Eutrophication

When too many nutrients enter water and disrupt its ecosystem.

excessive nutrients in a lake or other body of water, usually caused by runoff of nutrients (animal waste, fertilizers, sewage) from the land, which causes a dense growth of plant life; the decomposition of the plants depletes the supply of oxygen, leading to the death of animal life; "he argued that the controlling factor in eutrophication is not nitrate but phosphate"