 2021). The results were completely unexpected: it was established that there are two species of loaches inhabiting Kozjak Lake, which was a real surprise. Moreover, these are representatives of two different genera within the Cobitidae family—the genera Cobitis and Sabanejewia, i.e., the Italian spined loach and Italian golden loach. The Italian spined loach is distributed in Italy, Switzerland, France, Slovenia and Croatia. In Croatia, it has been confirmed in only two localities: Kozjak Lake and the Zrmanja River. The Plitvice Lakes area is the only native site of distribution of this species in the Black Sea Basin. In Lika, the known loach species is the Jadova spined loach Cobitis jadovaensis Mustafic and Mrakovcic, 2008, a stenoendemic, critically endangered species that is only found in the Jadova River. As such, the Italian spined loach was not expected to be found in Lika. The second loach species found in the Plitvice Lakes is the Italian golden loach, and Kozjak Lake is the only place this species is found in Croatia. Prior to this interesting find, it was believed that the Italian golden loach is endemic to Italy!

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Fig. 5 Italian golden loach (above) and Italian spined loach (below) on the hand of a researcher immediately upon capture in Kozjak Lake

Loaches are interesting fish that for decades have been attracting the attention of biologists of varying disciplines. They live in slow flowing waters and standing waters, in areas with soft sediments (sand, silt, small gravel), where they burrow, making it difficult to detect them. Their lifestyle keeps them hidden further, as they spend most of the day buried in the sediment, and only become active at dusk. The body shape and colouration are excellently adapted to the benthic habitat and burrowing into the soft substrate. The body is elongated, cylindrical in form, the head is also elongated, and the colouration is pale yellow to grey, with darker spots and markings. The number, shape and position of those markings is often typical for a certain species, enabling the differentiation of genera. Members of the genus Cobitis have markings distributed into four pigmentation zones along the flanks, called Gambetta zones, while members of the genus Sabanejewia have one or two laterally positioned pigmentation zones along the flanks. Both genera have one row of dark stains stretching along the dorsal surface, which occasionally join into larger markings, and the head is covered with tiny dots and stripes. The Italian spined loach can easily be distinguished from other loach species in that its markings in the Gambetta zones are very clear and distinctive, and there are two clear, dark spots at the base of the caudal fin (Buj et al. 2014). On the other hand, the Italian golden loach differs greatly from other members of the genus Sabanejewia in that the basic body colour is grey, and not yellow, and the dorsal side of the body has a large number of small, rounded spots that often join together, with a dark spot on the lower lip and at least one additional series of small spots under the lateral pigmentation zone (Kottelat and Freyhof 2007). The Italian spined loach grows to about 10 cm in length, and it is interesting that the individuals from Kozjak Lake are somewhat larger than those in the Zrmanja River. This is likely due to the favourable habitat conditions for this species in the lake. The Italian golden loach is smaller, with the largest individuals caught in the Plitvice Lakes only about 8 cm long (Buj et al. 2021).

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Representatives of the family Cobitidae are also unusual in their way of life, taking up a very specific niche in the aquatic ecosystem. They feed by sifting the fine sediment, and they are able to separate tiny organic particles from the silt using mucous they produce in the mouth. The mouth is surrounded by three pairs of sensitive whiskers used to search for food. Due to their unique way of feeding, their importance in the freshwater ecosystem is very high, while on the other hand, they are closely tied to habitats containing fine sediments. Though they can actively swim well, they very rarely move away from these suitable habitats. In addition to fine sediment, optimal habitat for loaches are those that have sparse submerged vegetation, since this is necessary for spawning and the development of newly hatched larvae. During spawning, the male wraps his body around the female, surrounding and touching her pelvis. This stimulates the release of eggs and ensures spawning, even though fertilisation is external, as in most fish species. The eggs are laid in deep vegetation so as to protect the fry from predators and other dangers. Adults find protection from predators by burrowing into the sediment, since they are not fast swimmers, and additional protection is given by the suborbital spine on the side of the head, a spiny structure that can be secured in a skin fold when not needed, and erected for protection in the case of danger. Given the exceptionally specific and narrow ecological niche occupied by the loaches, it is very rare to have two different loach species in the same watercourse, particularly in the same habitat, as has been found in the Plitvice Lakes. Interesting, these two species are commonly found together in Italian waters, while in the Zrmanja River we find only the Italian spined loach, while the Italian golden loach is absent. It is possible that their ecological niches and feeding patterns do not overlap completely, as seen in the microhabitats they inhabit in Kozjak Lake. Though it is possible to find individuals of both species in a span of just 10 or fewer centimetres, the Italian spined loach is usually found in the silty sediment in shallower areas, while the Italian golden loach is more often in deeper water under the leaf litter. Though the research to date has cast a bit of light on the life of these mysterious residents of Kozjak Lake, many questions still await answers. The way that these, typically Mediterranean elements, colonised the Plitvice Lakes remains unknown, even though it can be assumed that they entered the waters of the Black Sea Basin from the Italian rivers via Slovenian rivers, and have since remained in the Plitvice Lakes area. The population of the Italian spined loach from the Plitvice Lakes is genetically more similar to the Italian population than to the population in the Zrmanja River, while the Italian golden loach is absent in the Zrmanja River (Buj et al. 2021). Which geological events caused such an unusual distribution of loaches, and which paths were taken in the spread of this species, is still unknown. However, it is certain that the loaches of Kozjak Lake represent an immeasurably valuable component of the biodiversity of the Plitvice Lakes and Croatia. Though their past is not clear, their future should not be allowed to be jeopardized.

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5.3 Minnows—Fish of Unknown Origin and Survival in the Plitvice Lakes Area In the earlier records on the fish of the Lika region, the most common fish, second to the trout, are the minnows, called suribe (loose translation—subfish) by the local people of Lika, who believed that minnows grew up to become trout. They also called a number of other fish genera minnows: the common minnows from the genus Phoxinus and the minnows and daces from the genera Paraphoxinus, Phoxinellus, Telestes and Delminichthys. The first data on the minnow fauna in Lika was given by Steindachner (1866), who listed the species Phoxinellus croaticus as present in the Lika and Novˇcica Rivers near Gospi´c, Otuˇca River near Graˇcac and the Riˇcica River near Štikada. The same species was mentioned by Brusina (1878), also for the Lika River. Brusina (1892) also reported minnows in the waters around Gospi´c, and in Konjsko Lake near Brlog. Langhoffer (1904) reported Common minnow (Leuciscus phoxinus) in the Dretulja, Jesenica, Munjava, Mrežnica and Jaruga Rivers (near Jezerane), and the species Paraphoxinus croaticus in the Lika and Novˇcica Rivers and in the Zelena pe´cina (Green Cave near Buni´c). Directorate of the Croatian Zoological Museum (1908) published a list of fish housed in the museum, listing the Common minnow in Jaruga River (near Jezerana), in the Plitvice Lakes, and in the Korana River (near Sadilovac), Lika River and Kostanjevica River (near Liˇcki Osik). The species Paraphoxinus croaticus was mentioned in the Lika, Novˇcica and Jadova Rivers, and on the Balatin Peninsula. It is interesting that the species Paraphoxinus adspersus was also reported on the Balatin Peninsula, while Paraphoxinus ghetaldii was reported for the Suvaja River (near Mekinjar). The greatest contribution to understanding the minnows of Lika was by Luka Trgovˇcevi´c, who in 1905 completed his doctoral dissertation entitled “Paraphoxinus Blkr. i Telestes Bonap. u vodama Like i Krbave” [Paraphoxinus Blkr. and Telestes Bonap. in the waters of Lika and Krbava]. He stated that Paraphoxinus croaticus was the most common species and present in the largest number of rivers and streams in Lika (around Gospi´c, Graˇcac and in the Krbavsko Polje field). Paraphoxinus adspersus was listed as present in two streams on the Balatin Peninsula near Liˇcki Osik and in Japoga stream near Vrepac, while Paraphoxinus ghetaldii was reported in the Suvaja stream near Mekinjare in the Krbavsko Polje field. In his later publications, Trgovˇcevi´c (1908, 1932) repeated that three minnow species were present in Lika, and that Phoxinus laevis had been found only in the Koreniˇcka stream. Taler (1951a) wrote about the fish that retract underground in the karst region, and made most mention of the Konjsko Lake near Kompolje and the minnows in it, that were the same as those in the Lika River from the genus Paraphoxinus. Today’s understanding of the minnows in Lika, and their taxonomy, is the result of new research on their morphology and molecular genetics. These include four species belonging to the genera Phoxinus and Telestes. The first species is the Italian minow P. lumaireul, while the remaining three species are the daces of the genus Telestes. In the Stajniˇcko Polje field, we find the karst dace T. karsticus Marˇci´c and Mrakovˇci´c, 2011; in the Jadova River (a tributary of the Lika River) and the Riˇcica catchment

