Eutrophication is the result of an increased presence of nitrogen and phosphorus in bodies of water. This nutrient enrichment of the water causes algal blooms and their decomposers use up and therefore deplete the water of oxygen, causing other organisms to die. Fertilizer runoff and sewerage leaking increases the rate of this otherwise natural process.
Sources
http://toxics.usgs.gov/definitions/eutrophication.html
http://www.eoearth.org/files/116401_116500/116490/300px-Algal_bloom_orielton_Australia.jpg