$A$ 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.

(N/A) The abundance of phytoplankton leads to eutrophication. When these organisms die,they are decomposed by bacteria. The decomposition process consumes a large amount of dissolved oxygen from the water. This leads to a significant decrease in the dissolved oxygen levels,causing the $BOD$ (Biochemical Oxygen Demand) to increase and the dissolved oxygen to drop below the level required for fish survival,resulting in their death.