Lakes with high biological activity undergo greater fluctuations than lakes with few plants and animals. Typical oxygen levels in a productive lake following summer stratification are shown in Figure 9 . Low oxygen levels in the hypolimnion mean that fish must live in the epilimnion and metalimnion. Typical oxygen and nutrient status of mesotrophic and eutrophic lakes after summer stratification. It happens in years when at least four inches of snow cover the lake, which prevents sunlight from reaching the water.
All photosynthesis stops and plants begin to die and decompose.
The extent of oxygen loss depends on the total amount of plant, algae and animal matter that decays. Drought increases the chance of winterkill by reducing the volume of water in the lake. This is the minimum amount of oxygen needed for fish to survive and grow. A smart angler would know that the lake in Figure 9 contains no trout and that it would be silly to fish for walleye in the deep holes in late summer. Carbon Dioxide Reactions.