Lakes in the offseason: A look under the ice

Learn about lakes this winter with Introduction to Lakes Online.

The air is brisk, snow is piling up in northern Michigan, and lakes are freezing over. Many think lakes are dormant during winter, but this is far from the truth. Lakes in winter are still very dynamic and support an abundance of life. Here are a few tidbits about lakes in the offseason that may peak your interest in learning more about lakes in the winter.       

Did you know that algal blooms can occur under the ice?

Light easily penetrates clear ice, which can lead to algal blooms that are almost comparable to summer levels. This is especially true where lake ice develops without much snowfall. Conversely, when there is an abundance of snow on the ice, light penetration is greatly reduced and algae levels can be very low to undetectable. Interestingly, some algae species can grow within and on the underside of the ice where they can absorb maximum light.

Did you know that in winter the water is warmest at the bottom of the lake?

During the fall months, most lakes in Michigan are fully mixed, meaning they have the same temperature and density from the top to the bottom. As the air cools, the surface water becomes colder than 39 degrees, which is the temperature where water is the most dense or heaviest. As the temperature decreases, the water becomes less dense, resulting in colder and less dense water sitting on top of the relatively warmer 39 degree water. Eventually ice, which is lighter than liquid water, forms and floats on the water’s surface.

Did you know the causes of winter fish kills?

Winter fish kills ultimately result from a lack of oxygen under the ice, which occurs for several reasons. Ice does not allow oxygen from the atmosphere to enter the water and although photosynthesis can occur, it may not be enough to replenish the oxygen levels necessary to support fish, particularly in productive lakes. This is because bacteria, which can be very active in the winter, use up the oxygen as they decompose dead algae and plants from previous years. The combination of these factors leads to fish dying due to insufficient oxygen levels.

Did you know there is an online class where you can learn more about lakes?

Michigan State University Extension’s Introduction to Lakes Online course is offered once a year in January. Introduction to Lakes Online is a nationally recognized six-week course offered in a convenient, self-paced online format. The course is designed for anyone interested in lakes, including lakefront property owners, lake users, local government officials, lake managers and educators.

The online format allows you, from the comfort of your home or office, to have week-by-week, 24/7 access to six online units that are complete with video lectures, activities, resources, discussion forums, quizzes and Ask-an-Expert webinar sessions with professionals from Michigan State University and the Michigan Department of Environment, Great Lakes and Energy. The course’s six units cover lake ecology, watersheds, shorelines, Michigan water law, aquatic plant management and community involvement.

For more details about the course and to register visit the MSU Extension Introduction to Lakes Online course webpage at

A version of this article appeared in the November 2020 issue of the Lakefront Lifestyles Magazine.

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