Seasonal lake mixing

Twice a year, unseen forces churn water from the depths of our deeper lakes and deliver oxygen and nutrients essential to aquatic life. This temperature-driven process of lake “turnover” allows aquatic life to inhabit the entirety of the lake as oxygen becomes more available. Without this natural process, our lakes would not be the vibrant and dynamic ecosystems we see today.

“I think of turnover as the lake taking a deep breath as everything is mixed. It’s like a fresh start every spring and fall.”

Robert Ladwig, Hydrologist
UW Center for Limnology

What is lake turnover?

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.

Did you know that deep lakes like Mendota and Monona are “dimictic,” meaning they undergo turnover twice during the year? On the other hand, shallow lakes like Lake Wingra maintain a more uniform temperature. Shallow lakes mix frequently and rarely experience stratification.

Why do lakes stratify?

Water is unique in the way it changes density at different temperatures. Unlike almost all other liquids, water is most dense at 39 degrees Fahrenheit (4 degrees Celsius), and is lighter at both warmer and colder temperatures. In other words, when water reaches the critical temperature of 39 degrees Fahrenheit, further cooling causes the water molecules to become less dense and rise to the surface. This unusual characteristic allows water to form distinct layers within an otherwise uniform liquid. This phenomenon explains why ice forms at the surface and does not sink.

There are two ways that help visualize this phenomenon. First, think of diving down into a lake in the summer. The top layer of water is very warm. As you swim deeper you often feel a distinct and sudden drop in temperature.

Now imagine it is winter and the lake is frozen over. Solid ice floats and stays on top of the lake because it is less dense than liquid water, despite the ice being colder than the water it floats on. The weather keeps the water near the surface cool, making it less dense than the warmer water deep in the lake. Ice also acts as an insulating blanket, preventing deeper lakes from freezing completely solid.

How and when does lake turnover happen?

Due to seasonal changes in sunlight intensity, surface water temperature begins to transition in the spring and fall. In the spring, the water surface warms. This causes the temperature of the top and bottom layers of the lake to equalize. With the help of strong winds, this new equilibrium breaks the thermal stratification, and the lake is able to mix. Oxygen from the surface mixes with the bottom, while nutrients trapped near the bottom are free to mix throughout the lake. This is why, sometimes, in the spring and fall the lake can smell unpleasant. Decomposing organic materials are churned up from the bottom of the lake, bringing a signature sign of lake turnover.

Similarly, turnover happens again in the fall within our deeper, “dimictic” lakes. The distinct layers established over the summer disappear as the top layer of the lake cools, and temperatures equalize. The lake is once again able to refresh nutrients and oxygen. Later, when temperatures stabilize, it becomes stratified through the winter, until spring.

What does this mean for oxygen levels and lake health?

Oxygen is most commonly depleted at the bottom of the lake by oxygen-hungry bacteria. These bacteria consume dead algae that sinks to lake floor.

“Once a lake stratifies, atmospheric oxygen, and oxygen produced by algae in the surface layer, have a hard time getting to the bottom of the lake.”

Robert Ladwig, Hydrologist
UW Center for Limnology

If oxygen is not replenished, the amount of viable habitat for fish would drastically decline. Hypoxic zones or dead zones are areas of low oxygen. These zones are only suitable for certain bacteria. The biannual cycle of lake turnover is essential in mitigating the negative impacts of low oxygen dead zones and providing suitable habitat for fish and aquatic organisms to thrive.

Read more about our five Yahara lakes.