Fish & Water - Sounds Wild
Lake Turnover

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Lake turnover

On a sunny day in late May in Interior Alaska, a lake is turning over. Covered with ice for the winter, over the past month the ice has been melting and the lake has been warming. The ducks and geese that have stopped on their spring migration north don't care about lake turnover, but the fish do. Lake turnover mixes the lake water, distributing nutrients from the bottom upward and oxygen from the surface down, and that's benefits the lake's aquatic inhabitants.

Most liquids become denser as they cool and are most dense when they freeze or solidify, but not water. Ice floats because it's less dense than water. If water acted like most liquids, during winter, ice would form on the surface of a lake and sink until the lake was frozen solid. But because ice floats it caps the lake and insulates the water from the frigid air.

Water is most dense at 39 degrees. Water that is warmer or colder than 39 degrees is lighter and this causes "turnover" in our lakes every spring and every fall.

In the fall, cold air chills the surface water, and at 39 degrees, it sinks below the relatively warmer water until the entire lake has "turned over." Once the lake has turned over and is at a relatively equal density, ice will form on the surface. In the spring the same thing happens in reverse - as the ice melts surface water warms to 39 degrees and sinks until the entire lake has turned over once again.