Dead Zone

A dead zone is an area of water so depleted of oxygen (hypoxic or anoxic) that it cannot support most marine life, formed where nutrient-driven blooms sink and decompose under a stratified water column. The northern Gulf of Mexico develops one each summer thousands of square kilometers in size from Mississippi-basin nitrogen; the Baltic holds one of the largest persistent zones. Mobile animals flee and sessile ones die, collapsing local fisheries.