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 consists of organic contaminants such as grass leaves etc. In water these materials are degraded by bacteria and a large amount of dissolved oxygen will be used up for this process. As the amount of DO drops below 6 ppm, fish cannot survive and they float dead.