Explaining lake turnover

Published 1:22 am Thursday, June 11, 2009

By Staff
The other day a reader e-mailed wondering about the why's, how's and wherefores of lake turnover. He and many other fishermen and lake property owners have noticed a number of lakes here in southern Michigan have just "turned over" causing drastic changes in water temperature, slime forming on the bottom and fishing success going out the window.
Most folks that interact with our waters are vaguely aware of lake turnover but few know the explanation behind this phenomenon.
Lake turnover is a temperature thing. In general, warm water is less dense than cold water. In late summer when temperatures are hot and the days are long and sunny the surface water of a lake is at maximum temperature and, therefore, lightest weight. Warming sunlight and air temperatures can't reach the deeper waters of a lake so they are much colder and heavier. The lighter surface water actually floats on top of the heavy bottom water. If a lake is deep enough where the wind can't keep the waters mixed the lake becomes distinctly stratified, or layered.
On top is the warm, oxygen rich epilimnion layer that (fishermen take note) supports much of the lake's life during the summer. This can extend as far as 20 feet down. In the middle is the narrow thermocline layer with distinctly colder temperature and less oxygen. If you're swimming and dive far enough down it's like hitting a frigid wall. Below the thermocline lies the hypolimnion, the coldest, deep waters of the lake that hold almost no oxygen, build up quantities of sulfurous gases and support little or no life at this time of year.
But come fall the days shorten, air temperature drops and the surface water begins cooling. Eventually the top layer becomes cooler, thus heavier, than the lower layers. At around 50 degrees the surface water breaks through the thermocline disrupting the layering. Aided by the wind all the lake's waters are thoroughly mixed together. As bottom water comes to the top the sulfurous gases are released, often causing a rotten egg smell. All the water becomes well oxygenated and fish can return to deeper water to overwinter. The more wind the faster and more pronounced this turnover happens.
Now enters another of nature's unique phenomenon. Water reaches its heaviest at 39 degrees. As it cools beyond that it becomes lighter. That's why ice floats rather than sinking to the bottom which would cause freezing from the bottom up, killing all aquatic life. As the surface water cools below thirty-nine degrees it floats on top and ice prevents wind from mixing the lake's water. By late winter the lake again becomes layered like it was in late summer only this time reversed with the cold water on top instead of the bottom. Come the balmy days of spring the surface waters begin to warm. When they reach the temperature (and weight) of the water below the lake again turns over. Ain't nature somethin'? This has recently happened within a number of our lakes.
Besides providing a healthy spring and fall cleaning of the lake, turnover triggers several other noticeable events. With all the lake's waters suddenly fully oxygenated fish are able to spread out, food sources are plentiful and fishing typically takes a nose dive. Particularly in the fall turnover the aforementioned sulfurous gases are released, stinking the place up for a while. During spring turnover the sun and warm water loving algae that has grown dense in the surface layer sinks to the bottom and dies. That and other disturbed decaying debris is the slime covering the bottom of the lake after a turnover. Of course, with shallow lakes the wind is better able to keep the water mixed and there is little or no layering so they may not experience turnover. Likewise with small lakes where water is circulated by strong inlet and outlet currents. So now you know. Carpe diem.