Have we had snow across the state this winter! While most people are concerned about the snowmelt and potential flood damage, some among us are also concerned about winterkill in some of our lakes. Perhaps like you, I have already smelled the tell-tale rotten egg smell on some smaller lakes and marshes. That smell means there has been a lot of anaerobic, or without oxygen respiration. The smell also is a major indicator of winterkill!

Occasionally I hear comments about winter kill being somehow caused by thick ice. That can be a factor, but the most common cause of winter kill is too much snow on the ice for too long a time period.

You may recall that photosynthesis in plants is the process by which carbon dioxide and water are used to produce sugars. Oxygen is a by-product of that process. It is the same for algae in a body of water. That oxygen is important in keeping the water well oxygenated for the animal life. Think of the algae as little aerators.

During the summer the lake water is oxygenated through wave action and algae carrying out photosynthesis. Life is good! Once the lake freezes over, however, about the only source of oxygen for the fish and other animals is that from photosynthesis. If all those microscopic aerators can’t do their job, there are going to be some problems.

During a winter with normal snowfall, there is usually enough sunlight getting through the ice and into the water to support the algae, thus oxygenating the water. Even when the ice is unusually thick, enough light typically gets through to keep the algae carrying out photosynthesis.

However, when a thick blanket of snow covers the ice, it may block enough light from getting into the water that the algae can not carry out their normal amount of photosynthesis. As a result, the amount of oxygen in the water begins to drop. If the oxygen level continues to drop a winter kill may eventually 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 the rotten egg smell.

Hopefully the winterkill will be minimal this year. Fishing in North Dakota is popular activity, and one that also help us better appreciate our Natural North Dakota.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.

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