WordNet (r) 3.0 (2006):

eutrophication
    n 1: 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"