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Author(s): Deanne Erdmann MS

Thermal Stratification in Lakes

Large bodies of water often have layers with different temperatures. In the summer, the top layer (epilimnion) is warmer than the bottom layer (hypolimnion). The boundary between the upper and lower layers is called the thermocline. Winter approaches and the upper layer cools, eliminating the thermocline. Winds create circulation and the top and bottom regions will mix (called fall overturn). In the spring, the temperature of the upper layer (now the coolest layer) warms and the layers mix in what is called the spring overturn. The mixing of oxygen and other nutrients between the upper and lower regions during seasonal overturns supplies essential ingredients for organisms in lake ecosystems.