We continue to hear lake ice cracking, groaning and booming. Why does this happen?

Once ice forms, it expands when warmed and contracts when cooled. Ice covers all Minnesota lakes except Lake Superior at this time of year. On most of the state's 15,000 lakes, the ice is now from 1 foot to 2 feet thick. Under this concealment, fish and other animals are sealed off from their replenishing supply of oxygen. Without the mixing of air and water by waves, and with photosynthesis occurring at a much lower level, very little oxygen is introduced into the water.

To survive under all the ice and snow, fish moderate their eating, growth, movement and reproductive patterns. As winter progresses, oxygen first disappears from the bottom of lakes, because bacteria use it to decompose leaves and other organic matter in the sediment.

Consequently, sunfishes, northern pike and other species are forced to move up the water column. Lakes that are very shallow, quite small or that have high decomposition rates eventually run out of oxygen entirely. The result is what is known as "winterkill," which may kill all the fish in a single lake.

Jim Gilbert's Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.