Lake

Water circulation is the mixing of water in a lake. Water mixes at the surface, within the top layer, the epilimnion, and among layers. The bottom layer of water is called the hypolimnion, and the water between the hypolimnion and epilimnion makes up the metalimnion. The metalimnion is also called the thermocline, because a drastic temperature change occurs the lower one goes in it. Mixing is facilitated by wind at the epilimnion and is possible due to water density variation between layers. When layers mix and change places, a lake is said to turn over. Turnover occurs when water in an upper layer is heavier, or denser than the layer of water underneath it. Lakes that turn over once a year are said to be monomictic. Lakes that turn over twice a year, once in spring and once in fall, are called dimictic. Lakes that turn over at least once a year are called holomictic. Some lakes do not fully turn over at all due to high salt content; the high salt lower layer prevents hypolimnic turnover in these meromictic lakes.

The most controlling factors in lake circulation are changes and differences in water temperature; however, salinity, wind, and lake shape each have a role in circulation as well. Bowl-shaped lakes tend to turn over more easily than oxbow lakes. Water temperature determines water density which, in turn, accounts for turnover. Water is at its minimum density in the form of ice. Warmer water is less dense than cooler water until cold water reaches 39.2°F (4°C), when it gets lighter. Deeper water is generally both denser and colder than shallower water—other than ice.

In fall, the surface is cooled in proximity to the surrounding air. As this surface water cools, it sinks, mixing throughout the epilimnion. The epilimnion continues to cool and eventually matches the metalimnion in temperature. Wind mixes these two water layers, which then cool to temperatures lower than the hypolimnion temperature. Then the hypolimnion water mixes in with the rest of the water and rises to surface. If hypolimnion water was oxygen-depleted, then it will obtain more oxygen at the surface during the winter. During winter, the hypolimnion is warmer than the epilimnion-unless the entire lake freezes. This process is called fall turnover.

In spring, the ice warms, melts, and mixes within the epilimnion. As the entire epilimnion warms, it becomes denser than the hypolimnion, the whole lake turns over, and mixing takes place. This is spring turnover. As summer progresses, the metalimnion warms and the three temperature layers are apparent until fall. If snow has piled high onto the surface over the winter and blocked photosynthesis, then much lake life may die, resulting in a phenomenon called winterkill.

Tremendous variability exists in turnover patterns and date of onset. Polar lakes warm later in spring and cool sooner in fall than similar lakes in tropical regions. Ice may only melt away from some lakes for two months a year, resulting in slow fish growth compared to warmer climates. High altitude lakes also warm later and cool sooner than equivalent low altitude lakes. Tropical, high altitude lakes lose heat continuously, do not develop layers, and overturn continually, whereas sub-tropical, low altitude lakes that never freeze only layer in summer and turn over in winter.