Winter Kill

There appears to have been a significant winter kill on the little lake, which explains the presence of so many eagles recently; they were gorging on the dead fish that were trapped in the rotting ice last week.

Hugh Burton shares some information about the biology of this phenomenon:

Winter kill results when oxygen dissolved in the water decreases below the point at which the fish can still retain life function. This degree of oxygen depletion occurs when the level of organic material in the water exceeds a critical level. The organic material is made up of dead leaves, trees lying in the water, dead: reeds, bulrushes, aquatic plants, and most significantly dead microscopic plants (phytoplankton) and dead microscopic animals (plankton). As these dead organisms decay the process uses up the oxygen present in the water releasing gasses such as carbon dioxide, hydrogen sulphide and methane. When the lake is covered with ice, the water beneath it is relatively static, and of course there is no surface area open to receive replacement oxygen. Natural water flow from inlet to outlet as well as mixing due to wind action is important in the maintenance of oxygen levels and these are reduced during winter conditions. Shallow eutrophic lakes (those with a high natural organic content) are particularly subject to winter kill as there is a much higher level of biodegradation occurring. This leads to a critical loss of oxygen and high level of resultant toxic gases and consequent die off of those animals that have the highest demand for dissolved oxygen. The longer the ice on period the more critical this becomes. Once the fish start to die under the ice they too begin to decompose further contributing to the oxygen depletion and toxic gas accumulation.

This recent evidence of winter kill goes a long way to supporting our concerns for Lac Le Jeune water quality, particularly for that of the little lake.