The effect of the heat interaction between a water column and sediments on the formation, development, and duration of existence of anaerobic zones in ice-covered lakes is estimated based on observational data from five frozen lakes located in northwestern Russia and North America. A simple one-dimensional model that describes the formation and development of the dissolved oxygen deficit in shallow ice-covered lakes is suggested. The model reproduces the main features of dissolved oxygen dynamics during the ice-covered period; that is, the vertical structure, the thickness, and the rate of increase of the anaerobic zone in bottom layers. The model was verified against observational data. The results from the verification show that the model adequately describes the dissolved oxygen dynamics in winter. The consumption rates of DO by bacterial plankton and by bottom sediments, which depend on the heat transfer through the water-sediment interface, are calculated. The results obtained allow the appearance of potentially dangerous anaerobic zones in shallow lakes and in separate lake areas, which result from thermal regime changes, to be predicted.

The consumption of oxygen in ice-covered lakes is analyzed and related to biological oxygen demand and sediment oxygen demand. An approach for computing dissolved oxygen concentration is suggested assuming horizontally mixed waters and negligable vertical dispersion. It is found that the depletion of dissolved oxygen is mainly due to the transfer of oxygen at the water/sediment interface. The morphology of a lake is very important for how fast the dissolved oxygen concentration is reduced during winter.

For 10 consecutive winters, measurements were carried out in Lake Vendyurskoe, Karelia, Russia. The aim of these measurements was to investigate some of the physical processes in this small shallow lake during its ice-covered period. Detailed measurements of ice thickness and condition, snow thickness, water temperature, salt content, and currents were developed into a database. Within this database are the winters 1994/95 and 1995/96, which are characterized by a very sharp change between positive and negative phases of the North Atlantic Oscillation (NAO). This note investigates and compares the effects of these two NAO phases on the regional climate and their relation to the thermal regime of Lake Vendyurskoe. It became clear that the ice-covered period length did not vary much from one winter to the other; however, a large difference was observed in the maximum average ice cover thickness and in the average water temperature at ice formation (e.g., larger thickness and colder water temperature during the 1995/96 winter).

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