A reconstruction of the effects of Post- Glacial Rebound on the lake system dynamics in the Baltic Basin since the Last Glacial Maximum to explain the high biodiversity anomaly. Image by NASA (1999) Alex Nap Universiteit van Amsterdam 04-07-2017, Amsterdam Supervisor: Dr. K.F. Rijsdijk Institute for Biodiversity and Ecosystem Dynamics Abstract The Baltic Basin has had a dynamic history since the Last Glacial Maximum 22 000 years ago. The post-glacial rebound following the retreating of the ice sheet changed the landscape of the peri-Baltic region significantly. This study aims at identifying the effects of these landscape changes on freshwater lakes surrounding the Baltic Basin, to help clarify the hypothesis that the connectivity between lakes and the Baltic Basin made an exchange of freshwater lake species possible. With digital elevation and ice sheet models in ArcGIS the connectivity and ice coverage of selected major lakes in the area were analysed. The results show when the selected major lakes became ice-free, if and how they were connected to the Baltic Basin and when they disconnected. The main conclusion drawn from this study is that some lakes in the peri-Baltic have been connected with the Basin for thousands of years and have been part in the dynamic changes of the Baltic Basin, suggesting that an exchange of species would have been possible until recently. Keywords: Baltic Basin, post-glacial rebound, Last Glacial Maximum, biodiversity, Fennoscandia, freshwater lakes. 1 Institute for Biodiversity and Ecosystem Dynamics Table of contents Abstract ................................................................................................................................................... 1 Introduction ............................................................................................................................................. 3 Methods and Data ................................................................................................................................... 6 Results ..................................................................................................................................................... 7 Overlook of the history of the Baltic Basin from 21 ka BP until 1 ka BP ............................................. 8 Closer look at some lakes’past .......................................................................................................... 14 Discussion .............................................................................................................................................. 19 1. Lake Districts ............................................................................................................................. 19 2. Peipus, Onega and Ladoga ........................................................................................................ 19 3. Lake Saimaa ............................................................................................................................... 20 4. Lake Vänern, Vättern and Hjälmaren ........................................................................................ 20 The post-glacial rebound and retreating ice sheet and stages of the Baltic Basin. .......................... 20 Improvements and the models’ relation to literature ...................................................................... 21 Consequences for biodiversity .......................................................................................................... 22 Conclusion ............................................................................................................................................. 22 Literature ............................................................................................................................................... 23 Acknowledgements ............................................................................................................................... 24 Appendix 1: Selection of lakes............................................................................................................... 25 2 Institute for Biodiversity and Ecosystem Dynamics Introduction The Baltic Basin has had a dynamic geographical history during the last glacial periods. During the Quaternary glaciation, the area around the Baltic Basin has been covered by ice sheets several times for thousands of years and this came with a continuous changing landscape (Lehman & Jones, 1991). According to Björck (1995) there are four main reasons why the scientific institutes, mostly from countries around the Baltic Sea - also peri-Baltic Region - are interested in the geographical past of the Baltic Basin since the Last Glacial Maximum – LGM – 22 000 BP: 1) The salinity records of since the Last Glacial Maximum – LGM - 22 000 BP, have varied widely for a relative short period of time and the subsequent species changes have varied as well. 2) There had been land bridges connecting and disconnecting different parts of the peri-Baltic Region and the waterbodies of the Baltic Basin, creating a way for flora and fauna to migrate to other areas. 3) The deglaciation events from the LGM until the forming of the current Baltic Sea as it is known today, it is the most recent large-scale glaciation of the Fennoscandian peninsula. 4) The Baltic Basin gives an extensive amount of varve chronology, allowing for detailed paleo- geographic conditions prevailing since the LGM. Figure 1: The area of The Baltic Basin (Björck et al., 2008) 3 Institute for Biodiversity and Ecosystem Dynamics The Baltic Basin is part of the Baltic Shield and is surrounded by the Fennoscandian peninsula. The depression that is today known as the Baltic Sea was formed by tectonically active horst and graben system that created the current low lying Finnish and Swedish lowlands (Sub-Cambrian peneplain) and the Baltic Basin (Gaál & Gorbatschev, 1987; Pira, Flodén & Mokrik, 2003). This basin was later filled with sediment and in the Miocene the Eridanos river system flowed through the Basin until the Middle Pleistocene, this could explain the slightly dendritic shape of the Baltic Basin, together with former tectonic activity (Pira, Flodén & Mokrik, 2003). During more recent, Quaternary, history the Baltic Basin has been covered three to seven times with an ice sheet, during the Elsterian, Saalian and Weichselian (Marks, 2004; Stewart & Lonegan, 2011). The focus in this study will be on the history of the Baltic Basin from the LGM until present. During the LGM the Fennoscandian ice sheet covered extensive parts of Northern Europe. An area from Northern Germany, Denmark, Norway, Sweden, Finland, The Baltic States and parts of North Western Russia were occupied by an ice sheet, measuring 3 – 4 kilometres in thickness in some parts (Svendsen et al., 2004). The weight of this ice sheet was large enough to press down the Earth’s crust considerable amounts. In the central part of the ice sheet this meant that the crust was pressed down more than 400 metres compared to today (Peltier, 1996; Fjeldskaar et al., 2000). This effect is known as isostatic depression. When around 18 000 BP the ice sheet started melting and retreating, the decline of the weight of the ice pressing down on the Earth’s crust began as well. Due to the elasticity of the crust, a vertical upward crustal motion started. This effect is called post-glacial rebound, this is a part of the isostasy theory (Peltier, 1996). This post-glacial rebound and declining size of the ice sheet led to some sudden and severe changes in the Baltic landscape. Not only through the post-glacial rebound but the glaciers also left behind huge end moraines in Central Europe (Marks, 2002; Kalm, 2012). Lobes on the end of the ice sheet carved out areas and left depression in the landscape, later filled up with lakes and sediments (Kalm, 2012). Underneath the ice sheet several forms of erosion and sedimentation occurred, examples are drumlins and subglacial meltwater riverbeds. All these effects of glacial erosion and sedimentation are still visible and significant in the landscape. This thesis will focus mainly on the forming of freshwater lakes and their relation to the retreating ice sheet and the newly formed waters of the Baltic Basin. In academic literature, there is consensus about a few stages of the Baltic Basin since the LGM, the system of Björck (1995; 2008) is considered a standard (table 1). Table 1: Stages of the Baltic Basin according to Björck (2008): Name of waterbody Salinity Age The Baltic Ice Lake Freshwater 15.0 – 11.6 ka BP Yoldia Sea Brackish 11.6 – 10.7 ka BP Ancylus Lake Freshwater 10.7 – 8.5 ka BP Littorina Sea / Baltic Sea Brackish/Salt 8.5 – present It is important to notice that the shift from Littorina Sea to the current Baltic Sea does not have a clear distinct border but rather a gradual transition from a larger waterbody containing about twice the amount of water and approximately 1¼ the size of the current Baltic Sea (Björck, Andrén & Jensen, 2008). The salinity of these stages has been researched through molluscs in sediments, hence the names of the last three lakes, these are all gastropod species. 4 Institute for Biodiversity and Ecosystem Dynamics As Björck (1995) postulated in the reasons why the Baltic Basin research is so extensive, floral and faunal migration is an important one. Recent research showed that there is an unexpected “hump” when one looks at the species richness graphs of freshwater lake Figure 2: The relation between