Avoid “winter kill” in your pond

This winter there were more than the usual number of sub-zero days; many backyard and garden ponds will be completely frozen over which can cause winter kill of pond fish and hibernating creatures.

Colder than normal winter temperatures for longer periods in Michigan sets the stage for winter kill in small lakes and ponds this year. “Winter kill” refers to the death and die off of fish, frogs and turtles after a hard winter where a pond or small lake becomes completely snow covered or freezes over completely, preventing the natural exchange of gasses. There is only so much water in a pond containing oxygen. If there is no way for the water to exchange gasses in the air, then the oxygen that is present when the pond freezes over is all that there is. Animals and decaying vegetation sealed under ice use up the oxygen in the water and the animals can suffocate. This is somewhat common in the natural world, but can be avoided in farm and garden ponds. The unfortunate part about a winter kill is that the true effects are not seen until spring when things begin to thaw off and the dead fish and frogs can be seen.

Preventive action before ice forms is the best way to control winter kill in your pond. Some steps to take after snow and ice form are:

  1. Maintain open hole(s) in the ice for the exchange of gasses.
  2. Add air bubblers and run filter or fountain pumps to both introduce air and circulate the water. This is best done in the fall before the freezing takes place. Keep in mind during installation that lakes and ponds are different temperatures at different depths. The deepest portion of lake or pond is the warmest in the winter (about 40 degrees Fahrenheit) and is where the fish are mostly dormant for the winter. Care should be taken not to disturb the bottom warmer layer of water in the pond; with the colder upper layers. Mixing the two can cause the bottom to freeze. Install equipment above this level by placing them on a platform or floats.
  3. Using an electric tank or pond deicer can be ok as long as it floats near the top of the water and is powerful enough to keep the ice clear. In Michigan, a higher wattage deicer is recommended to keep up with freezing temperatures over several days.
  4. If there are plants in the pond, even removing the snow so photosynthesis can occur can help alleviate oxygen deficiencies. A word of caution: if you are walking out on the ice, make sure it is safe to do so with at least 4 inches of ice depth for safety. Oxygen levels recover very quickly when ponds and lakes become ice free.

For more information about lakes, streams, and watersheds or invasive aquatic plants contact me, Beth Clawson, MSU Extension educator. To learn more about invasive organisms and invasive aquatic plants contact Michigan State University Extension Natural Resources educators who are working across Michigan to provide aquatic invasive species educational programming and assistance. You can contact an educator through MSU Extension’s “Find an Expert” search tool using the topic “Natural Resources” or “Water Quality.”

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