Areas with low levels of oxygen are referred to as dead zones. Red areas correspond to less oxygen.
A very large dead zone, an area of water with no or very little oxygen, is expected to form in the Gulf of Mexico this year — a trend in recent years, according to the National Oceanic and Atmospheric Administration (NOAA).
Computer models put together by scientists predict that the zone will cover an area between 7,286 and 8,561 square miles (18,871 to 22,173 square kilometers) this summer, the typical time for such zones to form. The large end of the estimate is roughly the size of the state of New Jersey, and would be the largest dead zone ever recorded. The biggest one recorded to date, in 2002, reached 8,481 square miles (21,966 square km).
Meanwhile, models predict the dead zone in the Chesapeake Bay will be smaller than usual.
The makings of a dead zone begin with nutrient pollution, primarily fertilizers and agricultural runoff. Once these excess nutrients reach the ocean, they fuel algae blooms. The algae then die and decompose in a process that consumes oxygen and creates lifeless areas where fish and other aquatic creatures can't survive. This zone can have serious impacts on commercial and recreational fisheries on the Gulf Coast.
For the rest of the story: http://www.livescience.com/37534-huge-dead-zone-expected.html