Environmental drivers and abrupt changes of phytoplankton community in temperate lake Lielais Svētiņu, Eastern Latvia, over the last Post-Glacial period from 14.5 kyr

Understanding the long-term dynamics of ecological communities on the centuries-to-millennia scale is important for explaining the emergence of present-day biodiversity patterns and for predicting possible future scenarios. Fossil pigments and ancient DNA present in various sedimentary deposits can be analysed to study long-term changes in ecological communities. We analysed recent compilations of data, including fossil pigments, microfossils, and molecular inventories from the sedimentary archives, to understand the impact of gradual versus abrupt climate changes on the ecosystem status of a regional model lake over the last ~14.5 kyr. Such long and complete paleo-archives are scarce in North-Eastern Europe. The study site lies in a sensitive area, both climatically and in respect to vegetation. Namely the maritime-continentality line runs west to east in the central Baltic area to NE Europe and its south- north transect lies within the gradual decay of the nemoral forest into a boreal environment. Therefore, the selected location is an ideal sampling point to decipher long term environmental changes in the temperate climate zone. The main objective of the present study was to find out external factors influencing phototroph dynamics at temperate Lake Lielais Sv etin ̧ u over the post-glacial period (~14.5 kyr). We were able to model climate change together with vegetation change and the appearance of anthropogenic forcing, either as a gradual change or as abrupt events that influenced the phototrophs, which are keystone groups within the lacustrine ecosystem. Most interestingly, the gradual increase of species richness of phototrophs was linked to similar increase in fungal parasites of the same group e phototrophs. Abrupt climate change in the Late Glacial period caused abrupt events in the ecosystem but equally abrupt events were caused by gradual changes during the stable period of the Holocene Thermal Maximum (HTM). In addition, we highlight the increased frequency and degree of perturbation in pristine lakes due to low impact human activity over a larger region. Both observations demonstrate an impaired relationship between gradual external drivers and ecosystem response and apply to future scenarios of climate warming and increased human impact in north-eastern Europe. lmar Tõnno and Rene Freiberg were supported by the Estonian Research Council (ETAg) IUT21-2, PRG705, PRG709 and PRG1167. Veljo Kisand was supported by grants PUT134 and PUT1389 (ETAg) and Institute of Technology, University of Tartu basic funding grant. Siim Veski was supported by IUT1-8 and PRG323 (ETAg). Normunds Stivrins was additionally financially supported by the Latvian Council of Science project No. LZP-2020/2e0060. lmar Tõnno and Rene Freiberg were supported by the Estonian Research Council (ETAg) IUT21-2, PRG705, PRG709 and PRG1167. Veljo Kisand was supported by grants PUT134 and PUT1389 (ETAg) and Institute of Technology, University of Tartu basic funding grant. Siim Veski was supported by IUT1-8 and PRG323 (ETAg). Normunds Stivrins was additionally financially supported by the Latvian Council of Science project No. LZP-2020/2e0060.