Tracing Environmental Change and Human Impact as Recorded in Sediments from Coastal Areas of the Northwestern Baltic Proper 9189109295, 9789189109292

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Tis thesis presents diatom and geochemistry stratigraphies from fve sediment records along the southeast coast of Sweden, northwestern Baltic Proper. Tese records cover time periods of 500 years to more than 2000 years. Te diatom stratigraphies and geochemical proxies allow for reconstruction of environmental histories at these sites. Te outcomes of this thesis highlight the importance of a longer time perspective than the environmental monitoring can provide. Lena Norbäck Ivarsson carries out research within the feld of environmental science with a focus on understanding past and present ecosystem responses to environmental changes. She holds a M.Sc. in biology from Stockholm University and this is her PhD thesis.

ISBN 978-91-89109-29-2 (print) / 978-91-89109-30-8 (digital) | Södertörn University | [email protected]

LENA NORBÄCK IVARSSON

Environmental Science, Environmental Studies, School of Natural Sciences, Technology and Environmental Studies & the Baltic and East European Graduate School, Södertörn University.

Tracing environmental change and human impact as recorded in sediments from coastal areas of the northwestern Baltic Proper

Te eutrophication of the Baltic Sea due to increased anthropogenic nutrient loads during the 20th century is well documented and studied. However, in the Baltic Sea drainage area, humans have afected the environment longer than the environmental monitoring can provide data for. Sediment records from lakes and seas can provide fundamental data on the environmental conditions before ecosystems were impacted by humans and give the range of natural variation.

SDD 178

Tracing environmental change and human impact as recorded in sediments from coastal areas of the northwestern Baltic Proper Lena Norbäck Ivarsson SÖDERTÖRN DOCTORAL DISSERTATIONS

Tracing environmental change and human impact as recorded in sediments from coastal areas of the northwestern Baltic Proper Lena Norbäck Ivarsson

Södertörns högskola

Subject: Environmental Science Research Area: Environmental Studies School of Natural Sciences, Technology and Environmental Studies & the Baltic and East European Graduate School

Södertörns högskola (Södertörn University) The Library SE-141 89 Huddinge

www.sh.se/publications © Lena Norbäck Ivarsson Cover image: Lena Norbäck Ivarsson, Epiphytic diatoms growing on Cladophora sp. Cover layout: Jonathan Robson Graphic form: Per Lindblom & Jonathan Robson Printed by Elanders, Stockholm 2020 Södertörn Doctoral Dissertations 178 ISSN 1652–7399 ISBN 978-91-89109-29-2 (print) ISBN 978-91-89109-30-8 (digital)

Till Berta, Aina och Birgitta

Abstract The eutrophication of the Baltic Sea due to increased anthropogenic nutrient loads during the 20th century is well documented and studied. However, in the Baltic Sea drainage area, humans have affected the environment longer than the environmental monitoring can provide data for. Sediment records from lakes and seas can provide fundamental data on the environmental conditions before ecosystems were impacted by humans and give the range of natural variation. This thesis presents diatom and geochemistry stratigraphies from five sediment records along the southeast coast of Sweden, northwestern Baltic Proper. These records cover time periods of 500 years to more than 2,000 years. The diatom stratigraphies and geochemical proxies allow for reconstruction of environmental histories at these sites. Overall, the results show that the environmental changes that have occurred in the coastal zone in recent centuries are unprecedented over the last two millennia. The records from the coastal zone show only minor variations in the diatom stratigraphies and nitrogen stable isotope signals through history until recent centuries. The results show no evidence of increased runoff of nutrients from land during medieval times. Temperature anomalies since 500 CE have had little or no significant effect on the diatom assemblages from the coastal sites, while increased nutrient input from land has had a significant effect. Anthropogenic nutrient runoff has affected the diatom assemblages most markedly during the 20th century. The results show a time lag of the onset of eutrophication of approximately 100 years between the coast and open Baltic Sea, highlighting how the coastal zone acts as a buffer for the open Baltic Sea. The timing for the onset of eutrophication in these coastal areas is site-specific. For several sites, reference conditions prevailed more than 200 years ago. Water transparency at this time allowed for extensive distribution of benthic diatom habitats, such as macrophytes. The years of maximum nutrient load to the Baltic Sea during the 1970s–1980s is recorded in the diatom stratigraphies, especially with regard to the concentration of diatom valves in the sediments. There has been a recovery in diatom absolute abundance since maximum pollution years. However, there is no indication of a recovery in diatom species composition in the investigated coastal sites, and these sites are thus far from reaching a “good environmental status” according to the EU Water Framework Directive. The outcomes of this thesis highlight the importance of a longer time perspective than the environmental monitoring can provide. Keywords: Baltic Sea, paleoecology, diatom stratigraphy, stable nitrogen isotopes, hypoxia, nutrient discharge, eutrophication, Medieval Climate Anomaly, Little Ice Age

Svensk sammanfattning Övergödningen av Östersjön under 1900-talet är väldokumenterad och har bland annat resulterat i sämre siktdjup, att cyanobakterieblomningar har blivit mer omfattande och vanligare, utbredd syrebrist i bottenvatten, och en förändrad artsammansättning av många organismgrupper. Systematiskt provtagna mätdata från miljöövervakningen finns bara tillgänglig från 1960–70-talet och därmed vet vi väldigt lite om Östersjöns ekosystem före människans storskaliga påverkan. Sedimentkärnor från sjöar och hav fungerar som ett historiskt arkiv som under årtusenden lagrat information om dåtidens ekosystem. I denna avhandling används bevarade subfossila kiselalger och geokemi för att spåra miljöförändringar längs svenska sydostkusten de senaste tvåtusen åren. Resultat presenteras från fem sedimentkärnor från Östersjökusten, från Stockholms skärgård i norr till Gåsfjärden i söder, längs en sträcka på ca 250 km. Alla stratigrafier tyder på stabila förhållanden i dessa kustområden under yngre järnålder (500 före vår tideräkning – 1050 efter vår tideräkning (evt)) och medeltid (1050–1500 evt), fram till 1700-talet. Varken förändringar i klimat eller markanvändning har påverkat dessa kustområden i någon större utsträckning tills för några hundra år sedan. Det finns inga tecken på effekter av mänsklig aktivitet som exempelvis jordbruk fram till mer nutida förändringar. Alla undersökningsplatser har påverkats av övergödning under de senaste århundradena. Den exakta starten för ökad näringstillförsel skiljer sig något mellan platserna. De första tecknen på övergödning är från slutet av 1700-talet, och i början av 1800-talet är artsammansättningen av kiselalger redan förändrad. Storskaliga förändringar i markanvändning skedde under 1800talet och fortsatte in på 1900-talet. Våtmarker och sjöar dikades ut, jordbruk med ängar och traditionell träda av jordbruksmark fasades ut till förmån för vallodling, till det kom konstgödsel i slutet av 1800-talet. Växande städer, industrier och reningsverk är punktkällor som i varierande grad har påverkat dessa kustområden. De första tecknen på övergödning syns ca 100 år tidigare vid kusten än i öppna Östersjön, vilket belyser kustzonens roll som näringsfilter. I öppna Östersjön har både klimatet och näringstillförsel från land påverkat artsammansättningen av kiselalger de senaste 2 000 åren. I kustområdet däremot har de direkta effekterna av klimatet spelat en mindre roll, och artsammansättningen av kiselalger har främst varit påverkad av näringstillförsel från land. Övergödningen har resulterat i ökad pelagisk primärproduktion och därmed lägre siktdjup, vilket har begränsat utbredningen av bottenlevande arter. I Östersjön finns inga opåverkade områden kvar, och därmed inga referensområden för att definiera referensvärden enligt EUs vattendirektiv. Resultaten som presenteras i denna avhandling visar att i flera av de undersökta kustområdena rådde ett miljötillstånd opåverkat av mänsklig aktivitet för mer än 200 år sedan. Maximal tillförsel av näring till Östersjön skedde under 1960–70-talet, vilket avspeglar sig i koncentrationen av kiselalger i sedimenten, något som kan användas som en proxy för primärproduktion. Lägre koncentrationer av kiselalger i sedimenten de senaste årtiondena indikerar en bättre vattenkvalité. Däremot syns ännu ingen förbättring i artsammansättning av kisel-

