question_answer 1)

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.

Answer:

Abundance of phytoplankton, i.e., excessive organic pollutants present in water are biodegradable. During the decomposition of the organic matter by bacteria requires the oxygen dissolved in water. When the quantity of the dissolved oxygen falls below 6 ppm, the fish cannot survive. Hence, the fish die and float on the surface of the lake.