Abstract
Summary1. The impact of long thermal stratification events on some key properties in a polymictic lake was studied by determining the mixing regime of Müggelsee, Germany, using water temperature profiles taken hourly over 4 years. The period included two exceptional summer heatwaves.2. Long thermal stratification events lasted from about 1 week to 2 months, and exhibited a high variability in thermocline depth and stratification intensity within and between events.3. During stratification events, hypolimnetic oxygen concentrations strongly decreased while hypolimnetic SRP accumulation increased, depending on the duration and intensity of stratification and on hypolimnetic water temperature.4. The impact of stratification on the functional phytoplankton composition increased with increasing stratification duration, but was rather different for the heatwaves.5. Stratification events were followed by strong nutrient pulses into the euphotic zone and intense phytoplankton growth, particularly after the heatwaves. Hence, the influence of the climate extremes counteracted effects of reduced external nutrient loading.
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