alger som indikerar en tillbakagång till referensvärden. Inte heller syns tecken på någon förbättring vad gäller siktdjup i undersökningsområdena.

Contents

List of papers.................................................................................................................................................13 Abbreviations and definitions....................................................................................................................15 Introduction..................................................................................................................................................17 Thesis objectives ...........................................................................................................................................21 Background ...................................................................................................................................................23 Holocene history of the Baltic Sea .......................................................................................................23 The present Baltic Sea ...........................................................................................................................24 Effects of climate on the Baltic Sea ecosystem ...................................................................................25 Climate during the Holocene ...............................................................................................................26 Records of early human impact in lakes .............................................................................................27 Agrarian history in southern Sweden during the last millennium .................................................28 Description of study sites ............................................................................................................................31 Kanholmsfjärden....................................................................................................................................32 Ådfjärden ................................................................................................................................................32 Himmerfjärden.......................................................................................................................................33 Bråviken...................................................................................................................................................33 Gåsfjärden ...............................................................................................................................................34 The western Gotland Basin ..................................................................................................................35 Material and Methods .................................................................................................................................37 Field work................................................................................................................................................37 Chronologies...........................................................................................................................................39 Radiocarbon dating .........................................................................................................................39 Lead dating .......................................................................................................................................40 Markers: cesium and mercury .......................................................................................................40 Correlation of cores and age-depth modeling.............................................................................41 Diatoms as a proxy for environmental change..................................................................................41 Lab procedure – diatom analysis...................................................................................................42 Geochemical proxies .............................................................................................................................43 Lab procedure – geochemical analyses .........................................................................................44 Data processing and statistical analyses .............................................................................................44 Results – summary of papers......................................................................................................................47 Interpretation and Discussion....................................................................................................................51 Signs of eutrophication in the coastal zone during the last centuries ...........................................51 Diatom life-form ..............................................................................................................................55 Diatom species composition ..........................................................................................................55 Diatom species richness..................................................................................................................57

Diatom absolute abundance...........................................................................................................57 Stable isotope δ15N...........................................................................................................................57 Causes for eutrophication in the coastal zone ...................................................................................58 Tracing human impact and climate change during medieval times .............................................61 Climate or nutrients as drivers of paleoecological trends in the Baltic Sea since 500 CE ..........62 Implications for the environment .......................................................................................................65 Conclusions...................................................................................................................................................67 References......................................................................................................................................................69 Tack ................................................................................................................................................................81 Paper I............................................................................................................................................................95 Paper II ....................................................................................................................................................... 113 Paper III...................................................................................................................................................... 137 Paper IV...................................................................................................................................................... 153

List of papers

This thesis is based on the following papers referred to in the text by the Roman numerals (I-IV): Paper I: Norbäck Ivarsson, L., Andrén, T., Moros, M., Andersen, T. J., Lönn, M., and Andrén, E. (2019). Baltic Sea Coastal Eutrophication in a Thousand Year Perspective. Frontiers in Environmental Science 7. doi:10.3389/fenvs.2019.00088. Paper II: Norbäck Ivarsson, L., Andrén, T., Moros, M., J. Andersen, T. and Andrén, E. Signs of early eutrophication in the Stockholm outer archipelago as evident in a 500-year-long sediment record. (Manuscript) Paper III: Ning, W., Nielsen, A. B., Norbäck Ivarsson, L., Jilbert, T., Åkesson, C. M., Slomp, C. P., Andrén, E., Broström, A. and Filipsson, H. L. (2018). Anthropogenic and climatic impacts on a coastal environment in the Baltic Sea over the last 1000 years. Anthropocene 21, 66–79. doi:10.1016/j.ancene.2018.02.003. Paper IV: Norbäck Ivarsson, L., Lönn, M., Andrén, T. and Andrén, E. Exploring paleoecological trends since 500 CE; a comparison between coastal and open Baltic Proper. (Manuscript)

Contributions of the author to the different manuscripts Paper I: Study design, performed field work and lab work (subsampling, prepared samples for dating, geochemical analyses and diatom analysis, performed the diatom analysis), interpreted the results, wrote the manuscript. Paper II: Study design, performed field work and lab work (subsampling, prepared samples for dating, geochemical analyses and diatom analysis, performed the diatom analysis), interpreted the results, wrote the manuscript. Paper III: Contributed with the diatom analysis and the interpretations of these results. I wrote the parts on this in the methods, results and discussion. I also contributed to the overall interpretations of results and writing of the manuscript. Paper IV: Study design, performed field work and lab work for the coastal sites. I am involved in computing the statistical analyses and interpreting the results, and I wrote the manuscript.

Related paper not included in this thesis Andrén, E., van Wirdum, F., Norbäck Ivarsson, L., Lönn, M., Moros, M., Andrén, T., 2020. Medieval versus recent environmental conditions in the Baltic Proper, what was different a thousand years ago? Palaeogeography, Palaeoclimatology, Palaeoecology 555, 109878. https://doi.org/10.1016/j.palaeo.2020.109878

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Abbreviations and definitions

BCE CCA CE DCA LIA MCA MoWP MSFD RDA WFD Cal. yr BP

Before Common Era Constrained Correspondence Analysis Common Era Detrended Correspondence Analysis Little Ice Age (1400–1700 CE, Mann et al. 2009) Medieval Climate Anomaly (950–1250 CE, Mann et al. 2009) Modern Warm Period (from 1850 CE, Harland et al. 2013) Marine Strategy Framework Directive Redundancy Analysis Water Framework Directive Calendar years Before Present (present defined as 1950 CE)

