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 deeper lakes during those times as oxygen becomes available at depth. Without this natural process, many deeper and more fertile lakes (like lakes Mendota and Monona) would experience year-long dead zones that would be inhospitable to most aquatic life.

“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 stratification?

For many lakes deeper than about 20 feet, distinct and thermally separated layers of water form during the majority of the year. These layers prevent the lake from mixing and aerating. This layering effect is known as “stratification.”

Stratification happens due to the unique relationship between the temperature and density of water. Unlike almost all other liquids, water is most dense at 39 degrees Fahrenheit (4 degrees Celsius), and decreases in density at both warmer and colder temperatures. In other words, water is less dense at any temperature warmer or cooler than 39 degrees. 

During summer, warmer and less dense water floats on the top of cooler, denser water at the bottom. You may have experienced a sensation like thermal stratification yourself. Imagine diving in a lake during the summer where it often feels colder as you dive deeper. The top layer of warm, less dense water is called the epilimnion, and the deeper water layer is called the hypolimnion.

During the winter, deeper lakes also stratify – but in reverse. 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 cold weather keeps the water near the surface ice-cold at 32 degrees, making it less dense than the slightly warmer 39 degree water deeper in the lake. Ice also acts as an insulating blanket, preventing deeper lakes from freezing completely solid.

How and when does lake turnover happen?

Turnover is the process that describes when a lake de-stratifies and is able to mix from top to bottom. Due to seasonal changes in sunlight intensity and ambient air temperature, surface water temperatures undergo large fluctuations in the spring and fall. 

In the spring, the water surface warms and approaches the 39-degree temperature of the bottom water layer, meaning that the density difference between the two layers becomes less and less. Eventually, with the help of strong winds, the lake is able to mix. During each turnover event, water from the surface mixes with the water on the bottom. 

Similarly, turnover happens again in the fall within our deeper, “dimictic” (or twice mixing) lakes. As the warm top water layer cools in the autumn, the density of this layer begins to increase. Eventually, the difference in water density between the warmer top and colder bottom layers is small enough that a windy day can result in lake-wide mixing. As a result, the lake becomes an intermixed soup of uniform density and temperature from top to bottom. The lake is once again able to refresh nutrients and oxygen. During winter, surface temperatures begin to cool beyond the critical 39 degrees Fahrenheit, allowing ice to form and the lake to become stratified once again until the spring turnover renews the annual cycle.

Within the Yahara Watershed, our larger lakes like Mendota and Monona are “dimictic,” meaning they undergo turnover twice during the year. Shallow lakes like Lake Wingra maintain a more uniform temperature throughout the year; because the shallow lakes mix frequently and rarely experience stratification.

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 and other organic matter that sinks to the lake floor.

During the summer, the warm epilimnion consistently receives fresh oxygen from the atmosphere and from photosynthesizing organisms, making it an oxygen-rich layer of water. Conversely, the deeper and darker hypolimnion is home to many oxygen-hungry bacteria and decomposer organisms. Because there is no way for oxygen to mix into this layer, the hypolimnion may become oxygen deprived during the summer. 

The creation of an oxygen-depleted, or hypoxic, zone of the lake has significant ecological impacts. First, the development of a large anoxic zone during stratification means that oxygen-loving organisms like fish are unable to utilize this space. Once the lake turns over, oxygen is replenished throughout the entire lake, providing suitable habitat for fish and aquatic organisms to thrive.

An oxygen-deprived hypolimnion also affects the chemical dynamics of a lake. In well-oxygenated waters, phosphorus binds with iron atoms and becomes unavailable for plants and algae to use. However, in the oxygen-lacking hypolimnion, the phosphorus and iron separate, making the nutrient available for algae to consume. By the end of summer, the bottom layer of our lakes often become loaded with this ready-to-consume form of phosphorus. Once the lake turns over in the fall, boom! Algae in the well-lit surface layer of the lake now have access to all this available phosphorus to rapidly grow and reproduce. This pulse of nutrients associated with lake turnover is why we often see late-season algae and cyanobacteria blooms. A similar phenomenon happens with the spring turnover, but often to a lesser extent. 

Read more about our five Yahara lakes.