Evolution of the temperature of the water column of boreal lakes against the background of changes in the regional climate

The results of studies of the effect of changes in the regional climate on the dynamics of the thermal regime of two small boreal lakes with different types of mixing during the open water period according to the data of long-term observations are given in the article. The main attention is paid to the stage of spring mixing (including spring under-ice convection and the stage of homothermy after breaking of ice), and revealing its role in the development of the thermal regime of lakes at subsequent stages of the annual cycle. It is shown that the spring under-ice convection began on lakes earlier and lasted longer in years with abnormally high winter and spring air temperatures than in years with temperatures of these months close to the climatic norm. This could provoke the formation of direct stratification by the end of the ice period and the disappearance of the stage of homothermy after ice-break. The duration of the spring homothermia phase is the most critical for dimictic lakes, since it affects the summer temperature of the hypolimnion and can therefore determine both the thermal and gas regimes at subsequent stages of the annual cycle. The obtained results allow us to conclude that, against the background of anomalously warm winter and spring months, polymictic lakes can acquire features of dimictic ones, leaving stably stratified after the melting of ice, and losing the stage of spring homothermy. Reduction of the ice-period on dimictic lakes due to earlier ice-off may lead to both a decrease and an increase in the duration of summer stratification, depending on the weather conditions of late spring and early summer.

boreal lakes, water temperature, spring mixing, under-ice convection, spring homot-hermia, mixing regime

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