Scientists led by Nancy Rabalais of the Louisiana Universities Marine Consortium say the area of low oxygen, called hypoxia, covers 5,840 square miles of the Gulf floor.
Scientists had expected a wet spring to bring more nutrients than usual down the Mississippi River, leading to a dead zone of up to 8,561 square miles. The largest dead zone on record was in 2002, when it spread across 8,481 square miles of the Gulf.
Rabalais says temperature and salinity measurements indicate high winds in early to mid-July mixed oxygen into deeper waters.
Copyright 2013 The Associated Press.