Why do lakes turnover in the fall

Simply put, lake turnover is the seasonal mixing of the entire water column. For many lakes deeper than about 20 feet, distinct, thermally stratified layers of water form during the summer. These layers prevent the lake from mixing and aerating. Warmer and less dense water floats on the top of cooler, denser water at the bottom. Temperature layering begins to weaken when outside temperatures cool. This allows the lake to mix when temperatures equalize throughout the water column.

Meaning, water from the bottom of the lake rises to the top, and water from the top of the lake sinks to the bottom. The process allows for oxygen to be replenished and nutrients to be distributed throughout the lake. On the other hand, shallow lakes like Lake Wingra maintain a more uniform temperature.

Shallow lakes mix frequently and rarely experience stratification. Water is unique in the way it changes density at different temperatures. The deep layer of water, called the hypoliminion is made of cooler water. The hypolimnion is the zone of decomposition, where plant material either decays or sinks to the bottom and accumulates. Dissolved oxygen levels are often very low in this layer.

The two layers are seperated by something called the thermocline. Its a small layer of water where the water temperature rapidly changes from warm to cold.

The middle layer, known as the thermocline , acts as an effective barrier to any mixing of the deeper waters. Toward the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place. As the days get shorter and cooler, and energy is transported away from and out of the lake, mixing becomes easier. At about 50 degrees, the cooler water with a higher oxygen content at the surface begins to sink into and through the thermocline, forcing warmer and less dense water to the surface, eventually erasing the temperature stratification built up over the summer.

At some point, the majority of the water in the lake reaches an approximately uniform temperature. Now, storms and sustained high winds can begin to perform the task of overturning and mixing all of the water in the lake -- referred to as fall turnover. The deep water contains an abundance of decaying matter and sulfurous gases; when it reaches the surface, it produces a telltale odor that indicates the process has begun.

Eventually the turnover mixes fresh oxygen into the entire lake mass, replenishing the deep waters with the life-giving stuff and cleansing the sulfurous fumes from the water, allowing fish to return to the depths where they will spend the winter months. As winter approaches, the water that has now reached 39 degrees sinks to the bottom, allowing colder and less dense, buoyant water to remain at the surface to freeze.

The ice thickens because it is not a good insulator; water in contact with the underside of the ice cools further and freezes, adding to the surface layer. A stratification similar to that of the summer months will occur in the water column during the winter months, but not to as great an extent. With oxygen level changes, a big shift in temperature, cloudy water, and numerous other chemical and biological shifts occurring, fish change their behaviors.

Warmer water fishes like bass and sunfish can still be found feeding but they hang out in shallow refuges of clear, warmer waters very close to the shoreline. In deeper areas these species are adjusting to low visibility conditions, cold water, and lower oxygen so it is not surprising they stop biting down there!

Pike and walleye head for the live weed beds and other panfish will group up to feed on aggregates of plankton. The fish shift their behaviors during fall turnover, and successful fall anglers know to do the same. The signs of fall approaching are all around. Leaves are just beginning to change, the mushrooms are popping, and pesky mosquitoes are disappearing. Watch for more fish along the shorelines, cloudier deep water, and a lot fewer kiddos willing to brave a swim.

These are sure signs that fall has arrived and that your lake might be turning. Search Query Show Search. Features and Podcasts.