A 'dead zone' is an area of low oxygen, where marine life can't thrive. The dead zone may be caused by an excess of nutrients (eutrophication) that results in a decrease in dissolved oxygen. The best example of a dead zone in the U.S. is the Gulf of Mexico dead zone, which occurs each year in the late spring and summer.
The Gulf of Mexico dead zone is caused primarily by runoff that flows down the Mississippi River into the Gulf. The excess nitrogen and phosphorus from the runoff causes huge algae blooms. Bacteria feed on the algae as they die and decay, consuming oxygen, which eventually leads to extremely low levels of oxygen.
The average size of the Gulf of Mexico dead zone over the past 5 years was about 6,000 square miles. The 2010 dead zone is predicted to be about 6,500 square miles.
