During spring, summer and autumn, the upper layer (epilimnion) of a lake is warmed by the sun, and the wind keeps it well mixed and enriched in oxygen. The lower layer (hypolimnion) is cooler, and has less oxygen.
In winter the epilimnion cools, and there is complete mixing of all the water in the lake. Such annual mixing allows oxygen to be spread throughout the lake, and is an essential part of keeping the lake alive.
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