Long-term changes in summer stratification of a shallow polymictic lake by climate change and anthropogenic water...

Climate change has a crucial impact on lake ecosystems, making the quantification of its long-term effects essential. Although shallow lakes are more exposed to external changes than deep lakes, they remain underrepresented in scientific literature, especially in examining the impacts of climate change on weak diurnal thermal stratification. This study investigates the long-term effects of climate change and water level regulation on a large, but shallow polymictic lake, Lake Balaton's vertical temperature dynamics on a diurnal scale employing a fully physics-based one-dimensional model. The model was calibrated and validated using in situ measurements and applied to simulate the summer seasons over a 40-year period. Our findings indicate that both climate change and water level regulation significantly affect the intensity of the shallow lakes' temperature dynamics. Considering their synergetic effect, slightly higher water levels amplify the impacts of climate change, substantially intensifying stratification and causing longer stratified periods. Furthermore, we found that the determinative parameters for stratification exhibit temporal variability, with shortwave radiation dominating in June and August, and water depth in July. Analysis of the mixed layer depth indicates its long-term stability, preserving its daily dynamics through the decades with no appreciable variations. Lastly, it was found that climate change also affects the turbulent heat fluxes, resulting in an enhanced evaporation at the expense of sensible heat flux and thus a declining Bowen ratio.