Ecology

Lake Turnover

The seasonal mixing event when surface and deep water layers in a stratified lake exchange positions, driven by temperature equalization.

What It Means for Your Lake

Lake turnover is the physical process by which the entire water column of a stratified lake mixes from top to bottom, driven by seasonal temperature changes. Most deep lakes in Minnesota and Wisconsin are dimictic, meaning they undergo turnover twice per year, once in spring (typically April to May, shortly after ice-out) and once in fall (October to November). During summer, lakes develop thermal stratification with three distinct layers: the warm surface layer (epilimnion), the transition zone with rapidly changing temperature (thermocline or metalimnion), and the cold deep layer (hypolimnion). As air temperatures cool in autumn, the surface water cools and becomes denser until it reaches the same temperature as the deep water. At this point, wind energy can mix the entire lake, bringing nutrient-rich and oxygen-poor deep water to the surface and sending oxygenated surface water to the bottom. Spring turnover occurs similarly: after ice-out, surface water warms from near-freezing to the temperature of maximum density (39.2 degrees Fahrenheit or 4 degrees Celsius), allowing the entire water column to mix. Turnover is ecologically essential because it redistributes dissolved oxygen throughout the water column and recycles nutrients from the lake bottom to the photic zone where algae can use them. The post-turnover nutrient pulse often triggers brief algal blooms in both spring and fall. Climate change is affecting turnover timing in Upper Midwest lakes, ice-out is occurring earlier, fall turnover is arriving later, and the overall stratification period is lengthening.

Frequently Asked Questions