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(near Lovinac), the Croatian dace or Croatian pijor T. croaticus (Steindachner, 1866) lives; while the Krbavsko Polje field is inhabited by the Krbava dace, also called the spring pijor T. fontinalis (Karaman, 1972). All three species are endemic to Croatia and more specifically, to the Lika region. However, there are two additional species found in the Lika region that were once considered and called minnows (belonging to the genus Phoxinellus), but which today belong to a new genus Delminichthys. There are two relatively recently described species: Jadova minnow D. jadovensis (Zupanˇciˇc and Bogutskaya, 2002) and Krbava minnow Delminichthys krbavensis (Zupanˇciˇc and Bogutskaya, 2002). As their names suggest, they inhabit the Jadova River and Krbavsko Polje respectively, and these are also very narrowly distributed species that are endemic to Croatia, specifically to Lika. Due to their very small distribution range and lack of data, these endemic species in the genera Telestes and Delminichthys have been included on the IUCN Red List in the category of endangered (EN) and critically endangered (CR), and they are strictly protected by law in the Republic of Croatia. Additionally, six areas of the Natura 2000 ecological network were proclaimed for the purpose of protecting these species. The waters of the Plitvice Lakes system are inhabited by a minnow from the genus Phoxinus. There are currently 21 known species of fish in the genus Phoxinus in Europe and Asia, extending from Spain to China (Fricke et al. 2021). These minnows are small fish that rarely grow to lengths of longer than 10 cm, though specimens are known of up to 15 cm. They achieve sexual maturity between the second and sixth years of life, depending on the elevation and ecological conditions, usually at a length of about 5 cm. The life span is usually about 6 years, though individuals over 10 years have been reported. The body is spindle-shaped with slight lateral flattening, and a large number of tiny scales. The lateral line is often incomplete, and the mouth is terminal or semi-terminal. Flanks bearing irregularly shaped markings that can be joined, forming a midlateral stripe. The upper part of the body is darker brown to green, while the belly is light grey to white. Body colouration varies depending on the reproductive period, particularly in males when colouration becomes more intense (Kottelat and Freyhof 2007; Froese and Pauly 2020). Sexually active males often display breeding tubercles or pearl organs on the head. Minnows and daces inhabit a range of aquatic habitats with colder water (up to 20 °C) that are rich in oxygen, like streams, rivers and lakes, at elevations of over 1000 m. They do not inhabit waters containing a large number of species, and often are found in waters containing only salmonid species (Kottelat and Freyhof 2007; Froese and Pauly 2020). They avoid areas with constant, fast flows, and prefer waters with calmer and slow-flowing waters that can form lake-like areas, particularly during overwintering. The highest population abundance is found in shallow lakes and slow flowing streams. They are very often found in schools in larger numbers, which depends on the quantity of available resources and predatory pressures. The diet is comprised of algae, plant material and different invertebrates such as bivalves, molluscs and insects, and they also feed on trout larvae. Prior to spawning in spring, they often migrate upstream to the appropriate habitats. Spawning is typically on gravel and small stones in shallow, well oxygenated, flowing water or on the shallow shorts of lakes with wave

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action. They lay up to 1000 sticky eggs (Kottelat and Freyhof 2007; Froese and Pauly 2020). In the past, minnows and daces were spread into new habitats in new areas as they were used by anglers as live bait in fishing for trout and other species (Sandlund 2008). They were occasionally incidentally introduced together with trout during stocking activities, after which they were able to spread throughout the lake or watercourse. Even intentional introductions have been recorded, with the aim of providing a food source for trout. However, though they are often prey for trout species, it is known that trout abundance is lower in places where stable minnow populations are present. Morphological analysis and comparisons are not sufficient for the successful taxonomic identification due to the high phenotypic plasticity of certain populations in different habitats and different ecological conditions (Ramler et al. 2017). On the other hand, data on their genetic diversity, intraspecific variability and relationships between populations and lineages are still incomplete. This is also true in Croatia, where the taxonomic status of members of the genus Phoxinus has not yet been fully resolved. This has an effect on recognising the wealth of fish diversity and the application of the necessary conservation measures. Until recently, it was believed that the only native Phoxinus species in the Plitvice Lakes is Common minnow (Phoxinus phoxinus). Newer molecular research, however, has indicated that Common minnow does not inhabit the waters of Croatia at all, and instead this is the Italian minnow (P. lumaireul). However, an interesting question that still needs to be resolved, and which is controversial at best, is the origin of the minnows in the Plitvice Lakes. There are literature data suggesting that the Italian minnow was introduced into the lakes in the late nineteenth century, with a claim that individuals were deposited “in a fishpond at the delta of Proš´cansko Lake” by owner of the local sawmill and restaurant (Frani´c 1910; Bogdanovi´c 1959). Rössler (1929) stated that the minnow was introduced into the Labudovac Lake in the late 1890s. Pauli´c (1923) also mentioned a large quantity of minnows introduced into the lakes as food for trout. Most authors agree that after these introductions, the minnow colonised all the lakes, and that it in fact represents a strong threat for the trout, as it feeds on trout fry and eggs. It is interesting that Würth (1956) mentions the presence of minnow in the Plitvice Lakes, but uses the unusual Latin name Paraphocinus croaticus. Accordingly, it cannot be ascertained whether the Italian minnow is indeed a native species that previously inhabited the Plitvice Lakes, or whether this is an introduced species that has since expanded its range. Therefore, it would be very important to determine the true taxonomic belonging of the populations of these species in the Plitvice Lakes, to determine whether there hides among them a new and undiscovered lineages or species, as was the case with the loaches.

