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Water is densest at 4°C.
In winter, the coldest water in the lake (0°C) lies just
below the surface ice; water is progressively warmer at deeper levels of the lake,
resulting in thermal stratification.
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In spring, as ice melts, the surface water warms to 4°C and sinks, eliminating temperature differences.
Winds mix the water, bringing oxygen to the bottom waters and nutrients to the surface.
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In summer, the lake regains thermal stratification,
with warm surface water separated from cold bottom water by a narrow zone of rapid temperature change,
called a thermocline.
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In autumn, surface water cools and sinks below the underlying layers,
remixing the water until the surface begins to freeze and the winter temperature profile is reestablished.
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