Giulia Wienhues, Hendrik Vogel, Wojciech Tylmann, Martín Grosjean
Lakes are highly sensitive to environmental changes, making them valuable archives for reconstructing past responses to human disturbances and ecosystem dynamics. While the effects of climate warming on lake oxygenation and primary production are well documented, the impacts of long-term land-use changes - particularly deforestation - on lake mixing and biogeochemical cycling remain poorly understood. This study investigates the response of Lake Rzęśniki (NE Poland) to Late Holocene land-cover changes, particularly the effects of massive 15th century deforestation, on lake productivity, redox conditions, and nutrient cycling. Our results indicate that closed forest canopy around the lake provides wind protection and supports long-term stable stratification, hypolimnetic anoxia and possibly efficient phosphorus recycling (through reductive dissolution) throughout most of the Early and Late Holocene at Lake Rzęśniki. After the 15th century CE, deforestation triggered a fundamental shift in the lake’s physical and biogeochemical state. The removal of the forest wind shield resulted in enhanced lake mixing, bottom-water oxygenation, and restructured the aquatic ecosystem. Enhanced soil erosion increased nutrient fluxes to the lake and eutrophication. 20th-century reforestation around the lake re-established the wind sheltering effect of the forest, renewed lake stratification, and hypolimnetic anoxia. This also increased aquatic primary production, suggesting possible chemical feedback mechanisms driving nutrient dynamics. Comparison with three other lakes shows that the responses of Lake Rzęśniki to forest cover changes are reproducible, systematic and a time-space transgressive regional phenomenon. Our results highlight the complex interactions between deforestation, lake stratification and anoxia, and eutrophication, providing critical insights into long-term anthropogenic impacts on aquatic ecosystems.