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5.4 Alien Fish Species in the Watercourses of Plitvice Lakes National Park—A Great Threat to the Survival of the Native Species, and Stability of the Aquatic Ecosystems As stated above, there are six fish species that have been introduced into the Plitvice Lakes by humans, either intentionally or accidentally. Unfortunately, these introduced species threaten not only the survival of the native species, but also cause changes to and degradation of the ecosystem. Among the alien species, three are not native at all to Croatia: Brown trout, Rainbow trout and Arctic charr. The remaining alien species have been introduced (translocated) from different watercourses, or from the downstream sections of the Korana River: Common chub, Common rudd and Northern pike (Fig. 6). Brown trout was likely introduced into the Plitvice Lakes multiple times. The only literature report of this pertains to an introduction from the Turkovi´c fish farm near Ogulin, at the time of stocking with Rainbow trout (Taler 1951b). There are no data on stocking of fish originating from other waters or fish farms, however, since Brown trout is not native to Croatia, these fish were likely introduced from another European countries, such as Slovenia or Austria, the most common places of obtaining fish for stocking during the period of the AustroHungarian Empire. Rainbow trout was first introduced into the Plitvice Lakes in 1935, when fry of this species from the Turkovi´c fish farm near Ogulin was released into Kozjak and Galovac Lakes (Planˇci´c 1938; Taler 1951b; Bogdanovi´c 1958). Arctic charr was introduced into Kozjak Lake in 1963 from the Bohinj Lake in Slovenia (Pažur 1970; Bristol et al. 1984), and it succeeded in adapting and forming a sustainable population that thrives here even today. Common chub is a common species in the Korana River, and is recorded in the river downstream of the Plitvice Lakes (Pauli´c 1923; Bogdanovi´c 1961, personal observation). In the Plitvice Lakes themselves, this species was first mentioned as present in the late twentieth century (Leiner 1999), while today it is the most abundant and dominant species (Mrakovˇci´c et al. 2018). It is assumed that it was introduced to the lakes as live bait for trout angling. Common rudd, as a species present in the Plitvice Lakes, was not mentioned in the literature until recently (Leiner and Pevalek-Kozlina 2009). However, today it is the second most common fish in the lakes, after chub (personal observation). It is not known where it was introduced from, or why, but it is believed to have been introduced for the same reasons as chub. Over the past 15 years, Northern pike has also been present in the park, even though there is no knowledge as to how, when or why it was introduced, though this was likely illegal introduction for angling purposes. It is interesting that Pauli´c (1923) listed Pike as a species present in the Korana River, downstream of the last lake. Another interesting fact is the known stocking of the Korana River with Grayling Thymallus thymallus (Linnaeus, 1758), where Bogdanovi´c (1961) mentions the idea of possibly stocking this river with Grayling, while Pažur (1970) stated that an introduction had been carried out but was not successful.

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Fig. 6 Pike at the lake edge

In addition to the successful stocking with alien species, there are also some reports of failed attempts. Planˇci´c (1938) lists two attempts to stock the Plitvice Lakes with whitefishes from the genus Coregonus from Austria. The first was in 1937 with fish originating from the Wörthersee Lake in Austria, though all eggs and fry died. One year later, eggs were obtained from the Attersee Lake, but it is not known whether these whitefish survived in the lakes and for how long. There is also no information as to which whitefish species was used in the stocking, though it can be assumed that the local species from these lakes was used, which belongs to the Coregonus lavaretus complex. According to the available data, there is another, still undescribed species Coregonus sp. Kärnten (https://kis.ktn.gv.at/Flora-Fauna/Fische?ffid=59) present in the Wörthersee Lake, while the Attersee Lake is inhabited by the species Coregonus atterensis Kottelat, 1997 (Kottelat 1997). It is concerning and indicative that two-thirds of the fish species in the waters of this protected area, Croatia’s oldest national park, are alien species, and that they largely dominate the aquatic habitats, both in abundance and in biomass. This is the consequence of unconscientious stocking practices, poor management, misunderstanding of the idea and purpose of a protected area, and degradation of the aquatic habitat and the ecological conditions within. The fact that two new species have been introduced in just the past 20 years is disconcerting, despite strict protection, better education and understanding of stakeholders, improved public awareness, and the conservation efforts invested by the park management. Therefore, it is imperative to continue responsible and science-based management of aquatic systems within the national park in order to protect the native species, populations and lineages of freshwater fish, and to control and where possible eliminate alien and introduced species.

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6 An Investment in the Future—How is Plitvice Lakes National Park Guiding Fish Protection in Croatia? The natural distinctiveness, biological diversity and aesthetic value of the Plitvice Lakes has led to protection of this area at three levels—it is protected under the national legislation as a national park; it is included in the Natura 2000 network, the European network of protected areas; and it is protected at the international level by its inclusion in the UNESCO World Heritage List, as the value of this area surpasses state borders and represents a wealth for all of humanity. Despite the triple protection and exceptional natural wealth, this area, like many others, has not been spared the negative impacts of anthropogenic actions. These negative impacts are evident in various aspects of the sensitive aquatic ecosystems of the Plitvice Lakes, and this is perhaps most pronounced in its native fish community. A series of anthropogenic pressures, the most significant of which are habitat change including raising watercourse barriers, introduction of alien species and eutrophication, have all result in the reduced abundance and stability of the native fish species, and cause alien species to flourish. This balance in the fish community can tip to such an extent that natural salmonid communities are dominated by the alien, cyprinid fish species. The natural fish community of the Plitvice Lakes is not rich in species, though it does contain several distinctive elements, such as the described loach species. Its value instead ensues from its simplicity—the specific habitat conditions and isolation even from other parts of the Korana River catchment, has favoured the development of a salmonid community. In this community dominated by the Black Sea trout as the key species, we find only loaches (Karaman 1932; Taler 1952; Pažur 1970) and perhaps the minnows. In the past, the trout populations were stable, and much denser than today. There are also claims of the presence of two morphological types: one associated with the lake ecosystems and the second found in the flowing waters. The fundamental cause for the pronounced changes in the fish community of the Plitvice Lakes is the introduction of alien species, and today, the lakes are instead dominated by chub and rudd, with other introduced species also abundant. These introduced species inflict a range of negative impacts on the native ichthyofauna— chub feeds on small trout individuals, Rainbow trout and Brown trout inflict strong competition pressure on the Black Sea trout, many introduced species are likely predators of loaches and minnows, and their metabolic processes alter the habitat conditions. The biomass of alien populations in the lakes is much higher than it would be in natural conditions, characteristic for an oligotrophic lake system, and all these fish need oxygen to breathe, and they release carbon dioxide, ammonia and other compounds, thus altering the physico-chemical conditions in the lakes. Furthermore, substantially more oxygen is required for the microbial decomposition of dead biomass than in the natural lake conditions, when the fish populations were significantly less dense. Conditions of reduced oxygen concentrations, and increased concentrations of substances that cause eutrophication are not optimal for the native salmonid community; however, they are well tolerated by these alien species. As a

