Reconstructing and modelling lake mixing regimes in southern Finland

By 2100AD, it is predicted that approximately 16% of lakes worldwide will experience less frequent mixing and become permanently stratified (meromictic) as a result of climate change. Within the Arctic and subarctic, it is anticipated that increases in global air temperature will be magnified leading to strong feedback effects on the climate system, drastic changes in ecologically sensitive aquatic systems and increasing carbon emissions from lakes. Finland contains a dense network of lakes of differing mixing regimes, including meromictic lakes, enabling the opportunity to understand their response to warming temperatures. Hydroclimatic reconstructions have been undertaken, however they have not been focused on mixing regime changes and they have not been combined with modelling efforts to fully understand changes across past, present, and future timescales. Here, we present preliminary results from reconstructions of lake mixing regimes from sediment cores and initial modelling results from five lakes in the Evo National Park, southern Finland. We take a multi-proxy approach to reconstruct lake mixing regimes, including analysis of geochemistry (micro-XRF), chironomids, bacterial pigments, radiocarbon dating and varve counting. Ultimately understanding how lakes have responded to past changes in climate will enable baselines to be established against which to assess any future changes and will also enable models projecting future mixing regime changes to have appropriate boundary conditions set. The General Lake Model, forced with field data provided by Lammi Biological Research Station and from two fieldwork seasons in 2023, is used to understand contemporary and predict future mixing regimes. This research provides a full chronology of lake mixing regimes from the mid Holocene until 2100 linking climatic events with mixing regime changes through combining palaeoenvironmental reconstructions and modelling efforts.