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

Presence of excess phosphate and nitrate compound increases the growth of phytoplankton (organic pollutants such as leaves, grass, trash etc.).
A large population of bacteria decomposes this organic pollutants. During this process, they consume the dissolved oxygen of water which is of course very much essential for the life of sea animals particularly fish. When the level of dissolved oxygen falls below 6 ppm, the fish cannot survive. Hence a large number of fish are found floating dead on the lake.