Ice covers all Minnesota lakes except Lake Superior this time of year.
On most of the state's close to 11,800 lakes, the ice now is up to 2-feet thick, but the amount of ice can vary greatly, and ice covers should never be considered safe. Always use caution. Some of the problem areas are pressure ridges: areas of slush between the heavy snow cover and the ice on some lakes. Sometimes bad ice is caused by springs.
Under icy concealment, fish and other animals are sealed off from their replenishing supply of oxygen. Without the mix of air and water by waves, and with photosynthesis at low ebb, little oxygen is introduced into the water.
To survive under all the ice and snow, fish moderate their eating, growth, movement and reproduction. As winter progresses, oxygen first disappears from the bottom of lakes because bacteria is using it to decompose leaves and other organic matter in the sediment. Consequently, sunfishes, northern pike and other fish move up the water column. Lakes that are very shallow, quite small, or that have high decomposition rates eventually run out of oxygen. The result is what is known as winterkill, which can kill all the fish.
Some other observations:
- Those of us who are close to nature during the length of a Minnesota winter can appreciate the subtle spring signs and take joy in each occurrence. The wonderful whistled "what-cheer, cheer, cheer" in January — songs of the northern cardinal — makes us take note. Our minds think of warmer days. Yes, hearing a cardinal is an early sign of spring. They sing in response to lengthening days. The female's song is like the male's but softer, so always look up and see who is singing.
- A beaver lodge is a communal home. The inside of the lodge can become 60 degrees warmer than the outside temperature because of the animals inside it. Eastern chipmunks may wake up on cold days but will stay inside and eat from the supply of food they stored in their underground burrows.
- Over much of northern Minnesota, at this time, male common ravens do elaborate courtship flight maneuvers, including steep dives, tumbles and rolls. Pine siskins, pine grosbeaks, red-breasted nuthatches and Canada jays are among the birdfeeder birds.
Jim Gilbert has taught and worked as a naturalist for more than 50 years.
