Question

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

Answer 1

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 they consume the oxygen dissolved in water. Water has already limited dissolved oxygen (≈10 ppm) which gets is further depleted. When the level of dissolved oxygen falls below 6 ppm, the fish cannot survive. Hence, they die and oat dead in water.