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 a reason for the fish kill.

Explanation

Excessive phytoplanktons (i.e., organic pollutants such as leaves, grass, trash, etc.) present in water is biodegradable. A large population of bacteria decompose this organic matter in water. As a result, they consume the oxygen dissolved in water and therefore, the concentration of dissolved oxygen becomes low. If the concentration of dissolved oxygen in water becomes below 6 ppm, the survival of fish becomes difficult. Hence the fish die and float dead on lake.