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Winterkill in North Dakota lakes

With all the snow this winter, there is increasing speculation about spring flooding, as well as winterkill in some of our lakes — particularly some of the more shallow lakes. Winterkill occasionally occurs in some North Dakota lakes. Some people will blame thick ice for winterkill, and that may be a factor, but the most common cause of winterkill in North Dakota lakes is too much snow on the ice for too long a time.

What is winterkill?

During the summer months, the lake water is oxygenated through wave action. Plus, the algae in the water are carrying out photosynthesis which, you may recall, gives off oxygen as a byproduct. All those microscopic algae cells function like little aerators, and they are important in keeping the water well oxygenated for the fish and other animals in the lake.

But once the lake freezes over, about the only source of oxygen for the fish and other animals is the oxygen given off by the algae carrying out photosynthesis. If all those microscopic algae cannot carry out photosynthesis, oxygen levels are going to drop, and that could be problematic for the fish and other organisms.

What causes winterkill?

Clear ice covering the lake (even thick ice) generally will allow enough light to pass into the water column to keep the algae carrying out photosynthesis and giving off oxygen to support the fish. But as the depth of the snow cover increases, less light can get down into the water column. As a result, the amount of photosynthesis carried out by the algae drops. So of course, the oxygen levels in the water will begin to drop as well.

If the oxygen levels continue to drop below what is required by the fish, a winterkill will occur. Plus, if a lot of algae die, the decomposition process uses up even more oxygen. When oxygen is no longer available, the anaerobic (or without oxygen) decomposers take over and give off a rotten egg smell. That smell means there has been a lot of anaerobic respiration going on, and an indicator of winterkill.

It may be too late for some lakes this winter. But hopefully most lakes will make it through this winter in good shape. Fishing in North Dakota is a popular activity, and one that also helps anglers better appreciate a little Natural North Dakota.

Chuck Lura has a broad knowledge of "Natural North Dakota"and loves sharing that knowledge with others. Since 2005, Chuck has written a weekly column, “Naturalist at Large,” for the Lake Metigoshe Mirror, and his “The Naturalist” columns appear in several other weekly North Dakota newspapers.
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