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consequence, the populations of alien species are flourishing and blooming, thanks to their high reproductive potential, while the native species populations continue to decline. Unfortunately, this is not the only problem for the native species. The reproductive success of trout has also been reduced due to the construction of barriers built on several of the watercourses during the twentieth century, which partially or completely disable trout migration into upstream parts of the watercourse and the suitable spawning grounds. This means that fewer individuals are hatched each year, and the negative effects of fragmentation are also seen in the lack of gene flow that should naturally be present between the different localities within the Plitvice Lakes system. It also reduces the genetic diversity of the populations, and imposes other negative consequence in the populations’ structure. In many situations, even if river continuity were to be re-established and all alien species removed, the structure of many populations would already be so damaged and their viability reduced that they likely would not be able to survive without effective conservation assistance. And as though the problems outlined above are not enough, one of the alien species that has been introduced into the Plitvice Lakes area is the Brown trout (Salmo trutta), a species that is physically so similar to the Black Sea trout, it is virtually impossible to distinguish them by appearance alone. In mixed populations, the Brown trout is a stronger competitor than the Black Sea salmon, and it can often cause genetic pollution of native individuals due to the possibility of their interbreeding. This issue is not exclusive to the Plitvice Lakes area, but is also present throughout Croatia and Europe. On the other hand, there are populations still containing only Black Sea trout individuals that possess the complete genetic material belonging to that species since no genetic pollution has occurred. Such populations are an exceptional rarity in Europe, and their presence is considered a special value that should be conserved. Despite all the threats and negative impacts of human thoughtlessness, this unique area still stands out from many salmonid habitats throughout Europe in several ways. Firstly, though in poor condition, all the elements of the native fish community are still present, including several distinctive species. Secondly, even pure populations of Black Sea trout are still present, making the Plitvice Lakes area exceptionally important for conserving the species as a whole. Thirdly, and most importantly, a small group of enthusiasts entrusted with conserving the wealth of the Plitvice Lakes has launched concrete activities aimed at resolving each of the individual threats observed in the fish communities of this area, with the aim of restoring the natural condition of these unique ecosystems, preventing the extinction of fish species, and ensuring their unhindered evolution. In addition to their great effort, courage and determination, they also have the most modern scientific principles at their disposal, applying scientific findings on the state of the fish populations of the Plitvice Lakes, and the newest conservation methods to achieve the most valuable goal—ensuring the survival of the native biodiversity. In order to turn around the negative trend observed in the native fish species in the Plitvice Lakes and to secure their survival, the park management and ichthyologists from the Faculty of Science, University of Zagreb have decided to apply contemporary principles of conservation biology and conservation measures that have the

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greatest change of improving the population density and viability of the native fish populations. This in particular pertains to the Black Sea trout, as the keystone and indicator species of this ecosystem. In order to determine which measures are needed and in what intensity, an analysis of population viability was conducted. This is a computer simulation to determine the possible outcomes of the population in the next 100 years, based on specific conditions. This analysis indicated that most of the trout populations in the Plitvice Lakes do not have a bright future, even if no new threats should appear, if the current conditions are retained (Buj et al. 2020). On the other hand, the simulation clearly identified the removal of alien species, restoration of river continuity (and consequential increase in the reproductive success of trout), and targeted augmentation of the population to improve its density and genetic diversity as measures that would give the greatest likelihood of leading to population stabilisation and securing their survival. A number of activities are planned, and some are already being implemented. Artificial barrier removal and habitat restoration will restore the continuity of the river courses, and trout will again be able to swim to the optimal spawning grounds in the upstream areas of the Plitvice Lakes system, such as the habitats in the Ljeskovac and Bijela rijeka Streams. These habitats have rocky and gravelly bottoms, and crystal clean water rich in oxygen, which are ideal conditions for trout spawning. By securing the reproductive migration and spawning in optimal habitats, the reproductive success of trout can be expected to increase, and with it, their population densities will improve. Re-establishing river continuity will enable the gene flow between populations necessary to achieve a stable population structure. However, since these populations are currently in poor condition with a low chance of long-term survival, measures are also being implemented to increase population density and improve the genetic diversity, using the principles of conservation genetics. Precisely selected males and females with a specific genetic constitution are spawned in captivity, and their fry, carrying the desired genes, are introduced to a specific population so as to increase the genetic diversity and prevent the negative consequences that arise from excessively low genetic variability, as is currently the case in many populations. With the aim of ensuring the highest possible likelihood of survival of these re-introduced fry, they are released in the appropriate localities in three age categories: just hatched larvae whose survival is secured in incubation boxes, two-month old fry, and one-year old individuals. Though this population restoration programme currently underway in the Plitvice Lakes area includes the most modern knowledge of conservation genetics, and is a unique application of scientific principles in protecting threatened species in Croatia and beyond, the greatest problem remains the removal of alien species. Alien species have a particularly high reproductive potential and are exceptionally strong in utilising the available resources, which is why many attempts to remove them from an ecosystem have failed; they quickly and easily replace all the captured individuals. In order to properly control their populations, and if possible to eradicate them completely, it is necessary to catch them so intensively so as to destroy their structure and disable reproductive success. This is the plan of the Plitvice Lakes National Park

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management. Only by resolving the present threats and restoring the native populations is it possible to secure their stability and viability. These activities are planned and underway so as to preserve the unique biological diversity, and to ensure the stability of the exceptional aquatic ecosystems of the Plitvice Lakes. Conservation activities and programmes aimed at preventing extinction of the native species and ensuring the restoration of natural communities, and with that restoring the structure and function of the natural ecosystem, are very complex. Such programmes are not implemented anywhere else in Croatia or in the broader area, even though their application will certainly be necessary to conserve a number of other fish populations and communities in many places. It is perfectly fitting that Plitvice Lakes National Park, as Croatia’s oldest national park, is the first to apply contemporary conservation methods, and to lead by example in protecting fish populations in Croatia. Acknowledgements We would like to thank Sven Horvati´c, Roman Karlovi´c, Siniša Vajdi´c, Lucija Ivi´c, Lucija Onorato and Milorad Mrakovˇci´c for their help in the fieldwork and assistance. We would also like to thank Linda Zanella for proofreading the manuscript.

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Caves in Plitvice Lakes Kazimir Miculini´c, Tvrtko Dražina, Nikola Marki´c, and Neven Boˇci´c

Abstract In the karst terrains to which the Plitvice Lakes National Park belongs, there are two sides: the face and the reverse. Tectonics, carbonate dissolution, and gravitational processes have created a variety of aboveground and subterranean forms. The underground is full of smaller and larger empty spaces, the majority of which are inaccessible to humans. Caves are natural entrances to the underground that allow us to explore it. For the inhabitants of karst areas, the movement of water from the surface to the subsurface, underground water reservoirs and groundwater levels are important factors in living conditions. Also, caves have been interesting to people since prehistoric times, mostly as places of safe stay. Interest in caves continues to this day, as they represent fascinating places of perpetual darkness, absence of flora and hard-to-see fauna of unusual beauty. Viewed from a biological perspective, subterranean environments form a whole range of habitats that are inhabited by organisms that are associated with them in varying degrees of adaptation. Research, knowledge and monitoring of the underground are carried out in karst areas worldwide, and special attention should be paid to them in protected areas. Keywords Plitvice Lakes National Park · Caves · Speleological cadastre · Biospeleology · Speleogenesis K. Miculini´c (B) · N. Marki´c Public Institution National Park Plitvice Lakes, Josipa Jovi´ca 19, 53231 Plitviˇcka Jezera, Croatia e-mail: [email protected] N. Marki´c e-mail: [email protected] T. Dražina Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia e-mail: [email protected] The Croatian Biospeleological Society, Rooseveltov trg 6, 10000 Zagreb, Croatia N. Boˇci´c Department of Geography, Faculty of Science, University of Zagreb, Maruli´cev trg 19, 10000 Zagreb, Croatia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. Miliša and M. Ivkovi´c (eds.), Plitvice Lakes, Springer Water, https://doi.org/10.1007/978-3-031-20378-7_13

