A Dead Zone is a hypoxic area in the ocean where marine life is very sparse. They are caused by excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water.
1970s oceanographers began noting increased instances of dead zones. These occur near inhabited coastlines, where aquatic life is most concentrated. In March 2004, when the recently established UN Environment Programme published its first Global Environment Outlook Year Book, it reported 146 dead zones in the world’s oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometer, but the largest dead zone covered 70,000 square kilometers. A 2008 study counted 405 dead zones worldwide.
Aquatic and marine dead zones can be caused by an increase in chemical nutrients (particularly nitrogen and phosphorus) in the water, known as eutrophication. These chemicals are the fundamental building blocks of single-celled, plant-like organisms that live in the water column, and whose growth is limited in part by the availability of these materials.
Eutrophication can lead to rapid increases in the density of certain types of this phytoplankton, a phenomenon known as an algal bloom.