THERMAL STRUCTURE OF THE LAKES OF THE NORTH-WEST OF RUSSIA DURING THE FREEZE-UP PERIOD
On the basis of analyzing long-term field data, we investigated the vertical thermal structure of 66 lakes of the North-West of Russia during the freeze-up period. The largest variability in thermal structure is characteristic for low-drainage shallow water bodies. Detailed data were obtained at self-contained buoy stations located in a small Lake Vendyurskoe (southern Kare lia) were used to identify water temperature changes on the horizons for a winter season as well as making assessments of heat flows at the water-bottom and water-ice interfaces. The interannual water temperature variability on the horizons reaches 2 °C. The main geographical factors influencing the formation of thermal stratifications in the 66 lakes used in the study during the winter period are their mean depth, area, water residence time and geographical latitude. The largest vertical water temperature gradients are characteristic for the group of the smallest and shallow lakes, in the bottom layers of which the water temperature exceeds the temperature of maximum density, whereas in the deep lakes (more than 15 m) the water temperature is below 4 °C. The lowest values of water temperature are observed in large lakes. The water temperature in the upper layer (up to 10 m) of drainage water bodies also decreases to 0-1 °C as the result of the removal of heat with the river discharge. According to the thermal stratifications, the lakes are categorized as small (shallow, deep and drainage lakes), medium-sized and large shallow and large deep lakes. The suggested regression model permits a typical water temperature to be assessed on standard horizons at the end of a winter season for any water body in the study region using available geographical information. The verification of the model is done from independent data for eight lakes of Finland.