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1 Introduction Plitvice Lakes National Park is located in the Dinarides, which are a world-renowned karst area from a geomorphological and biospeleological point of view. The science of karst, karstology, has its origin in the Dinarides area. External Dinarides are the largest karst area in Europe, covering about 60,000 km2 (Mihevc and Prelovšek 2010). They are built mostly of Mesozoic and Paleogene well karstified carbonate rocks. Karstification of Dinarides was influenced by Pleistocene glaciation. Karstification and groundwater circulation was also influenced by the rise of the sea level in the Upper Pleistocene and Holocene by about 135 m. Dinaric karst is characterized by a number of diverse surface karst forms including dolines, uvalas, karst poljes, blind valleys and plateaus (Mihevc 2010; Boˇci´c 2019). Speleological exploration of the Dinarides has a long tradition and has been systematically carried out in almost the entire Dinarides area. Through a long tradition, tens of thousands of caves have been explored. The longest of them is the Crnopac Cave System, 53.7 km long, and the deepest is the Lukina jama—Trojama cave system, 1431 m deep. Both are located on Velebit Mt. in Croatia. Although speleological research of Croatian karst is intensive and has a long tradition, the area of Plitvice Lakes National Park is speleologically relatively poorly explored. The branch of biology related to the underground—biospeleology, developed comparatively with the development of speleological techniques and equipment. The cradle of biospeleology, as well as the first described cave animal, comes from the Dinarides: world famous amphibian Proteus anguinus Laurenti, 1768 and beetle Leptodirus hochenwartii Schmitd, 1832. Furthermore, the Dinarides are word hotspot of subterranean faunal diversity (Culver and Pipan 2009). Deharveng et al. (2012) compared 7 large karst areas of Europe and North America, and the Dinaric karst is by far the richest in the obligate subterranean fauna, when comparing areas of approximately the same size. Biodiversity of subterranean fauna (both aquatic and terrestrial) are not evenly distributed: two hotspots can be indentified, northwestern and southwestern part of Dinarides (Zagmajster et al. 2008, 2010; Bregovi´c and Zagmajster 2016). This biodiversity pattern cannot be simply explained. Complex biological and geological history of the Dinaric mountains, the large amount of available subterranean habitat, together with possible invasion of enhanced fauna during interglacials of the Pleistocene are probably most important drivers of biodiversity in this region (Deharveng et al. 2012). So far, more than 500 animals have been described from more than 300 different caves and pits in Croatia (Jalži´c et al. 2010, 2013). These speleological objects, called type localities, are extremely important because new animal taxa were originally found and described according to the samples found in these habitats. Importance of type localities is not only taxonomical, but also, they are important in evolutionary and conservation biology. Thus, protection and monitoring of these objects should be one of the main goals at the state level, as they possess unique and, in most cases, endemic fauna.

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Unexpectedly, but because of the Park’s status as a protected area, speleological and biospeleological research has been insufficient, which has changed significantly in recent years. Unfortunately, all the information presented in this text is based on old data and the first data from systematic speleological and biospeleological research in progress, as well as other recent cave-related research. Much better and more comprehensive knowledge of the Park’s underground is expected in 3–4 years.

2 Cave Research The caves in the area of the present National Park were known only to the local population until the beauty of the lake zone of the Park attracted a larger number of people. The increased interest in spending time in nature in the second half of the nineteenth century, led to initiatives for the tourist arrangement of paths and bridges along lakes and watercourses (Markovi´c Vukadin and Franjkovi´c 2019). The construction of paths in otherwise difficult to access areas allowed naturalists of that time to access and describe caves, which also served to interpret the incision of the Korana riverbed and the formation of the lakes. In the book Plitvice Lakes Frani´c (1910) describes 12 caves, only two of which are not located in the canyon of watercourses and lakes. Almost the same caves, but much more detailed, were described by Poljak (1914). In his work he gives descriptions of the interior, speleothems, sediments, archeological and paleontological finds, and analyzes speleogenesis. He also gives maps of seven most interesting caves, including Golubnjaˇca (Fig. 1), located at the very edge of the Korana River kanyon and Rodi´ca pe´cina (Fig. 2), outside of the lake zone, near Rodi´ca poljane. More intensive speleological research in the area of the then already proclaimed National Park begins in the 1950s (Redenšek 1953; Marjanac 1957), when speleological techniques also made it possible to descend into the deepest cave in the ˇ Park—Cudinka. In 1957 the measured depth was 203 m (Marjanac 1957), but latter 196 m were mapped (Kuhta 1983). Although occasional research has been conducted to date, the area of National Park has been one of the less speleologically explored parts of Croatia. In 2007, there were 101 known caves in the Park (Šiki´c 2007), but most of the caves were not mapped or the maps were not of sufficient quality. Coordinates were also not accurate. An example of the insufficient research is the number of caves from the Park recorded in the Speleological cadastre of Croatia. In the cadastre, only caves with sufficient quality of the map, coordinates, photos and the obligatory set of 32 information about the cave can be entered. Out of the total 3401 caves in the Speleological cadastre of Croatia, only 10 are from the Park (Cave Cadaster of the Republic of Croatia 2021). In order to improve the knowledge about the caves of the Park, several projects of the Public Institution National Park Plitvice Lakes were started from 2019. Since the Park has LiDAR image of the entire area with a buffer zone of 1 km around it, it was decided to perform remote detection of cave entrances based on a relief model.

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Fig. 1 Speleological map of Cave Golubnjaˇca

A total of 347 potential cave entrances were detected, of which 233 were located in the Park. To assist in locating the cave entrances, the speleologist was provided with coordinates as well as a point cloud visualization of each potential cave entrance with its surroundings (Fig. 3). Field verification of cave entrances, which is ongoing, has so far yielded 67% accuracy in locating the cave using this method (Grozi´c et al. 2019). Potential cave entrances based on LiDAR data were only a digital aid for systematic speleological work. In Croatia, as in most countries, speleological explorations are carried out by speleological associations. Because of the large area, the Park invited all Croatian speleologists to work here, and 11 speleological clubs signed a contract with the Park. In the initial phase, about one third of the Park’s territory (102 km2 ) was devided to the speleological clubs for reconnaissance and research. In less than a year 151 cave maps (Fig. 4), together with all other requested information and photographs (Fig. 5), have been delivered to the Park, and about seventy more are in the process of

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Fig. 2 Speleological map of Cave Rodi´ca pe´cina

being delivered, in only one-third of the Park territory. Of this number, 73 caves were found using LiDAR data, and 78 caves were found using traditional field surveys. By analyzing LiDAR data, it is possible to find only vertical entrances to caves, and the above ratio is an indicator of very good terrain reconnaissance. Even at this initial stage, speleological data provided sufficient information for two biological projects in subterranean environments—biospeleological explorations and environmental DNA (eDNA), both of which have just begun. Preliminary results of eDNA show that the composition and relative abundance of Metazoan groups are quite different in groundwater of spings and in the cave. The most abundant group are arthropods with more than 80% abundance in the cave and more than 60% in the spring. Following arthropods, the most abundant groups in the cave are annelids, cnidarians, sponges and rotifers. There is just one species of annelids, cnidarians and sponges known from caves in the world, and all are from the Dinaric karst. Whether the eDNA signals are coming from those known species or some yet unknown remains to be determined (Bilandžija 2021, personal communication). Biospeleological exploration are about to have their first field work. One of the main goals, except from the inventory of invertebrate cave animals is photographing and filming the underground fauna, which is poorly known to the general public, and otherwise impossible to be seen.

