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.
Water has limited amount of dissolved oxygen. This oxygen is depleted due to abundance of phytoplanktons. The bacteria that decompose phytoplanktons, require large amount of oxygen. They consume dissolved oxygen. The BOD level of water drops below 6 ppm which inhibits the growth of fish and causes excessive fish kill
