Eutrophication

Eutrophication is a process whereby water bodies, such as lakes, estuaries, or slow-moving streams receive excess nutrients that stimulate excessive plant growth . This enhances plant growth, , reduces dissolved oxygen in the water when dead plant material decomposes and can cause other organisms to die.

http://toxics.usgs.gov/definitions/eutrophication.html

this is the process

fertilizers not absorbed by plants are washed into near lakes, the nitrates and phosphates facilitate plant growth , this added nutrients enhance the growth of algae and water plants , the submerged algae and plants die due to insufficient sunlight, dead bodies get decomposed by bacteria , giving rise to growth of bacteria using up the oxygen in the water thus the other organisms die or are affected eventually