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Fig. 3 Poind cloud of the cave with surrounding 30 m. Image by D. Grozi´c

Monitoring of the bats has been carried out for five years in four caves and one building (Kipson et al. 2020). From that monitoring, along with some previous data, 17 bat species are known in the Park. Archeological and paleontological excavations are also activities that have been carried out in the caves. During selection of the most suitable caves for excavations, the value of the obtained data for the reconstruction of speleogenesis and the formation of the surrounding terrain was also considered. The results of the excavations in the caves Mraˇcna špilja (Fig. 6) and Velika pe´cina showed traces of the oldest human inhabitants in the wider mountain area of Croatia, dated to the Epipalaeolithic or Mesolithic (Tresi´c Paviˇci´c et al. 2020). Due to locations of the caves in the bottom and near the top of the Korana River canyon, stratigraphy of excavated sediment, in depth of 2.5 in cave Mraˇcna špilja and 3.4 m in cave Velika pe´cina (Fig. 7), gave valuable information on genesis of the lake and river zone.

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Fig. 4 Map of tufa cave Kostelˇceva špilja. Map by H. Cvitanovi´c

Near the Park, less than 15 km from the borders, is a longest partialy submerged cave in Croatia: Panjkov ponor—Vari´cakova špilja, 12,922 m long (DDISKF 2010), as wel as caves Ponorac (2834 m) and Adios (1969 m), also with water courses (Cave Cadaster of the Republic of Croatia 2021). Therefore, at least some cave with a water course or a larger body of water was expected in the Park area. Till now, in the search for the water in caves, the speleological survey did not help, as none of the researched caves has even a permanent pond or watercourse. The potential for such a caves was found in the depths of the lakes. After almost 70 years (Petrik 1958), new

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Fig. 5 Cave Turˇci´c jama. Photo by D. Stopi´c

bathymetry of the lakes conducted in 2019 found oval depresions in several lakes (Fig. 8) (Kordi´c 2019). These depressions were thought to be active or former underwater springs (vrulje) or sinkholes (ponor), which could be either in form of passages or diffuse type of spring/sinkhole. The digital resolution was not sufficient to identify possible cave passages, so diving explorations were carried out. Of 11 depressions, only one was determined to be a spring with 4 °C lower temperature than lake water. It is diffuse type of the spring, which mean that the water is comming through the sediment into a lake. Seven depressions showed no visible passages, water currents or difference in water temperature. Nevertheless, interesting shapes were found in three depressions in Lake Kozjak (Fig. 9).

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Fig. 6 Entrance of cave Mraˇcna špilja with the Korana River in front. Photo by H. Cvitanovi´c

Among 11 dolines explored by diving, 5 are located in the area where limestone, and 6 are in the area where dolomite is found as a geological base. Although there are an extremely large number of dolines in the Park, all depressions in the lakes cannot be defined as dolines due to bedrock below them. The assumption that these were submerged springs or sinkholes proved to be correct in only one case, although it is possible that they lost their function due to their closure by watertight deposits. In three cases of depressions with cave-like passages, the bottom of the lake is remarkably reminiscent of a doline within which a cave continues, except that everything is in unconsolidate clay-like sediment. These three could be dolines since they are situated in limestones or on the fault between dolomites and limestones. The sediment that surround cave-like passages is relatively solid, but when it rises to the surface, it disintegrates due to pressure changes (Jalži´c et al. 2019). Sampling of sediment cores from the bottom of Kozjak showed that the minimum thickness of sediment is 12 m, which was the limit of the drilling rig (Srdoˇc et al. 1986). Openings in unconsolidated sediment that resamble small cave entrances remains unexplained. New research is planned for this summer when more such forms will be sought. The diver’s entry into the passages is too dangerous, so the caves will be remotely explored with a guided camera. Climatic changes have also affected the undrground of the Park, which is visible in some caves that used to have a significant amount of snow and ice. In order to better monitor climate change and its effects, underground microclimatic measurements are being conducted. In order to establish a network of measuring stations in caves, measuring devices (data loggers) were placed in 7 caves to collect data

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Fig. 7 Sediment profile in Cave Velika pe´cina

on microclimatic parameters and basic physico-chemical properties. The monitored indicators are: temperature, relative humidity and air flow, dew point temperature, concentrations of CO2 and radon in the air, and temperature and depth of water in caves where it is present. The data collected will provide valuable information about the underground habitat and general ecological conditions (Buzjak 2020).

3 Geology and Caves For now, there is no complete data on the caves of the National Park because research is underway. There are currently two data sets. One is LiDAR scanning data. According to them, 347 potential caves were detected in the wider area, and 233 within the boundaries of the Park itself. Field validation of these data showed their 67% reliability for now. A comparison of these data with a geological map (Polšak et al. 1976; Veli´c et al. 1970) (Fig. 10) indicates that, although the entire

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Fig. 8 LiDAR image and bathymethry of the Lake Kozjak with visible oval depressions

Fig. 9 Diver sketches of the three depressions (K 10, K 11 and K 12) with cave-like passages. Sketches by B. Jalži´c

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Park is built of carbonate rocks, differences in the lithology significantly affect the appearance of caves. Most of them are in Upper Cretaceous limestone (Senonian) with rudists and Lower Cretaceous limestone. There are slightly fewer of them in the Upper Cretaceous limestone and dolomites (Cenomanian–Turonian) and in the Lower and Middle Jurassic limestone and dolomite. They are very rare in the Upper Jurassic dolomites and limestone, and only one has been recorded in the Upper Triassic dolomites. The second data set is a register of explored caves in the Park area. The difference between these data and those obtained by interpreting the LiDAR points cloud is multiple. Due to the large angle of the laser beams, LiDAR makes it easier to detect pits than horizontal caves. The LiDAR data set cover the entire area of the Park, and speleological exploration has been conducted where the density of potential entrances is highest. In the speleologically researched areas, caves from the LiDAR data set were explored, and many caves were found directly in the field. The advantage of this data set is that it provides accurate and reliable data for the explored areas, but the disadvantage is that there is very little data for other zones. However, few caves were known in that areas before, LiDAR scanning detected much fewer potential entrances, and a good part of the terrain is geomorphologically unfavourable for cave development (it has a developed surface drainage network). For these reasons, we believe that the given set of explored caves sufficiently gives a representative picture of the caves characteristics in the Park. Of the total of 151 caves investigated in the Park area, almost 70% are pits (vertical caves), and the rest are (horizontal) caves (Fig. 11). This is a consequence of the relatively large altitude range of the terrain within the Park and the relatively low position of the groundwater level and the local erosion base. In such conditions, the vadose circulation of groundwater prevails, which are good conditions for the development of pits. This is especially noticeable in the higher terrains, and there are ˇ well-known pits deeper than 100 m in the Park area. The deepest is Cudinka (196 m), followed by the Jama na Vršˇci´cu (159 m) and Turˇci´c jama 107 m deep. Horizontal caves are mostly related to the Lower Lakes and the Korana River canyon. They developed along faults lines and bedding planes, and most likely they had the function of conduits of groundwater towards the Korana River. The longest are Mraˇcna pe´cina (173 m), Golubnjaˇca (145 m), Šupljara (116 m) and Špilja vile Jezerkinje (104 m). In the higher areas, the caves are smaller and most likely fragments of denuded cave conduits. The exception is the Jama na Vršˇci´cu, which is the longest cave in the Park (330 m), and also the second deepest. It is located at an altitude of 752 m, and consists of a network of mostly vertical channels formed in the vadose zone. Data on the explored caves were also compared with data on geological settings of the area (geological data according to Polšak et al. 1976; Veli´c et al. 1970) (Fig. 12). The impact of differences in the lithology is significant and further confirms the data obtained by the LiDAR scan. Over 76% of the explored caves were found in Upper Cretaceous (Senonian) limestone formed by 97–100% of calcite (Polšak et al. 1978). A large proportion of caves are also found in Lower Cretaceous limestone built of 90–99% of calcite (Polšak et al. 1978). In the area built of Triassic dolomites, only one cave is known for now. Significant is the impact of faults because even 25 caves are located at a distance of up to 150 m of the known faults.