Anoxia Hypoxia

≤0 ml/l O2, H2S present ≤2 ml/l O2

15

Introduction

The eutrophication of the Baltic Sea due to increased anthropogenic nutrient loads during the 20th century is well documented and studied (HELCOM, 2009). Consequences of this nutrient enrichment and increased production of organic material include decreased water transparency, more intense cyanobacterial blooms, widespread sea bottom hypoxia and altered species composition for several organism groups (Elmgren, 2001). Since the 1950s, hypoxia has also increased in the Baltic Sea coastal zones (Conley et al., 2011; Persson and Jonsson, 2000). Some of the recorded changes resulting from present eutrophication are not new phenomena for the Baltic Sea. Pigment analyses in sediments have shown cyanobacteria blooms to be natural reoccurring features of the open Baltic Sea, since marine water started to enter the Baltic Sea Proper some 8000 years ago (Bianchi et al., 2000; Funkey et al., 2014). Since no major fauna can live in a hypoxic environment, the sediments are not bioturbated, and thus sediments deposited during oxygen depletion may exhibit laminae (Jonsson et al., 1990; Persson and Jonsson, 2000; Zillén et al., 2008). A compilation of laminated sediment sequences used as proxy for hypoxia show evidence of three time periods with extensive areal distribution of hypoxia in the open Baltic Sea; 8000–4000 cal. yr BP, 2000–800 cal. yr BP and from 1800 CE to present (Zillén et al., 2008). The European Union Water Framework Directive (WFD) was adopted in 2000 CE and concerns groundwater and surface waters, including transitional and coastal waters. The main objective of the WFD is for all waters in the EU to reach good or high ecological status. A key concept for determining ecological status under the WFD is reference conditions. Reference conditions are defined in the WFD as: “… a description of the biological quality elements that exist, or would exist, at high status, that is, with no, or very minor disturbance from human activities” (Anonymous, 2000). In the case of the Baltic Sea, there are no existing undisturbed sites (reference sites) to use (Andersen et al., 2011), and thus historical data, models and expert judgement are used to determine reference conditions (European Commission, 2003). Systematic environmental monitoring of the Baltic Sea in Sweden started in some sites in the 1960s, and has been continuously developed and expanded (SMHI, 2019). Yet humans have affected the environment in the Baltic Sea longer than this (Gustafsson et al., 2012). In addition, instrumental records of climate variables, such as temperature, only span the last few centuries. This is not enough of a time span to capture the natural variability in the environment. Sediment records from lakes and seas can provide fundamental data on the environmental conditions before ecosystems were impacted by humans and on the range of natural variation. Further, significant information about the speed and direction of ecosystem changes is gained

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TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

if a historical point of view is considered (Saunders and Taffs, 2009; Willis and Birks, 2006). There is a need to improve our understanding of the natural variability of the Baltic Sea and its response to climate, as well as human-induced, forcing. A long-term perspective concerning how environmental changes affect the marine system will provide us with a deeper knowledge of possible future scenarios, highly important for the management of the Baltic Sea (Kotilainen et al., 2014). The hypoxic event that occurred 2000–800 cal. yr BP coincides with a climatic warm event named the Medieval Climate Anomaly (MCA), as well as with the Roman Warm Period, and has been suggested to have been caused by an increase in human population and large-scale changes in land use in the drainage area at this time (Zillén and Conley, 2010). Around the turn of the last millennium, a series of technological innovations, including the iron plow, promoted an expansion of agricultural land (Myrdal, 1999). The favorable climate conditions during the MCA, with temperatures similar to those of today, also enabled a demographic expansion (Ljungqvist et al., 2012; Myrdal and Morell, 2011). It has additionally been suggested (Kabel et al., 2012) that the warmer climate during this time caused intensified cyanobacterial blooms (cyanobacteria are favored by warm and calm waters), resulting in hypoxia. The following oxygenation of the bottom waters in the open Baltic Sea has been explained by the onset of the Little Ice Age (LIA), and the colder sea surface temperatures during this time that would suppress cyanobacteria blooms (Kabel et al., 2012). However, it has also been suggested that this oxygenation event was due to an infectious disease, the so-called Black Death, that decreased the population by half around 1350–1450 CE (Zillén and Conley, 2010). Sediment cores from the open Baltic Sea have been studied on several occasions with the aim of tracing environmental history (e.g. Andrén et al. 2000a; b; Bianchi et al. 2000). Studies from the coastal zone of the Baltic Proper include two studies from the Archipelago Sea that show an onset of eutrophication in the early 1900s (Jokinen et al., 2018; Tuovinen et al., 2010). In the Gulf of Finland, Weckström (2006) reports the onset of eutrophication at around 1900 CE in two urban sites, and later (1940s– 1980s) in more rural sites. Andrén (1999) reports the onset of eutrophication at circa 1900 CE in the Oder estuary, at the Polish/German border. Except for Jokinen et al. (2018), these studies only cover a time frame of the last century/centuries. Studies of long-term trends in the coastal zone are lacking. If we want to understand human impact on the Baltic Sea through history, more focus should be placed on studying the long-term perspective in coastal sites. It is reasonable to assume that a land-based human activity will affect the coastal areas first, before the effects are recorded in the sediments from the open Baltic Sea. In this thesis, sediment records from five coastal sites in the northwestern Baltic Proper are presented. These sediment cores have recorded environmental changes during the last millennia, and they have been analyzed with respect to paleoecology (diatom relative and absolute abundance), and geochemical parameters. The records from the coastal zone are further compared to a record from the open Baltic Proper 18

INTRODUCTION

(Andrén et al., 2020). In order to accurately assess ecosystem responses of the Baltic Sea in a future climate scenario, it is of high importance to increase our understanding of ecosystem responses in history. Paleoecology has the potential to fill this gap in knowledge (Andersen et al., 2004).

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Thesis objectives

The overall objectives of this PhD thesis are 1) to put the present severe environmental situation in the Baltic Sea, in terms of excess nutrient loads and climate change, in a thousand-year perspective, and 2) to contribute to an improved understanding of the natural variability at coastal sites along the southeastern coast of Sweden. The specific objectives of this PhD thesis are to: • •

investigate and date long-term environmental changes in the coastal areas of northwestern Baltic Proper. identify the onset of anthropogenic environmental change, and how this is registered in sediment records from these coastal sites.

•

produce a scientific base for inferring reference conditions at these sites.

•

compare sediment records from the open Baltic Sea and the coastal zone, to explore if there is a synchronicity in interpreted environmental changes between these areas.

•

assess the relative importance of human activities versus climate variability in causing environmental change in coastal ecosystems of the Baltic Sea over the last millennia.

