The Effect of Morphometric Characteristics and Geographic Zonality on Thermal Stratification of Lakes N. I. Pal’shin, T. V. Efremova, and M. S. Potakhin Выполнила: Асанова Анастасия

Introduction Water temperature is among the major factors that govern the rate of chemical and biological processes in water bodies. Its vertical distribution determines exchange with energy and with all nutrients. Variations in temperature is the resultant heat flux in energy exchange between a water body and the atmosphere and the lake morphometry affects the processes of water mixing water mass. More than 275 thousand lakes are located in the territory of the Northwestern Russia and Finland, only about 100 of them are subject to regular observations. So, the task was: to establish the regularities in variations in the thermal regime of different types of water bodies depending on the physico–geographic factors and to develop methods for their assessment based on the analysis of field data.

Epilimnion Field data collected by K. Patalas (1984) used to examine dependence of epilimnion thickness h e on the effective wind fetch L for small and large lakes located in different climatic zones, including northwestern Russia : h e = ϕ logL (62°N) h e = 4.6L 0,41 (from 54° to 59°N)

Thermocline Lathrop and Lillie proposed the empirical formula for the assessment of thermocline depth h t in small lakes in the period of maximum heat content for ~60 lakes in the central Northern America The density differential is taken equal to 1.4 kg/m3.

Types of thermal stratification The examination was carried out by using field data collected at Roshydromet hydrological stations (1958–1989 ) in 62 water bodies of northwestern Russia and 8 lakes in Finland. The ratio of the wind-induced mixing depth to the maximum depth of a water body reflects the type of its thermal stratification. hypothermic, Θ < 0.3 metathermic, 0.3 ≤ Θ ≤ 0.7 epithermic, Θ > 0.7

The values of the thickness of epilimnion he, depth of thermocline ht, and the depths required for the formation of thermal stratification in lakes hs, h1, h2, by data from different sources. Summary table

Biological summer The “biological summer” is assumed to be the period when the temperature of the top water layer in lakes exceeds 10°C The data used to assess the dependence of “biological summer” characteristics on various geographic factors included 10-day averages of daily data on water surface temperature in 52 lakes in the European Russia, collected from 1945 to 1980 at Roshydromet stations,and two lakes in Finland studied in 1961– 1991.

Biological summer

Conclusions Empirical dependences of the thermal regime of lakes on different geographic factors were established by the generalization of long-term observational data on water temperature collected at Roshydromet stations in different types of water bodies in northwestern Russia. The duration of the biological summer averages ~80 days in lakes of Kola Peninsula 100– 120 days in Karelia, and 120–140 days in the area south of 60° N. The duration of biological summer obviously decreases for large, deep water bodies.

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