Lake Stratification and Turnover

By Tess Flynn-Belles, Natural Resource Specialist

    39.2°F.  What is the significance of this temperature? We hear “32°F” and automatically think, “Ah, yes, that’s when water freezes”. So, why is 39.2°F so unique? You may be aware that as water cools, it becomes denser and less dense when it warms. A practical example of this would be if you’ve ever tried to see how deep you can swim down in a lake on a hot summer’s day. You can feel the temperature change from warmer to cooler as you dive deeper. In winter, however, once water reaches the temperature of 39.2°F, its molecules actually start to expand and become lighter again, sitting on top of the warmer water!! At this point, the water can continue to cool to 32°F and turn to ice. If you are a pond or lake owner, the following facts will help you begin to understand why, during certain times of the year, your pond goes from crystal clear to cloudy and brown in a matter of days.
​

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.