21

Background

Holocene history of the Baltic Sea The Baltic Sea has a short but very dynamic history driven by glacio-isostatic land uplift and eustatic sea level changes. The last deglaciation of the Baltic basin started roughly 20,000 cal. yr BP resulting in an ice lake forming in front of the melting ice sheet around 16,000 cal. yr BP, the Baltic Ice Lake (Andrén et al., 2011). This lake was initially dammed 10 meters above sea level and was drained two times when the melting ice sheet retreated from the northern tip of mount Billingen, situated between Lake Vänern and Lake Vättern (Björck, 2008; Andrén et al., 2011). The first time was during the end of Allerød, roughly 13,000 cal. yr BP (Muschitiello et al., 2015). This event was followed by the colder Younger Dryas when the ice sheet re-advanced and the lake was again dammed. The final drainage of the Baltic Ice Lake took place about 11,700 cal. yr BP. This was a dramatic event, with drainage occurring over the course of 1-2 years and resulting in the lake being lowered 25 meters down to the level of the world oceans (Jakobsson et al., 2007). The next phase is called the Yoldia Sea. The melting of the ice sheet provided a constant feed of freshwater to the basin and it was only during a short cooling of the climate that marine water could enter from the North Sea (lasting max. 350 years) (Andrén et al., 2007). The Yoldia Sea lasted only about 1000 years and ended when its inlet through middle Sweden was closed due to the isostatic rebound, around 10,700 cal. yr BP. A freshwater lake was formed, the Ancylus Lake. This lake had an early outlet westward through middle Sweden, but the isostatic rebound came to close this outlet and the lake level started to rise. Eventually the lake found a new outlet in the southern Baltic basin through a complex river system through Denmark with river channels, levées, and lakes (Bennike et al., 2004; Björck et al., 2008; Andrén et al., 2011). The ongoing melting of both the Scandinavian and the Laurentian ice sheets caused a sea-level rise of the world oceans and when this sea-level rise caught up with the isostatic land uplift in the south of Scandinavia marine water could enter through the Danish sounds, approximately 9800 cal. yr BP (Andrén et al., 2000b; Andrén et al., 2011; Berglund et al., 2005). The following brackish phase, Littorina Sea, shows a peak in salinity around 6000–5000 cal. yr BP, coinciding with the Holocene Thermal Maximum (Gustafsson and Westman, 2002; Seppä et al., 2009; Willumsen et al., 2013). Because of the isostatic land uplift in the area and a simultaneous lowering of the global sea level, the Danish sounds became gradually shallower. The inflow of marine water decreased, eventually leading to a Baltic Sea with conditions more like today (Andrén et al., 2000a, 2011; Berglund et al., 2005).

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TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

The present Baltic Sea The Baltic Sea is a semi-enclosed brackish water basin, the second largest in the world after the Black Sea (Leppäranta and Myrberg, 2009). This inland sea has a constant inflow of freshwater from rivers. Intrusions of salt water from the North Sea occur irregularly through the Danish Straits and Öresund and feed the Baltic Sea with saline water. Due to this, there is a salinity gradient in the Baltic Sea, ranging from 0-4 in the northern parts, to about 15 in the Belt Sea (Leppäranta and Myrberg, 2009). There is also a vertical salinity gradient with a permanent halocline forming at around 4080 meters depth in the Baltic Proper. The halocline prevents mixing between the upper, more fresh water, and the lower, more saline water. The average depth of the Baltic Sea is about 54 meters with a maximum depth of 459 meters in the Landsort Deep (Leppäranta and Myrberg, 2009). Since the Baltic Sea is a geologically young sea, there are only a few macroorganisms truly adapted to the brackish water conditions. Instead, most organisms are either marine or limnic, living on the edge of what they can tolerate in terms of salinity. Because of this, the Baltic Sea is characterized by poor macro-species richness and biodiversity and the ecosystem is therefore sensitive to changes in the environment (Elmgren and Hill, 1997). The primary production is dominated by diatoms and dinoflagellates in the spring and cyanobacteria blooms during warm summer months (Klais et al., 2011). The spring bloom in the Baltic Proper is nitrogen limited, while the summer blooms of cyanobacteria are limited by the availability of phosphorous (Granéli et al., 1990). Following the decay of the cyanobacteria blooms, diatoms and dinoflagellates dominate during the autumn (Andersson et al., 2017). During recent decades, the growing season of phytoplankton has been extended with more than 100 days in the southwestern Baltic Sea. Further, the biomass produced during autumn months has increased significantly (Wasmund et al. 2019). The Baltic Sea has a large catchment area, with about 85 million inhabitants in 14 countries. There is severe pressure on this ecosystem due to human activities. Increased anthropogenic nutrient loads from e.g. sewage treatment plants, industries, and diffuse sources such as agriculture and burning of fossil fuels during the twentieth century, have led to a number of problems (Elmgren, 2001; Gustafsson et al., 2012). For example, increased primary production leads to more decomposition and consumption of oxygen. The oxygen depletion has severe effects on marine ecosystems, for example by killing benthic fauna and altering biogeochemical cycles (Diaz and Rosenberg, 2008). Approximately 18% of the deep water in the Baltic Proper, including the Gulf of Finland and the Gulf of Riga, is today hypoxic (≤2 ml/l O2) and approx. 8% is anoxic (≤0 ml/l O2) with H2S present (Hansson and Andersson, 2014). The Baltic Sea has shown a rapid increase of hypoxic sediments since the 1940s and the hypoxic area now covers about 80,000 km2, corresponding to about two times the size of Denmark (Carstensen et al., 2014a; Hansson and Andersson, 2014; Jonsson et al., 1990). Hypoxia is a globally increasing problem, not only in Europe but also, for

24

BACKGROUND

example, along the east coast of North America, but the Baltic Sea constitutes one of the largest so-called dead zones in the world (Diaz and Rosenberg, 2008). In an anoxic sediment, phosphorous cannot be retained and buried together with iron but is instead released back to the water masses. Increased phosphorous levels stimulate the growth of nitrogen-fixating cyanobacteria during the warm summer months. The cyanobacteria supply the sea with nitrogen (Karlson et al., 2015) and when they decompose, oxygen is consumed and phosphorous is released from the sediment (Carstensen et al., 2014b). A feedback loop is thus created, which sustains the eutrophic and hypoxic states of the Baltic Sea (Conley et al., 2002, 2009; Kemp et al., 2005). Since no organisms except for bacteria and archaea can live in an anoxic environment with H2S present, the sediment is not being bioturbated. Laminated sediments can therefore be used as a proxy for anoxic conditions (Jonsson et al., 1990). Although human impact has caused eutrophication of the Baltic Sea, both hypoxia and cyanobacteria blooms are natural features of the Baltic Sea and have been present periodically during the last 8000 years (Bianchi et al., 2000).

Effects of climate on the Baltic Sea ecosystem Climate in the Baltic Sea region is to a large extent controlled by atmospheric conditions and is thus affected by both the global climate as well as regional circulation patterns. The weather conditions in the Baltic Sea region are highly variable due to its location in the extra-tropics of the northern hemisphere. In general, westerly winds predominate in this region. The large-scale circulation patterns in the Baltic Sea are influenced by the North Atlantic Oscillation (NAO) (The BACC II Author Team, 2015). During a positive phase of NAO, the westerly airflow is strengthened, which brings warm wet winters to northern Europe, while during a negative NAO, westerly airflows are weaker, and this brings colder and drier winters to northern Europe (Hurrell et al., 2003). The NAO shows seasonal and interannual variability, and prolonged periods of negative or positive phases are recorded. Wind conditions in the Baltic Sea region shows some connection to the NAO, with more calmer periods during negative NAO phases and more storminess during positive phases of NAO. Precipitation in the Baltic Sea region varies greatly between regions and seasons, and no clear trend has been observed during the last century. A clear increasing trend in air temperature has been observed in the Baltic Sea region since 1871 CE. In the area investigated in this thesis, this increasing trend is of a magnitude of 0.08 ˚C per decade, which is greater than the average global increase of 0.06 ˚C between 1861– 2005 CE (The BACC II Author Team, 2015; IPCC 2007). The trends are positive for all seasons, and the temperature increase has also increased the duration of the growing season (The BACC II Author Team, 2015). The climate warming in recent centuries has led to changes in the catchment and in the Baltic Sea itself. No clear trend in the overall river runoff to the Baltic Sea has been detected. However, a decreasing trend has been observed for the southern catch-

