Eutrophication occurs when chemical nutrients, usually containing nitrogen or phosphorus, are added to an ecosystem. These chemicals change the habitat and the plants and animals that live in the ecosystem. Excessive growth occurs, which can end up contaminating water supply and compromise the sustainability of resources in the area. Oxygen is consumed more quickly, and the bottom waters can become low in oxygen or lose all oxygen, which can make conditions impossible for marine life to survive in.
Sources:
http://www.eoearth.org/view/article/51cbedc37896bb431f693ba8/
http://sciencebitz.com/wp-content/uploads/2009/11/eutrophication.png (featured image)