Dead zones are hypoxic (low-oxygen) areas in the world's oceans and large lakes, 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. (NOAA)".[2] Historically, many of these sites were naturally occurring. However, in the 1970s, oceanographers began noting increased instances and expanses of dead zones. These occur near inhabited coastlines, where aquatic life is most concentrated. (The vast middle portions of the oceans, which naturally have little life, are not considered "dead zones".)


Dead zones are bodies of water that do not have sufficient oxygen (3) levels in order to support most marine life. Dead zones are caused by oxygen-depleting factors which include, but are not limited to, human pollution (4). This is a process called eutrophication, where oxygen levels decrease as elements such nitrogen and phosphorus increase. A healthy river will have increased amounts of oxygen for consumption by organisms (1). As nitrogen increases, algae (5) produce large amounts of oxygen, but die from increased nitrogen. Decomposers then use all of the remaining oxygen decomposing the algae, resulting in no oxygen left and no oxygen being produced (2).
In March 2004, when the recently established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003), 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 kilometre (0.4 mi²), but the largest dead zone covered 70,000 square kilometres (27,000 mi²). A 2008 study counted 405 dead zones worldwide.[3][4]