Tech Talk 56
Most of us are aware that aeration systems are primarily required in summer, when the oxygen consumption rate is highest and stratification is most severe. In some lakes and ponds, winter fish kills are also quite common.
When ice covers a lake, oxygen transfer from the atmosphere is eliminated. If there is a significant accumulation of snow on top of the ice, the photosynthetic oxygen production can also be virtually eliminated. If the ice and snow cover persists long enough and the oxygen consumption rate underneath the ice is great enough, there will be a fish kill. Clean lakes, that is, those with a low trophic state index, may not require winter aeration.
During the summer, thermal stratification occurs when warmer water floats above cooler, heavier water. Near freezing, the situation is reversed. As water approaches the freezing point, it expands and floats on top of warmer water.
The oxygen consumption that takes place in a lake is the result of biological and chemical reactions. Since the rate of these reactions is related to temperature, the winter aeration system needs to be only about 20 percent of the size of a summer aeration system.
If it is acceptable to have an open hole in the ice, the simplest way to prevent a winter fish kill is to keep a small area ice free, either by using air or an electric de-icer. The electric de-icer may work best at the shoreline, perhaps mounted on a dock, while air-powered systems seem to be preferred where longer distances are involved. When the air compressor is mounted on the shore, the air line is buried below the frost line into the lake and directed to the area that is to remain ice-free. Caution must be taken due to the hazards of open water in the winter, where people or animals may be on the ice. Also, be careful not to open too large a hole in the ice, which can result in the loss of too much of the lake's heat.
Experience is necessary when designing de-icing systems, so we recommend contacting the Pentair AES Lakes Department at 407-472-0520 for assistance.
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