<P> 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)". In the 1970s oceanographers began noting increased instances 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".) </P> <P> 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 . </P>

What causes a dead zone in the ocean