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 growth occurs in water because of the presence of organic matter like leaves, grass, trash etc. in water. It is likely to consume a lot of oxygen dissolved in water which is of course very much essential for the life of sea animals particularly fish. If the level of dissolved oxygen in water is below 6 ppm, this means that the oxygen is not sufficiently available to the variety of fish living in water. They are likely to perish or die. This might have happened in this particular case.