25

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

ments and rivers over the last century. A reduction of ice cover and earlier ice breakup has been recorded for rivers in the drainage area. In addition, the duration of annual snow cover has become significantly shorter in western Scandinavia over the last century (The BACC II Author Team, 2015). When it comes to climate effects on the Baltic Sea itself, water temperature and sea ice respond quickly to higher air temperatures. There is a clear warming trend in sea surface temperature over the last century, with the greatest increase observed in summer months (Gustafsson et al., 2012; Mackenzie and Schiedek, 2007). The extent and duration of sea ice is associated with NAO, with more ice during negative ice phases, and less ice during positive phases (The BACC II Author Team, 2015). Salinity in the Baltic Sea is mainly governed by two factors: net precipitation and river discharge, and water exchange with the North Sea. The so-called major Baltic inflows of marine water through the Danish straits occur sporadically, usually during the winter and spring months and thus bring saline, but also cold and oxygen-rich water into the Baltic Sea (Mohrholz et al., 2015). However, in recent decades, several inflows have occurred during the summer, which brings water that is saline, but warm and low in oxygen content. No trend has been detected during the last century of salinity measurements (The BACC II Author Team, 2015). However, a reconstruction of annual mean salinities since 1500 CE indicates that the salinity has slowly increased by 0.5 salinity units since 1500 CE, peaking in the mid-eighteenth century (Hansson and Gustafsson, 2011). When it comes to wave activity and storm surges, no significant change has been recorded in the last century. The change in mean sea level in the Baltic Sea results from the combined effects of post-glacial isostatic rebound of the continental crust, global sea level rise and thermal expansion of water. It is not clear how climate change during the last century has affected the sea level in the Baltic Sea, but the rise is probably in pace with the global sea level rise, approx. 1.5 mm per year (The BACC II Author Team, 2015). Results from models predicting a 2-4˚C warming by 2100 CE indicate that ecosystem changes in the Baltic Proper will include e.g. increased primary production, larger cyanobacteria blooms and lower oxygen concentrations in bottom waters (Andersson et al., 2015).

Climate during the Holocene Historical changes in climate are reconstructed using proxies from ice cores as well as lake and ocean sediments (e.g. oxygen isotopes, pollen and insects), tree ring widths and densities and, during the historical time frame, written records on e.g. weather extremes. Climate oscillations during the Holocene are driven by astronomical conditions, solar activity, volcanic eruptions, concentration of greenhouse gases in the atmosphere, changes in albedo of the sea and changes in the vegetation on land (The BACC II Author Team, 2015). Summer solar insolation peaked approx. 7000–6000 years ago, and during the Holocene Thermal Maximum (approx. 8000– 4000 cal. yr BP), the temperature in the Baltic Sea region was approximately 1-3.5 °C

26

BACKGROUND

warmer than today. As already mentioned, this was a time period with higher salinity in the Baltic Sea (The BACC II Author Team, 2015). Due to a decreasing trend in summer solar insolation, the climate has become more unstable and a general cooling trend is recorded the last approx. 4000–5000 years (The BACC II Author Team, 2015). The climate during the last 2,000 years has fluctuated a lot, with general warmer temperatures in the Northern Hemisphere approx. 0-300 CE (the Roman Warm Period), and colder temperatures approx. 300800 CE (the Dark Age Cold Period) (Ljungqvist, 2010). The climate during the last millennium has been roughly divided into three periods: Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and Modern Warm Period (MoWP). There are regional differences in the intensity and exact timing of these climatic periods (Ljungqvist et al., 2012). Throughout this thesis, the definitions in Mann et al. (2009) are used, which means MCA=950-1250 CE, and LIA=1400-1700 CE. MoWP is defined as after 1850 CE (Harland et al., 2013). Proxy data show evidence for higher sea surface temperature and more intense cyanobacterial blooms during MCA (Funkey et al., 2014; Kabel et al., 2012). This could conceivably be explained by warm and dry conditions (Luoto and Nevalainen, 2018). Changes in the input of freshwater has shown to be an important factor in governing salinity changes over the last 8500 years (Gustafsson and Westman, 2002). However, reconstructing precipitation is not as straightforward as temperature reconstructions (The BACC II Author Team, 2015). In a modeling study by Schimanke et al. (2012), fresher conditions during MCA due to wetter conditions were reconstructed. Uncertainties in reconstructing past environmental conditions and ecosystem responses in the Baltic Sea are tightly coupled with uncertainties in future climate scenarios. Understanding the past is key to understanding the present, and essential to being able to predict the future.

Records of early human impact in lakes Humans have affected landscapes substantially on local scales for millennia, mostly by disturbing vegetation cover by deforestation (Roberts et al., 2018). The earliest signs of landscape disturbance (in pollen records from lakes) are recorded approx. 9000-6000 years before common era (BCE) in America, Oceanica, Asia, Africa and Europe (Dubois et al., 2018). The very first signs of eutrophication of lakes are recorded approx. 6000–3000 years BCE in Europe and America (Dubois et al., 2018). In the drainage area for the Baltic Sea, the first signs of landscape disturbance are recorded approx. 5000 years BCE in Finland, approx. 3800 years BCE in Estonia, approx. 2500 years BCE in Latvia, approx. 1000 years BCE in Poland and as early as approx. 6000 years BCE in northern Germany (Dubois et al., 2018). While the very first signs of eutrophication of lakes do sometimes occur simultaneously with vegetation disturbance (for example in Germany and Estonia), they are often recorded much later. The first signs of eutrophication in lakes are recorded in Finland approx.

27

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

2500 years BCE, in Latvia during the 13th century CE, and in Poland circa 1000 CE (Dubois et al., 2018). In a study from Denmark, early landscape disturbance was recorded as early as approx. 4000 BCE, but the lake remained undisturbed until the major deforestation occurred from approx. 500 BCE. Eutrophication of this lake was further intensified during medieval times, about 1050–1500 CE (Bradshaw et al., 2005). In Sweden, signs of human activities have been registered in pollen records from approx. 3600 BCE in the southernmost part of Sweden, with signs of agriculture from approx. 2600 BCE (Åkesson et al., 2015). Closer to the area of interest in this thesis, land use changes have been interpreted from pollen stratigraphies circa 800 CE, with a simultaneous response in the diatom assemblages of these lakes (Risberg et al., 1994; Karlsson and Risberg 2006). In southwestern Sweden, the first signs of landscape disturbance are recorded approx. 300 BCE, simultaneous with an increased pH of the lake, attributed to these changes in land use (Renberg et al., 1990). In northern Sweden, continuous agriculture is recorded from the 13th century and the lake response is simultaneous with dramatic changes in the diatom assemblages (Anderson et al., 1995). Lake Mälaren has been culturally eutrophicated since the isolation of the lake circa 1200 CE, as inferred from a diatom-based Total Phosphorous reconstruction (Renberg et al., 2001). This makes it impossible to define reference conditions according to the WFD. This is also highlighted in Willén, (2001), where it is suggested to instead define an acceptable deviation from an assumed pristine state.

