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In lakes and ponds with sufficient depth, when surface waters warm and become more buoyant in the spring and over the course of the summer, vertical differences in water temperature can cause the water column to stratify, or separate into distinct layers (Fig. 2). The top layer, known as the epilimnion, consists of warmer water and is generally where photosynthesis occurs. The bottom layer, or hypolimnion, usually consists of cooler, heavier water that can become depleted of dissolved oxygen by the end of summer. The metalimnion, also known as the thermocline, is a transition zone and acts as a barrier between the epilimnion and the hypolimnion keeping them from mixing. As the season changes from summer to fall, the warm, more oxygenated surface waters begin to cool, making them denser. When the epilimnion and the hypolimnion reach an equal density, the waters mix, or “turn over” (Fig. 1), bringing much needed oxygen to the bottom waters and allowing fish to return to deeper waters where they will spend the winter.
When mixing occurs, the water in the lake reaches an approximate uniform temperature throughout the water column. When the surface waters eventually reach 32°F and cap the waterbody with ice, everything beneath the surface of the ice never gets any colder than 39°F. During the winter months, stratification similar to that of the summer months will occur, but not to as great an extent. Because of the smaller temperature difference in the water column when spring approaches, the melting of the ice layer, winds, and storms are able to create spring turnover with little difficulty.
Shallow ponds and lakes may never stratify and turnover because winds keep the waters circulating throughout the year. Deeper lakes will appear clear during late summer, early fall, and early spring. As water temperatures change and turnover occurs, the waterbody will appear murky or dirty. This is because the sediment on the bottom of the lake has been disturbed by turnover and distributed throughout the water column. Monitoring the water temperature as well as the air temperature throughout the seasons may help you get a better idea when fall and spring turnover will take place.
Various staff at the Bradford County Conservation District