A similar phenomenon may occur in the winter under ice cover. In a hypereutrophic lake, algae may become so abundant that they suffer from self-shading. When a great abundance of phosphorus is available in a lake, nitrogen may become limiting. They dominate the algal community until another nutrient, or usually light, becomes limiting. In many infertile lakes in northeastern Minnesota, both phosphorus and nitrogen may be extremely low during midsummer.
Eutrophic lakes show wide seasonal changes in their biological and chemical conditions.
Because of the great amount of organic matter produced in these lakes, the decay rate is high in the hypolimnion, causing oxygen to be depleted. Clearly, fish and most other animals cannot live in the hypolimnion of such lakes. Warm-water fish that can live in the epilimnion, however, can be quite productive. Bass, panfish, northern pike, walleye, carp, and bullheads thrive in many of Minnesota's eutrophic lakes. Therefore, virtually complete anoxia below the thermocline does not necessarily mean that the lake is eutrophic.