Fall Lake Turnover

Lake turnover is the process by which the water at the surface and the water at the bottom become the same temperature. The significance of this is that it allows nutrients that have been trapped in cold water at the bottom to mix with the water above, allowing those nutrients to be distributed throughout the entire depth of the lake.

Here’s what happens. During the spring and summer, the ice on top of the lake melts and the water warms from the surface down. Complete warming never reaches all the way to the bottom (except in shallow lakes) so water near the bottom remains colder than the water above. Colder water is denser than warmer water so the colder water stays at the bottom while warmer water remains above the colder water. The formation of these two layers, a cold layer and a warm layer, means that the lake has stratified. Shallow lakes and bays do not stratify or stratify only weakly. The boundary between the higher density water at the bottom and the less dense water above forms an effective barrier that traps nutrients at the bottom preventing them from mixing with the water above. The consequence of this is that nutrients in the upper layer are consumed by organisms faster than they can be replenished. By the end of summer, nutrients, especially nitrogen, are depleted in the upper layer. This sets the stage for possible cyanobacteria (formerly called blue-green algae) blooms because cyanobacteria are able to incorporate nitrogen from the air, like peas and beans. Doing so enables them to outcompete other types of algae and they may come to dominate life at and near the water surface. Turnover breaks down the stratification, removing the barrier to upward nutrient movement and allowing replenishment of surface water.

In winter, a “reverse stratification” takes place.

Water is a peculiar and interesting substance that behaves strangely as it freezes. In particular, frozen water floats, which is most fortunate for life in the lake … and beyond.

Doug Pribyl, PhD
Pengilly MN