Water under the ice: winter layers and oxygen levels

We’ve actually had a real Minnesota winter this year, and we currently have a good thick ice cover. Today is the third installment of my articles about lake layers and mixing. We already talked about summer stratification and fall turnover; so what happens to the water under the ice in the winter?

As a reminder, water is most dense at 39 Fahrenheit (4 degrees Celsius), and as water warms or cools from that mark it gets less dense. This has implications for a lake’s structure because the denser water is heavier and will be at the bottom of a lake while the less dense water is lighter and will generally Lake_layers_winter be at the top of the lake.

In the summer, the sun warms the surface of the water making the surface-water less dense. This warm water then floats on top of the cold water at the bottom of the lake that doesn’t receive sunlight. You can sometimes feel this difference when you’re swimming and the top of the water is warm while down by your feet it’s cold.

In winter, the exact opposite happens since the lakes are covered with ice. Most of the water under the ice is 39 Fahrenheit; however, there is a thin layer of water under the ice that is colder than 39 and therefore less dense. This thin layer of water floats on top of the lake under the ice throughout the winter, but this stratification is not quite as stable as in the summer because the density difference is much smaller. This concept is called inverse stratification because cooler water is sitting on top of warmer water. Under the ice, the water cannot mix because it is not exposed to wind.

In especially long, harsh winters a winter fish kill can occur. During periods of prolonged ice cover, the lake is sealed off from the atmosphere and cannot be recharged with oxygenated air. Furthermore, ice and snow reduce the amount of sunlight reaching aquatic plants, thereby reducing photosynthesis and oxygen production. All the fish and aquatic organisms in the lake use up the oxygen, and when it does not get replenished, oxygen levels can get too low for fish to survive. Winter kill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish which usually show up as the water warms in early spring.

Trout are most sensitive; walleye, bass, and bluegill have intermediate sensitivity; and northern pike, yellow perch, and pumpkinseed are relatively tolerant. Bullheads, carp and certain minnows are very tolerant. It is important to remember that winterkill is usually a natural phenomenon, and fish from stream inlets are usually able to re-populate the lake in the spring. Only for extreme die-offs is fish restocking necessary.