Some Physical and Environmental Aspects of Shallow Ice Covered Lakes : Emphasis on Lake Vendyurskoe, Karelia, Russia

Sammanfattning: The ice cover presence on the surface of a lake insulates the water body from the atmosphere. This prevents or reduces the influence of processes which depend on the exchange between the atmosphere and the water surface. This implies significant reduction or elimination of some processes taking place at the open water surface; for instance, a substantial reduction in solar energy penetrating to the water body. This reduction of heat input from the atmosphere is partly compensated for by heat flux from sediment. Wind induced currents are replaced by oscillating currents due to wind action on the ice cover. Sediment heat flux generates slow density currents along sloping bottoms. This study is almost exclusively devoted to Lake Vendyurskoe, a small lake in the Russian Republic of Karelia. On Lake Vendyurskoe, ice is usually formed in November-December and it remains for a duration of six to seven months. The maximum ice thickness is 60-80 cm. The ice growth can be well described using a degree day equation. The water temperature was measured continuously in several vertical profiles in the lake. Heat fluxes at the water-ice interface and at the sediment-water interface were determined by measuring temperature gradients. Due to gain of heat from the sediments, the heat content of the ice covered lake increased throughout the ice covered period. At the time of freeze-over, the water temperature is less than 0.5 oC all the way to the bottom. In April, the temperature profile is almost linear from 0 oC at the underside of the ice to 4 oC or more at depth below 8 m. The heat flux conducted from water to ice soon after ice formation is about 1 Wm-2 , but it increases in the course of the winter and can reach 5 W/m2 in early spring. The sediment heat flux to water increases throughout the winter and is highest in early winter and at shallow bottoms, where the sediment is warmest and the water is coldest. Typical values are, directly after the ice formation, 2-6 W·m-2 and by early spring 1-2 W·m-2. Oscillation of the ice cover due to wind action produces small horizontal currents and mixing. These currents were measured during several campaigns with an acoustic meter. They had an average magnitude of about 2 mmsec-1 and a maximum value of 7 mm.sec-1.In early spring and in absence of snow on the ice, considerable amount of solar radiation penetrates the ice cover and introduces hydrodynamic instability and convective mixing. A vertically homogeneous temperature layer develops, which grows downwards. The depletion of oxygen and development of dissolved oxygen profiles during winter were investigated. The dissolved oxygen content at the time of freeze over was close to saturation over the entire water body. The dissolved oxygen reduces throughout the winter, but much more at deep water than at shallow water. Near the sediments, the concentration drops to low values. It is found that diffusion of oxygen into the sediments is the dominating consumption process. When there is convective mixing under the ice in early spring, the dissolved oxygen is redistributed over the homothermal layer. This can, because of the water movements, be associated with increase of the diffusion into the sediments which leads to a decrease of dissolved oxygen also at shallow water. Keywords: convective mixing, dissolved oxygen, heat exchange ice–water, ice cover, oscillatory currents, sediment heat fluxes, water temperature