Parameterization of surface water temperature and long-term trends in Europe's fourth largest lake shows recent and rapid warming in winter

Changes in the ice phenology, seasonal temperature and extreme events are consistent evidence of climate change effect on lakes. In this study, we analyzed multiannual variability, determined long-term trends and detected changes in the frequency of extreme events in the surface water temperature (LSWT) of Lake Peipsi (Estonia/Russia) for nearly seven decades (1950-2018) and aimed to trace how the LSWT responded to the climate change. Dynamic water temperature parameters were calculated using the smoothed water temperature curve fitted to daily water temperatures. Our results showed that, although the average LSWT did not increase significantly on an annual basis since 1950 it rose rapidly in the winter season during the last decade (similar to +0.5 degrees C). Ice formation exhibited a marked (similar to 15 days) delay since 2007 resulting in a longer open water period. Extreme LSWT events did not occur more frequently. We noticed however significant fluctuating in winter LSWT in time series, starting from 2007 and also causing an increase in stochasticity. The consequences of the on-going winter warming and changes of ice cover phenology are expected to be crucial for Lake Peipsi ecosystem functioning and impact on lake biota, especially temperature-sensitive native fishes.