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Fig. 10 Geological map of National Park with locations of potential cave entrances detected by airborne LiDAR scanning data

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Fig. 11 Percentage ratio of horizontal caves (caves) and vertical caves (pits) in total number of the speleological phenomena

Fig. 12 Number of caves in litostraigraphic units 1-Upper Cretaceous (Senonian, K2 3 ): limestones, 2-Upper Cretaceous (Cenomanian–Turonian, K2 1+2 ): limestones, 3-Lower Cretaceous (K1 1+2 ): limestones and dolomites, 4-contact zone beetwen 3 and 5, 5-Upper Jurassic (J3 2,3 ): limestones and dolomites, 6-Upper Triassic (T3 ): dolomites

4 Cave Types and Characteristics Three types of geologically classified caves can be distinguished in the Park, which is quite unique compared to other areas in Dinarides. The first type, which is the most common in the world, is carbonate caves. Thay are secondary caves formed by solution of carbonates after diagenesis. Second type is tufa caves. Those caves are primary caves, formed simultaneously with tufa (Bögli 1980). The third type is caves in unconsolidated sediment at the bottom of the Lake Kozjak. Since is not possible to put this cave in any classification, it is worth to describe it.

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Fig. 13 Lengths of the caves in the Park

4.1 Carbonate Caves According to the data of the cave cadastre and geological map (Polšak et al. 1976, 1978; Veli´c et al. 1970), vaste majority are the limestone caves which were formed in Jurassic and Cretaceous limestones. Only one cave, located on a fault between dolomites and limestones, has its entrance in dolomites. According to speleologists data so far, the caves in the Park area are usually small in size. This is supported by the percentage of smallest caves, 5–10 m long/deep, such as 27.2% in the Park (Fig. 13), while in the Speleological cadastre of the Republic of Croatia there are 15.3% of caves of these dimensions.

4.2 Tufa Caves This type of caves is deveolped inside of the tufa barriers. Unlike carbonate caves, which are the product of carbonate dissolving and collapsing, tufa caves are formed by the growth of the barrier (Fig. 14). They form in the zone of waterfalls and cascades, within tufa barriers. Cave formation begins at the edge of the waterfall. The water at the edge deposits tufa outward and downward, forming curtain-like shapes. Lowering of the tufa curtain to the ground closes the space within the tufa barrier and forms a tufa cave (Pentecost 2005). For example, below the 200 m wide outlet barrier of Lake Proš´ce there are numerous cavities, caverns and caves formed by the growth of the tufa, called the Špiljski park (Cave Park). Unlike ordinary caves, tufa caves are very rare and found in only a few locations in the world. Until now, only two tufa caves are mapped, Kostelˇceva špilja (Fig. 4) and Velika pe´cina.

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Fig. 14 Inside of the tufa cave in Špiljski park. Photo by A. Bionda

4.3 Unconsolitade Sediment Caves Despite the fact that this type of cave could not be found in the literature, it is worth describing, as a rare and still unexplored phenomenon. It is interesting that all three underwater depressions have their caves starting at very similar depth. Two have entrances are at 28 m and one on 26 m deep. Depth of passages is unknown due to low visibility coused by darkness and dissolved particles. Passages were visible from 4 to 5 m. Dimensions of entrances are 1.5 × 0.7 (K 10); 1.5 × 1.5 (K 11) and 1.2 × 0.6 (K 12) meters (Fig. 15). It is possible that passage in unconsolidated sediment is continuing with passage in limestone. Further exploring might give more information of their length, possible function and genesis.

5 Biospeleology and Subterranean Environments As in the case of speleological research, biospeleological research in this area was conducted occasionally, trough whole 20th century. Fist species described from cave in Plitvice Lakes National Park was arachnid from the order Pseudoscorpiones, Neobisium speluncarium (Beier 1928), found in Šupljara cave (Beier 1928). The species is describes based on subadult specimen, and never been confirmed since. New data and modern molecular methodology are needed to clarify and confirm taxonomic status of this Park stenoendemic species (Ozimec et al. 2009). Despite modest research effort, there are four type localities (i.e. caves and springs from which specimens were collected and described) in the Park area, with five subterranean taxa

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Fig. 15 Entrance to the cave-like passage K 10 in Lake Kozjak. Photo by B. Jalži´c

(Beier 1928; Karaman 1962; Strasser 1966; Pretner 1970; Hlavaˇc and Lakota 2004; Jalži´c et al. 2010, 2013): Šupljara cave (with above mentioned N. speluncarium), Izvor in Glibovita draga spring (with amphipod species Niphargus rucneri Karaman ˇ 1962), Ledenica in Cudina uvala cave (with two described taxa, beetle Astagobius angustatus driolii Pretner 1970 and millipede Attemsia likana Strasser 1966) and Rodi´ca Cave (with beetle Machaerites udrzali Hlavaˇc and Lakota 2004). The latter species (Fig. 16) is most interesting faunistically data from this area. It is described in 2004, indicating high possibility for finding new endemic subterranean taxa for the Park or wider area of Croatia. Kuhari´c et al. (2021) gave a short overview of previous speleological and biospeleological research and listed a total of 19 troglobionts (animals living exclusively in subterranean habitats, where they spend their entire lives). Most diverse taxonomical group are troglobitic beetles, with 8 confirmed taxa: Anophthalmus scopolii F.J. Schmidt, 1850, A. a. driolii, Bathysciomorphus likanensis likanensis (Reitter, 1890), Duvalius kodrici (Scheibel, 1938), M. udrzali, Parapropus sericeus (Schmidt, 1852), Redensekia likana Karaman 1953 and Typhlotrechus bilimekii (Sturm, 1847). In relation to the surrounding karst areas, certain specifics related to the underground environment can be discussed. The caves of the Park are mostly small, and there is no water in them. The relatively small length or depth of the caves is probably the main factor affecting the underground environment. Shallow caves with simple morphology in the Park are exposed to daylight, and the floor is filled with organic material from the surface. Slight temperature differences are present, but from a biospeleological standpoint these habitats are under great surface influence, hence most of fauna are troglophiles (species able to complete their life cycles within a cave, but also occur in ecologically suitable habitats outside caves) or trogloxenes

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Fig. 16 Machaerites udrzali Hlavaˇc and Lakota (2004) (Coleoptera; Staphylinidae; Pselaphinae), small beetle (approximately 1.7 mm long), endemic to the Plitvice Lake National Park. Photo by B. Jalži´c

(organisms introduced into caves accidently or entering in search of a mild climate; they may survive in subterranean habitant only temporarily). In 2021, a multi-year project of inventory and detailed biospeleological research of caves and pits of the Plitvice Lakes National Park began, and results of this project will confirm or reject stated hypothesis of such subterranean faunal composition in small caves. ˇ In the Park area there are also several deeper caves: Cudinka (196 m deep), - cestama (75 m deep), Golubnjaˇca (near Jama na Vršˇci´cu (159 m deep), Jama medu village Homoljac, 63 m deep) and Opaljena jama pod Opaljenim vrškom (55 deep), among others. These and other deeper and larger caves possess stable microclimate and diverse microhabitats (such as damp sediments and walls, decomposing organic material, guano deposits, pools and cave streams). Thus, they are important from biological view, as they can sustain a diverse community of the obligate subterranean fauna.