Agrarian history in southern Sweden during the last millennium Around a thousand years ago, a series of technological innovations, including the iron plow, promoted an expansion of agricultural land (Myrdal, 1999). The favorable climate conditions during the MCA, with temperatures similar to today, also enabled a demographic expansion (Ljungqvist et al., 2012; Myrdal and Morell, 2011). It is difficult to estimate changes in population this far back in time, but between 1000 and 1300 CE the numbers of households in Mälardalen, an area around Lake Mälaren, at least doubled (Myrdal, 1999). In 1350 CE, Sweden was hit by the plague. Several more outbreaks took place during the following century, decimating the population in Sweden by half (Myrdal and Morell, 2011). This strongly altered the land use, especially in more remote areas, where peripheral farms were abandoned in favor of more fertile lands. These changes in demography are recorded in pollen diagrams from the highlands of Småland, a wooded area in the south of Sweden (Lagerås, 2016). However, the more fertile plains were not usually abandoned, at least not for long. They were instead taken over by people migrating in search of land for agriculture (Myrdal and Morell, 2011). Following the medieval agrarian crisis, there was an increase in the population and by circa 1600 CE it had reached the same level as before the plague (Myrdal and Morell, 2011). During this time when both the population and the standard of living increased, the climate became colder (Schimanke et al., 2012). This might seem

28

BACKGROUND

contradictory, but except for the northernmost parts of Scandinavia, it was not the temperature, but the amount of precipitation that was of greatest importance for the size of the harvest (Charpentier Ljungqvist, 2015). Unfortunately, our knowledge of the changes in precipitation is not as extensive as that of changes in temperature. Model simulations show wetter conditions (compared to the mean annual precipitation from 800 to 1900 CE) during the MCA, and more dry conditions during the 14th and 15th centuries, i.e. at the beginning of LIA (Ljungqvist et al., 2016). However, other studies have reconstructed dry conditions during MCA (Helama et al., 2009; Luoto and Nevalainen, 2018). Early landscape disturbances and changes in agricultural practices have had an effect locally in lakes (Dubois et al., 2018). However, the question remains as to whether coastal seas have also been affected for such a long time. It has been suggested that major changes in the open Baltic Sea ecosystem during medieval times were caused by human activities on land (Åkesson et al., 2015; Schimanke et al., 2012; Zillén and Conley, 2010). In this thesis, the long-term effect on coastal areas of the Baltic Sea is traced using diatom stratigraphy and geochemical proxies.

29

Description of study sites

The study area for this thesis is the northwestern part of the Baltic Proper, which means the Western Gotland Sea and southeastern coast of Sweden (Figure 1, Table 1). The coast of southeastern Sweden consists mostly of rocky archipelago coastline. This area is still influenced by the last glaciation and the isostatic rebound of the area is approx. 2-4 mm/year (Harff and Meyer, 2011). The coastal sites in this study are located along a 250-km stretch of the southeast coast of Sweden (Figure 1). In most winters, all coastal sites in this study have about 90-150 days with sea ice. During summer months, all sites experience stratification due to the development of a thermocline. All sites are considered sheltered according to the classification system of wave exposure in the WFD (VISS, 2019).

Figure 1. Map showing the Baltic Sea and its drainage area (shaded in grey), and the study area with the five sampling sites along the Swedish coast, and the sampling site in the western Gotland Basin. Number 1 is Lake Mälaren, number 2 is Stockholm, number 3 is Södertälje and number 4 is Norrköping.

31

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

Kanholmsfjärden Kanholmsfjärden is a large and deep basin located in the outer parts of the Stockholm archipelago, with a surface area of approx. 35 km2 and mean and maximum water depths of 52 and 104 m, respectively (SMHI, 2019). The basin is separated from the open Baltic Proper by a rosary of islands. Salinity in surface waters is 4.3-5.5, while the deep-water salinity is 6.5-6.7 (SMHI, 2019). The stratification of the water masses prevents mixing of bottom and surface waters and the deep waters are more or less permanently hypoxic (SMHI, 2019). The islands surrounding Kanholmsfjärden to the north, east and south are very sparsely populated. The larger islands to the west are peri-urban and connected to the mainland by roads. A small sewage treatment plant is located in the southwestern part of Kanholmsfjärden, which is dimensioned for approx. 6000 people (Värmdö kommun, 2019). Kanholmsfjärden is located in the outer part of the Stockholm archipelago, and the drainage area of the basin itself is restricted to surrounding islands. However, this site is influenced by the outward-flowing freshwater from Lake Mälaren passing through Stockholm. This drainage area is large, approx. 20,140 km2, and consists of 21% agricultural land and 65% wooded areas (SMHI, 2020). Further, treated sewage water from approx. 1.5 million people has its outlet in Stockholm. Environmental monitoring has been performed through recipient controls along the main fairway to Stockholm, passing through Kanholmsfjärden, in the Stockholm archipelago since 1968 CE. This was later extended to many other sites in the Stockholm archipelago. This has resulted in a relatively long time series of monitoring data being available for Kanholmsfjärden. However, since 2015, Kanholmsfjärden has no longer been a part of the recipient controls. Kanholmsfjärden is today, due to its low biovolume of phytoplankton, classified as having a moderate ecological status while the chemical status is classified as bad (VISS, 2019).

Ådfjärden Ådfjärden is located in the southern part of the Stockholm archipelago, approx. 30 km south of Stockholm, and approx. 60 km southwest of Kanholmsfjärden. This water basin has an area of approx. 5 km2, a maximum water depth of 30 m, and thresholds in the north and south are 12 m and 19 m, respectively. About 10 km of archipelago separates this site from the open Baltic Sea. Salinity in the larger area of Horsfjärden, of which Ådfjärden is a part, is 5.1-5.7 in surface waters and 5.6-5.9 in bottom waters (SMHI, 2019). Ådfjärden is not an administrative area in the environmental monitoring and therefore the information about this water basin is sparse. The drainage area for Ådfjärden is the smallest of all sites in this study, 158 km2, of which 72% is wooded areas and 14% agricultural land (SMHI, 2020). There are no large urban areas within the drainage area nor in the fairway through Ådfjärden, which means it is relatively unaffected by boat traffic. Ådfjärden is today classified as

32

DESCRIPTION OF STUDY SITES

having a moderate ecological status and is thus affected by eutrophication (VISS, 2019).

