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The results of an integrated study of the radiocesium behaviour in Lake Tapeliai by using not only conventional data on radiocesium activity concentrations in lake water and sediments but also a complex data set on seasonal variations and vertical profiles of standard water variables are presented. Radiocesium activity concentrations in lake water and a vertical structure of the water column considerably depend on radiocesiumenriched inflows of the coloured waters from the lake's swampy watershed. The global fallouts are mainly responsible for radiocesium inventory in lake sediments, where maximum values of radiocesium are found in the upper part of the water column above the ~5.4 m depth. The maximum values of radiocesium inventories in lake sediments are consistent with the respective densities of its deposits in the nearest forest soils. The main process of sediment activation is a direct sorption of radiocesium onto the sediment surface. Sedimentation rates in the lake mainly vary in the range of 3.5-5 mm yr -1 . The lake bottom feeding sources located mainly on the southern terrace as well as their related near-bottom flows reduce respective sedimentation and radiocesium inventories in sediments. The thermal regime of the near-bottom water in Lake Tapeliai in winter strongly depends on the meteorological conditions in autumn and may be classified as varying from super warm to moderately warm. Under conditions of a super warm regime, the elevated radiocesium concentrations in the near-bottom waters may be partially due to the thermodynamic mechanism of its release from the bottom sediments.

The results of an integrated study of the radiocesium behaviour in Lake Tapeliai by using not only conventional data on radiocesium activity concentrations in lake water and sediments but also a complex data set on seasonal variations and vertical profiles of standard water variables are presented. Radiocesium activity concentrations in lake water and a vertical structure of the water column considerably depend on radiocesiumenriched inflows of the coloured waters from the lake's swampy watershed. The global fallouts are mainly responsible for radiocesium inventory in lake sediments, where maximum values of radiocesium are found in the upper part of the water column above the ~5.4 m depth. The maximum values of radiocesium inventories in lake sediments are consistent with the respective densities of its deposits in the nearest forest soils. The main process of sediment activation is a direct sorption of radiocesium onto the sediment surface. Sedimentation rates in the lake mainly vary in the range of 3.5-5 mm yr -1 . The lake bottom feeding sources located mainly on the southern terrace as well as their related near-bottom flows reduce respective sedimentation and radiocesium inventories in sediments. The thermal regime of the near-bottom water in Lake Tapeliai in winter strongly depends on the meteorological conditions in autumn and may be classified as varying from super warm to moderately warm. Under conditions of a super warm regime, the elevated radiocesium concentrations in the near-bottom waters may be partially due to the thermodynamic mechanism of its release from the bottom sediments.

Our paper presents data on the annual course of the temperatures of surface sediments in deep bottom areas of a lake that has been assigned a "moderately-warm" designation. It is shown that the highest temperatures of those sediments are reached in autumn when near-bottom waters are involved with the processes of the gravitational mixing of the water column. Mixing conditions of the water column were estimated using additional information on the vertical profiles of standard water variables (pH, oxygen concentration, and conductivity). It is shown that depending on the amount of heat stored in the sediments of the deep bottom areas and the thermodynamic processes of its redistribution, the lake may be designated "super-warm" or "moderately-warm. " This implies that existing thermal classification of lakes is too formal and does not account for the complexity of thermodynamic processes of heat redistribution. Measurement data is evidence of the implication of the inflows from the bottom-feeding sources in the formation of the highly mineralized near-bottom water layer in those bottom areas.

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