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

Phytoplankton (plants which are not firmly attached to any surface and can easily flow with water) is biodegradable and is decomposed by a large population of bacteria. During this process, bacteria consume considerable amount of oxygen dissolved in water and thus deplete water of dissolved oxygen. Aquatic animals like fish need dissolved oxygen for their survival. When the level of dissolved oxygen in water falls below a particular level (6 ppm), fish die and are found floating dead on the surface of water.