A large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping but you find an abundance of phytoplankton. Suggest reason for the fish kill.

All aquatic plants and animals depend on the dissolved oxygen in the water. The solubility of oxygen in water varies from 15 ppm (at 0°C) to 9 ppm (at 20°C). The aquatic plants and animals need a certain level of oxygen in the water, fish needing the highest level of oxygen. Excessive phytoplankton (organic pollutants such as leaves, grass, trash etc.) present in water is biodegradable. A large population of bacteria decomposes this organic matter in water. During this process, some of the oxygen dissolved in water has been consumed. As a result, the level of the dissolved oxygen goes down from 9 ppm to 6 ppm or even below it. When the concentration of the dissolved oxygen falls below 6 ppm, the fish can not survive. Hence, they die and float dead on the lake.