Author(s): Matti Lepparanta
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Abstract: In the boreal zone, tundra, and high mountain regions in lower latitudes, lakes freeze over in winter. The ice season may last more than half a year, and the thickness of the ice can reach one meter. Solid ice cover has been an excellent base for traffic across lakes and to transport cargo. Until about mid-20th century the cold content of lake ice sheet was utilized. The evolution of a lake ice season is primarily a thermodynamic process, where snow accumulation is an essential factor. Ice breakage and mechanical displacements take place in very large lakes and also in smaller lakes when the ice is weak. Decay of lake ice cover is a thermo-mechanical process, starting from the shoreline. Melting of near-shore ice releases the ice from solid boundaries, and the ice cover may shift as forced by winds and currents. Solar radiation provides a strong downward flux of heat, and the ice melt water with its impurities is released into the water. Lake ice formation and growth have influence on the water quality as well as on the lake ecology. The concentration of suspended matter decreases from the water column during the ice season. At the time of ice breakup, a peak load of impurities takes place. Sunlight is the main limitation for life under an ice cover. Primary production is paused for the polar night, and in the case of a thick snow cover on ice, the dark season can be much longer and extend to sub-polar latitudes. Another critical factor is that the ice cover reduces the level of dissolved oxygen in the water body by cutting the influx from atmosphere. This results sometimes in anoxic conditions in the deep water and fish kills. Lake ice is a degree more simple as compared with sea ice and river ice. In particular, fresh water lake ice is poor in impurities, and lake ice cover is mostly immobile and circulation in the water body beneath the ice cover is weak. But lakes with strong through flow may have river ice type processes, ice structure in brackish and saline lakes is similar to sea ice, and drift ice occurs in very large lakes.
Year: 2014