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?

[prasanna]

(i) Excessive phytoplankton growth within the water of the lake is the cause of water pollution, though it is bio-degradable.
(ii) In cold water concentration of dissolved oxygen can reach upto 10 ppm. That is why even a moderate amount of organic matter when decomposed in water can deplete dissolved oxygen of water. Thus fish dies.