Himmerfjärden Himmerfjärden is an estuary with a surface area of approx. 31 km2, located on the other side from Ådfärden of the peninsula Södertörn, approx. 40 km south of Stockholm. The bathymetry of Himmerfjärden is steep since the estuary is the result of a tectonically induced fault in the bedrock. Threshold depth at the inlet is approx. 15 m, and the mean and maximum water depths are 15 and 45 m, respectively. Salinity in Himmerfjärden is 5.2-5.9 in surface waters and 5.6-6.2 in bottom waters (SMHI, 2019). The drainage area of Himmerfjärden is 432 km2, and consists of 62% wooded area and 20% agricultural land (SMHI, 2020). The city of Södertälje, with around 100,000 residents, is located upstream from Himmerfjärden. Several industries were established in Södertälje in the 19th century, and these expanded and became more numerous in the early 20th century. This enabled population growth in Södertälje and surrounding areas, especially during the 1940s (Nordström, 1968). Lake Mälaren has its main outlet in Stockholm, but when the water level is exceptionally high, it is also drained through a lock in Södertälje, into the Himmerfjärden estuary (Elmgren and Larsson, 1997). The lock opened in 1819, and was rebuilt in 1924 (Nordström, 1968). Prior to the opening of the lock, Lake Mälaren had been isolated from the Baltic Sea since around 1200 CE (Price et al., 2018). According to the Swedish Maritime Administration, 2700 commercial ships and 9000 leisure boats pass through Himmerfjärden, in and out of Lake Mälaren, every year. In 1974, a sewage treatment plant opened with around 90,000 people connected, using the Himmerfjärden estuary as recipient. Since its opening, there have been several full-scale experiments with nutrient discharge in the bay, and the water quality in Himmerfjärden has been closely studied by recipient control environmental monitoring, with respect to e.g. nutrient levels, redox conditions of bottom waters and phytoplankton biomass (Elmgren and Larsson, 1997; Savage et al., 2002). Today the sewage treatment plant serves around 314,000 people and most of the wastewater from the southern suburbs of Stockholm ends up here (Winnfors, 2019). Himmerfjärden is at present affected by eutrophication and is classified as having a moderate ecological status (VISS, 2019).

Bråviken Bråviken is a long, approx. 50 km, and narrow bay with a surface area of 130 km2. This bay is large but is divided into smaller administrative areas. The sampling site is located in the inner part of Bråviken, where the mean and maximum depths are 9 and 39 m, respectively. The bay is separated from the open Baltic Sea by thresholds of approx. 16 m. Salinity in surface waters fluctuates a lot with seasons, from 1.1 to 5.6, while the salinity of bottom waters is more stable at 6.2-6.9 (SMHI, 2019). The Motala 33

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

Ström river drains into Bråviken through Norrköping. The drainage area for inner Bråviken is large, approx. 12,724 km2, with 22% agricultural land and 67% wooded areas (SMHI, 2020). The southern part of Bråviken is flat and very shallow (10-39 days2

>40 days2

N/A

>40 days2

>40 days2

Exchange of water

N/A

90-1502

N/A

90-1502

90-1502

N/A

90-1502

90-1502

Days with ice

Permanently stratified

Thermocline during summer2

N/A

Thermocline during summer2

Thermocline during summer2

N/A

Thermocline during summer2

Thermocline during summer2

Stratification

Table 1. Characterization of the water bodies in this thesis

w. Gotland Basin

References: 1) SMHI “Vattenwebben” (salinity sampled every month during the years 2004–2018), and SMHI “Tabell över havsområden” 2) VISS, 2019 3) Information from nautical charts 4) Leppäranta and Myrberg, 2009.

32

Material and Methods

Field work Sediment coring was conducted in 2011 from R/V Ocean Surveyor, in 2012 from R/V Skagerak, in 2014 from M/S Fyrbyggaren and in 2017 from R/V Maria S. Merian (Table 2). Slightly different equipment and techniques were used during the different cruises. However, the main methodology was to sample one long sediment core from each site and one complementary short core, to also retrieve the topmost soft and unconsolidated sediments. The following describes sediment coring during the 2012 and 2014 cruises: The long cores from each site were retrieved by a 3- or 5-meter-long piston corer (PC) using 20 to 30 kg of lead weights and a freefall of 0.5 meters. PVC liners with an inner diameter 4.6 cm were used. After retrieval, the cores were cut into sections 1 m (respective 1.25 m) long, sealed with end caps, marked and stored cold for later analyses in the laboratory. To recover the topmost sediments, a short, 1 m long, gravity corer (GC) with an inner diameter of 8 cm was used. The GCs were sliced onboard into 1 cm slices, marked and stored in plastic Ziploc bags. During the 2014 cruise, a second short core was collected and pushed out on deck, where it was documented by photography. All sediment core sections were stored in the cold room before opening. In the laboratory, the piston core liner sections were opened by cutting them lengthwise. One half of the core was placed into a core tray for visual inspection and documentation, both through a written core description and by photography. The other half of each core section was used for sub-sampling for radiocarbon dating, diatom stratigraphy and geochemistry. In Gåsfjärden, the complementary short core was sampled with a Gemini corer. The long core was opened and documented on board. Otherwise the methods were the same as those described above. For the western Gotland Basin, different gravity core devices were used to retrieve a continuous sediment record. A short multicorer was used to ensure sampling of the uppermost soft sediments. The sediment cores were kept in a cold room and transported to a lab in The Leibniz Institute for Baltic Sea Research in Warnemünde, Germany for opening and sub-sampling. The cores were opened lengthwise, scanned in a multi-sensor core logger and subsampled for radiocarbon, chemical and paleoecological analyses.

37

June 2014 M/S Fyrbyggaren

M/S Fyrbyggaren

Piston corer

Gravity corer

Piston corer

28

28

101

101

depth (m)

Water

5

0.72

~3

0.68

~3

length (m)

Core

N58˚59.616'

N59˚00.570'

N59˚00.573'

N59˚20.414'

N59˚20.414'

Lat

E17˚43.295'

E18˚02.289'

E18˚02.282'

E18˚47.138'

E18˚47.138'

Long

Papers I and IV

Papers I and IV

Papers I and IV

Papers I and IV

Papers II and IV

Papers II and IV

Results presented in

Table 2. Sediment cores used in this PhD thesis.

June 2014 M/S Fyrbyggaren

Gravity corer

44.1

Papers I and IV

Equipment

UPP5C

UPP5B

June 2014 M/S Fyrbyggaren

Piston corer

E17˚43.291'

Ship

Kanholmsfjärden

Kanholmsfjärden

UPP6A June 2014 R/V Skagerak

E16˚23.664'

Date

Ådfjärden UPP6D June 2012

N58˚59.629'

Core label

Ådfjärden PC1208

N58˚38.832'

Site

Himmerfjärden

0.77

Papers I and IV

4.5

E16˚23.636'

Papers III and IV

45

N58˚38.834'

16˚31'26''E

Papers III and IV

24

0.70

56˚34'23''N

16˚31'26''E

Paper IV

Piston corer

23.9

5.78

56˚34'23''N

E17˚57.37'

Paper IV

Gravity corer

Gravity corer

31

0.52

N57˚58.64'

E17˚57.56'

R/V Skagerak R/V Skagerak

Piston corer

31

7.94

N57˚58.74'

R/V Skagerak

June 2012

R/V Ocean Surveyor

Gemini corer

218

0.42

June 2012

GC1212 August 2011

R/V Ocean Surveyor

Gravity corer

203

June 2012

Bråviken VG31L August 2011

R/V Maria S. Merian

Gravity corer

PC1205

Gåsfjärden VG31D March 2017

R/V Maria S. Merian

GC1219

Gåsfjärden MSM62-1-60SL

March 2017

Bråviken

w. Gotland Basin MSM62-1-60-2MUC

Himmerfjärden

w. Gotland Basin

35

MATERIAL AND METHODS

Chronologies In order to correctly interpret results from sediment archives, and to be able to compare results to other published records and timings of climatic or historical events, it is highly important to establish reliable chronologies. Age models have been constructed using a combination of different radiometric methods. The youngest sediments, 100 years earlier in Paper IV than in Paper I). Considering the uncertainties of various aspects in dating and age-depth modeling, it is a strength of this study that it includes results from several records.

