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The Lake Where You Live: All charged up

November 01, 2019 by Ted Rulseh

Something invisible but critically important is happening in your lake right now. It’s charging up with the oxygen that fish and other creatures need to get through the winter.

In summer, unless your lake is uniformly shallow, the water stratified, a warm, less-dense, oxygen-rich layer on the surface, and a cold, denser layer below where the oxygen supply was slowly depleted as algae and other organic matter sank into the depths and decomposed.

Then as summer turned to autumn chill, the recharging with oxygen began. The surface water cooled until it was essentially the same temperature and density as the deeper water. At that point (the fall turnover) the two layers mixed, and the lake became for practical purposes a uniform bowl of water. Therefore, winds and wave action could stir and mix the water, infusing it with oxygen.

At this time of year it’s worth thinking of your lake as a giant, partly empty scuba tank slowly getting refilled. The oxygen will serve as the lake’s life-support system when the ice sets in and the lake mostly is sealed off from resupply with air.

By now, in early November, the lake is well mixed, and yet it can still gain oxygen as the wind and waves continue to move the water around. That’s because the colder water gets, the more oxygen it can hold. It’s a concept similar in a way to relative humidity, except in that case warm air can hold more moisture than cold.

Water at the freezing point of 32 degrees F can hold nearly twice as much oxygen as water at, say, 80 degrees F in summer. So by the time ice covers the lake, the water will hold more oxygen than at any other time of year. That is, the scuba tank will be full to capacity. From there on, the oxygen supply will decrease as it gets used up by water creatures and the decomposition of dead organic matter.

Fortunately, the rate at which the oxygen gets consumed will be lower than at any other time. That’s because the cold water slows the processes of life across the board. Bacterial activity declines, and so does dead material’s decay. 

Fish, frogs, turtles and other creatures living underwater are, of course, cold-blooded. That means they are less active. Their metabolism slows down. They eat less and, because they are using less energy, they require less oxygen to live. So there’s more oxygen available to them than ever, in a time when they need it the least.

So life will go on beneath the ice, the processes of biochemistry functioning in slow motion, the scuba tank depleting very gradually. In some shallow lakes, if winter persists, the fish could be at risk of winter kill from suffocation. But the life in most lakes, no matter how long the ice hangs on, will have enough oxygen to get by. 

Then comes spring, the ice melts, wind and waves go to work, and the scuba tank fills again.

Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected]

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