| Summary: | We studied vertical distribution of oxygen under the ice of 5 medium-sized, morphologically variable lakes that cooled well below 4 °C before freezing. In the upper part of the water column, dissolved oxygen and dissolved inorganic carbon concentrations generally remained vertically almost uniform, but in the deepest water, concentrations changed rapidly near the bottom. The coincidence of the changes with an increase in deep water temperature shows that they were due to advection of water made heavier by the heat flux from the sediment. Consequently, water with low concentrations of dissolved oxygen and high concentrations of dissolved inorganic carbon accumulated in the deepest part of the lake (i.e., outcome of sediment respiration on a large area was focused to a limited volume of the lake). This conclusion was supported by the results of an experiment in which water samples incubated at different depths showed no vertical differences in oxygen consumption. Our results show that temperature-dependent hydrodynamics affect under-ice oxygen conditions in medium-sized temperate lake basins. Interannual variation in water temperature and differences in morphology between lake basins probably cause significant variations in the accumulation of water in the deepest layers during winter.
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