6 History of the Show Caves During the development of infrastructure for visitors in the 1920s and 1930s, caves were seen as an opportunity for an additional tourist attraction. The proximity to the main attractions allowed easy access from the pathways. In two caves that have two entrances, the path even passes through them. Kaludjerova špilja cave, which is a natural tunnel, becomes part of the path leading down to the foot of the Veliki slap (Great Waterfall) in 1922 (Park’s archive). On the opposite side of the canyon, the Šupljara Cave, with one entrance at the top and the other at the bottom of the canyon, became a natural entrance to the Lower Lakes when a staircase was built in 1927 (Anonymus 1928).

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Downstream, in the canyon of the Korana River, is Golubnjaˇca, a cave with an impressive entrance hall with two large openings, one of which is 41.4 m high (Fig. 17). Considering the fact that there is daylight in this hall and there are no speleothems, except a concrete path that was made through that hall, in 1931 a 102 m long cave passage was arranged and illuminated for tourism. Although such a way of arranging the cave would be totally inappropriate nowadays, the esthetic impression, durability and technical solutions applied are amazing. The path and stairs are made of concrete, as is the bridge, which provides a one-way pathway through the cave passage (Fig. 18). In the same year, the Špilja vile Jezerkinje cave on the opposite bank of the Korana River was arranged for tourism and opened (Anonymus 1931). The Korana River enters the entrance passage of the cave in the length of about 20 m, and the water depth is more than 1 m, and therefore the cave was visited only by boat (Fig. 19).

Fig. 17 Huge entrance to the Cave Golubnjaˇca. Photo by R. Jug

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Fig. 18 Postcard from 30s of Cave Golubnjaˇca

The cave is also electrified, and in one part of the cave a concrete path has been built. One daily newspaper described the cave’s turistic arrangement as the culmination of bad taste due to the bars at the entrance, red light inside of the cave and gasoline smell from generator (Park’s archive). The Špiljski park (Cave Park), which consists of several caves formed in tufa on the Upper Lakes, has also been developed for tourism. The caves are located at the downstream end of the tufa barrier of Lake Proš´ce, and in order to enable their viewing, a path was built along the barrier (Fig. 20). In order to prevent possible destruction of tufa and speleothems, a metal door was placed at the entrance of the largest cave (Krajaˇc 1933). Daylight illuminates the space fairly well, so no lighting is installed. Below the Labudovaˇcki slap Waterfall is the Janeˇcekova špilja cave, which is more than 50 m long. This tufa cave has two halls and two corridors, as

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Fig. 19 Entance to the cave Špilja vile Jezerkinje. Photo by K. Miculini´c

well as a small 12 m2 lake that is 0.40 m deep. The tufa caves are hidden behind the curtains of the tufa barrier, and their established age is from 300 to 1600 years (Srdoˇc et al. 1982). Another cave created in tufa was arranged to receive visitors. It is the cave Kostelˇceva špilja, located inside the barrier of the Veliki slap Waterfall. In places where it was necessary, stone steps were made on the way to the cave. A lintel was placed at the entrance, as it was probably thought that the tufa at the entrance was not stable enough. No lighting was installed, and the interior of the cave was left untouched, with the exception of the installation of a mysterious 1.5-m bridge that crosses a small pond and leads to the cave wall (Fig. 21). It is possible that this bridge to nowhere, nowadays completely covered in flowstone, was built later, during the filming of the famous movie Winnetou (Apache gold) in 1963. In total, if the Cave Park is counted as one, six caves have been arranged for turists in approximately 5 years. The degree of intervention varied from minimal to the installation of a concrete path, stairs and a bridge, as well as the lighting of the entire passage in Golubnjaˇca. What can be said about these cave arrangements? What is their epilog? Have they stood the test of time? Only two of the six caves are still open to visitors, and those are the ones through which the paths lead. The paths through these two caves are also lit by daylight. The other caves are closed to visitors and are not intended to ever be placed in the visitor

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Fig. 20 Stairs that lead to the cave entrance in Špiljski park. Photo by A. Brozinˇcevi´c

system again. In the 1920s and 1930s, when the caves were arranged, there were a maximum of 20,000 visitors per year, and at that time no admission was charged to enter the area. Nowadays, with nearly 1,900,000 tickets sold, some activities are simply not possible. In a way the romantic days when people wanted to see live what the underworld looks like because they couldn’t do otherwise, are over. Gone are the days when only the wealthier could afford such trips, or those who were driven by a strong desire for knowledge or adventure. There came a time when many traveled, so without changing their size, some of the places became too small. As a result, the four former tourist caves are not only closed to the public, but the paths no longer lead to them. Instead of visitors coming to the cave by boat, beavers have found their safe home in the cave Špilja vile Jezerkinje (Fig. 22).

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Fig. 21 Bridge to nowhere in Kostelˇceva špilja. Photo by K. Miculini´c

7 Potential Threats Caves are the windows in the underground through which we can descend and see ˇ the inside of the karst. Apart from us, our garbage can also descend. Cisto podzemlje (Clean Underground) is an initiative of Croatian speleologists in which data about polluted caves are collected and cleaning actions are organized and carried out. According to their data, there are 4 polluted caves in the Park (Clean Underground 2021). Recent speleological surveys discovered 21 more caves, mostly with small amounts of garbage. A 2020 study (Novak et al. 2019) estimated the amount of waste in the 4 caves detected by Clean Underground, and in 2021 the first cave was cleaned. 24 cubic meters of waste were removed from the Jama kod uš´ca Plitvice

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Fig. 22 Beaver lodge in cave Špilja vile Jezerkinja. Photo by H. Cvitanovi´c

cave (Kovaˇc-Konrad and Jirkal 2021). In addition, 5 other caves with small amounts of garbage were cleaned. One of the most beautifle cave who share its name with above mentioned—Golubnjaˇca is also partialy polluted (Fig. 23). Fortunately, throwing garbage into the caves is a relic from the time without municipal garbage collection. Nowadays it is rare, so it is expected that the caves in the Park area will be cleaned in 5 years. Another type of threat is liquid phase pollution. Industrial activity is prohibited in the Park, and there is none in the wider area. Intensive agriculture and livestock breeding is prohibited, while small households with crops and livestock do not pose a significant threat. As a precautionary measure for possible pollution, truck driving is also prohibited, and these types of vehicles use detours around the Park. Finally, a possible threat are the waste waters. Local population is relatively small, but the number of visitors, especially during the high season, is high. Sewage treatment is required for tenants of accommodation that are not connected to the sewerage system, and on the main sewerage system a new treatment plant was installed. So far there are no data on water quality in caves, but within the framework of biospeleological research it is planned to carry out water analyses. The Conservation Service of the Plitvice Lakes National Park conducts long-term monitoring of the quality of surface water, once a month at 17 sites in the Park area (Fig. 24).

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Fig. 23 Speleothems in cave Golubnjaˇca. Photo by M. Kuhta

Bacteriological analyses have identified several sites near settlements where a slight increase in the number of bacteria, indicators of organic pollution (Brozinˇcevi´c and Vurnek 2011), occurs seasonally during the summer months. This increase is probably due to the still not completely regulated system of wastewater collection and disposal in the settlements. Such kind of pollution in greater extant may have a negative impact on groundwater, water in caves and the underground environment in general. The obtained data on physical–chemical factors show that the waters are of high quality, without significant anthropogenic pollution.

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Fig. 24 Monitoring of the surface water quality. Photo by A. Brozinˇcevi´c

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