Turnover is a phenomenon that occurs in terrestrial bodies of water, such as lakes and ponds, in which the water near the surface of the lake (epilimnion) is replaced with the water near the bottom of the lake (hypolimnion) to establish a homogenous mixture. Lake turnover occurs during both the fall and spring when the thermocline (temperature gradient relative to lake depth) is nearly uniform throughout the lake. As temperature of water directly affects density of water, an even water temperature through the entire depth of a lake allows for water to mix due the epilimnion and hypolimnion having very similar densities. With regard to the thermal properties of water and the role density plays, lake turnover has the power to cycle important nutrients and gases for aquatic flora and fauna throughout a lake.
During lake turnover in the fall, the temperature of the epilimnion will begin to lower as surrounding air temperatures decrease. The cooling surface water will begin to increase in density as the water molecules are condensed and this heavier water will sink toward the bottom of the lake. The sinking of the epilimnion will cause displacement of the warmer, less dense hypolimnion near the bottom and force it to rise to surface (lower density substances always sit upon higher density substances). The less dense bottom waters will carry with it large amounts of nutrients that have accumulated there from decomposing organisms, sediment, and runoff from land.
During lake turnover in the spring, the ice that once covered the surface waters during the winter is beginning to melt and exposing the epilimnion to oxygen and wind. The melting ice allows the water to increase in density again and sink down towards the bottom, while also carrying oxygen. This is similar to fall turnover as the warmer hypolimnion near the bottom can now be lifted upward carrying nutrients; however, the dense, sinking waters are replenishing the near anoxic waters with oxygen that was once beneath ice. The absence of ice also allows the body of water to be exposed to wind from the surrounding air, which aids in the mixing process of both nutrients and gases.
This is one of three articles written by Abby Good. Abby is a rising senior at the University of Iowa, majoring in Environmental Science with a focus in bioscience and minoring in biology. Her career interests are in conservation and air quality.