Diatoms as a proxy for environmental change Diatoms are single-celled algae that can be found in almost every aquatic habitat. The diatom cell wall is impregnated with silica (SiO2) and these walls are called frustules. Frustules are made up of two halves, valves, which fit together, resembling the structure of a petri dish with lid (Round et al., 1990). There are a number of reasons to work with diatoms. They are found in virtually all aquatic environments and often at high abundances and species diversity. The species composition of diatoms is very sensitive to changes in the environment, for example pH in lakes, salinity, tempe41

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

rature (as ice algae or sensitive to changed stratification), water depth and nutrient availability. Because of the silica walls of the diatom cells, they are preserved in accumulated sediments. All these factors combined are what make the diatoms excellent paleoenvironmental indicators (Smol and Stoermer, 2010). Diatoms can live either as plankton or attached to a substrate. How the life-form of diatoms can be used as a proxy for water quality is further elaborated in the Interpretation and Discussion section. Diatom species composition is presented as relative abundances in the diatom diagrams. Vegetative cells of Skeletonema and Chaetoceros spp. were counted when possible but left out from the base sum of diatom relative abundance because of mass blooms in some levels and possible fluctuating preservation due to their very thin frustules. Due to difficulties with species delimitations and possible hybridization, Diatoma spp. were merged. For the time frame covered by this thesis, diatom preservation was not a limiting factor. Based on visual inspection of diatom frustules during analysis, the preservation of diatom valves was considered sufficient in all analyzed samples. However, in some cases a poorer preservation with substantial dissolution of valves was noticed downcore. This would have been a problem in several sites if the focus would have been on a longer time perspective than 2000 years. Even though dissolution was not a problem, broken valves has been a challenge. The counting of diatom valves was consequently carried out according to the method described by Schrader and Gersonde (1978), so the results should not be affected by the number of broken valves. However, in some samples it was difficult to assign specimen to species level, and they are therefore merged into genus and presented as e.g. Epithemia and Mastogloia spp. All identified diatom species are listed in Supplementary table 1. Chaetoceros spp. is one of the most abundant and diverse marine planktonic diatom genera in the world oceans (Malviya et al., 2016). The lightly silicified frustules of the vegetative cells are rarely preserved in sediments, but their heavily silicified resting spores can be found. They form under unfavorable conditions; nitrogen deficiency after a bloom event in particular has been reported to be a crucial factor in forming resting spores for some Chaetoceros species (Oku and Kamatani, 1997). All Chaetoceros spp. resting spores were counted but left out from the base sum of diatom counts. The concentration (absolute abundance presented as number of valves per gram dry sediment) of Chaetoceros spp. resting spores is presented in each diatom relative abundance diagram.

Lab procedure – diatom analysis To clean the diatom valves and make permanent slides a quantity of approx. 0.1 g freeze-dried sediment was weighed and left overnight in 30% H2O2 and a couple of drops of 10% HCl to remove carbonates. The next day, the samples were put on a hotplate to oxidize organic matter until reaction had occurred and the samples had

42

MATERIAL AND METHODS

calmed down. The beakers were filled with de-ionized water. They were left overnight and were then rinsed several times over the following days using decantation (settling time 1 cm/hour) to get rid of clay particles. Diluted ammonium (0.5 ml of 25% NH3 per l water) was added to dissolve aggregates of particles and to keep the clay in suspension. After the last rinse, 1 ml of a microsphere stock solution with a concentration of 5.5603 x 106 microspheres/ml was added (Battarbee, 1986). The samples were then pipetted onto coverslips (#1 thickness) and left overnight to settle and dry. The following day, permanent slides were prepared using Naphrax™ as mounting media. Diatoms were analyzed under a light microscope using differential interference contrast and magnification x1000 with oil immersion. A minimum of 300 diatom valves were counted at each level. Floras used for identification include CleveEuler (1955, 1953a, 1953b, 1952, 1951), Krammer and Lange-Bertalot (1991a, 1991b, 1988, 1986), Snoeijs (1993), Snoeijs and Balashova (1998), Snoeijs and Kasperovičienė (1996), Snoeijs and Potapova (1995), Snoeijs and Vilbaste (1994) and Witkowski et al. (2000). Absolute abundance (concentration) of diatom valves was calculated as: 𝑚𝑖𝑐𝑟𝑜𝑠𝑝ℎ𝑒𝑟𝑒𝑠 𝑖𝑛𝑡𝑟𝑜𝑑𝑢𝑐𝑒𝑑 × 𝑑𝑖𝑎𝑡𝑜𝑚𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑 𝑚𝑖𝑐𝑟𝑜𝑠𝑝ℎ𝑒𝑟𝑒𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑

For the samples from Gåsfjärden, absolute abundance was not measured.

Geochemical proxies Geochemical proxies in this thesis include total organic carbon content (Corg), total nitrogen content (TN), and stable isotopes of nitrogen (δ15N) and carbon (δ13C). In the Baltic Sea, stable nitrogen isotope (δ15N) values at the sediment surface are generally higher in coastal areas (5-13‰) than in the open Baltic Sea (3-5%) (Voss et al., 2005; 2000). The cause for the elevated values in coastal areas is suggested to be anthropogenic nitrogen delivered by rivers and diffuse runoff (Voss et al., 2005). An increased δ15N signal has been interpreted as a result of eutrophication in different areas of the Baltic Sea (Ellegaard et al., 2006; Jokinen et al., 2018; Savage et al., 2010; Struck et al., 2000). Due to the differences in carbon sources for phytoplankton and land plants, they produce organic matter with different stable carbon isotope (δ13C) values. Marine phytoplankton produce organic matter with δ13C values of -18‰ to -22‰, while land plants show values of -25‰ to -28‰ (Kandasamy and Nagender Nath, 2016). However, in a coastal and brackish system where the primary producers are a mixture of marine and freshwater algae, the interpretation of δ13C becomes more complicated. According to Lamb et al. (2006), δ13C values of land plants and freshwater algae overlap, with land plants displaying values of -21‰ to -32‰, and freshwater algae 26‰ to -30‰. The C/N ratio can then aid in interpreting the sources of organic matter. Since terrestrial plants consist to a high degree of nitrogen-poor cellulose and

43

TRACING ENVIRONMENTAL CHANGE AND HUMAN IMPACT

lignin, the C/N ratios are usually >12. In algae, the C/N ratios are