Parameters of Water Column Stability in a Small Polymictic Lake in Years of Different Weather Conditions

The regime of lake mixing has a considerable effect on many aspects of the functioning of aquatic ecosystems, which increases the importance of identifying changes in water mass stability under the effect of climatic factors. This study is focused on the stability of the water mass of a small Vendyurskoe Lake (southern Karelia) in the open-water season in years with various weather conditions. Water temperature measurements in 2008–2013 and 2015–2022 at an autonomous station (thermistor chain) in the central vertical of the lake were used to assess the duration of stratification and the stability of water mass according to various criteria, including the temperature difference over water column of 1 and 2°C, and by threshold values of Schmidt stability, Wedderburn number, and Lake Number. To study the role of radiation mixing in weakening the stability of the lake water mass, the seasonal and interannual variability of the temperature difference between the upper layer of the lake and the air was analyzed using ERA-5 reanalysis data. For detail analysis of this parameter, an appropriate density function was constructed for each year, and its major characteristics were calculated. It was shown that, despite the continuing regional warming and an increase in the temperature of air and the top water layer in Vendyurskoe Lake, no increase in stability was recorded in 2008–2022. Limiting factors can be an increase in convective mixing due to radiative cooling, as well as increased wind load on the water body.