How brownification and eutrophication effects a drinking water source – with Lake Bolmen, Sweden, in focus

Sammanfattning

The ongoing human impact in form of deforestation, urbanization and intense agriculture is affecting the natural hydrology and our freshwater lakes negatively. Both eutrophication and brownification has seen to increase with the ongoing environmental change which can potentially have large impact on our freshwater ecosystems. Using a mesocosm experiment, this study investigates the individual and synergistic effects between eutrophication and browning. Specifically with focus on phytoplankton and zooplankton abundance and community structure, as well as toxicity of cyanobacteria. The mesocosm experiment was run over 32 days in Lake Bolmen, Sweden in July-August 2021. The result from this study shows that zooplankton diversity decreases, and zooplankton abundance increases with brownification. A community shift from Rotifera to Copepoda and then to Cladocera was also noticed independent of treatment. Green algae were the most prominent phytoplankton group in the experiment, followed by diatoms and lastly Cyanobacteria. Both Green algae and Cyanobacteria had highest abundance in the treatment only simulating eutrophication. The highest toxicity per cell, for cyanobacteria, was however found in the treatment simulating brownification and in the Control. According to results gathered from this study, synergistic effect between eutrophication and brownification may not pose a big threat to zooplankton or phytoplankton abundance and community structure in Lake Bolmen. However, as global warming also is an ongoing problem which both phytoplankton and zooplankton have seen to be affected by, there is still need for further research. Any synergistic affects between brownification, eutrophication and warming need to be investigated as all three environmental changes will continue in the future.