In some situations winter fish kills can be traced to nutrient pollution. When excess nitrogen or phosphorus enters a lake or pond during thaw events, it remains available until light becomes available under thinning ice, which quickly stimulates excessive algae or plant growth.
As algae die, the decomposition process consumes oxygen in the water, but ice cover limits how much new oxygen can enter from the air. This can cause oxygen levels to drop to the point where fish and other aquatic animals struggle to survive.
