The large area of low oxygen that chokes marine life comes in addition to the massive BP oil spill.
Microbes that eat the oil can deplete oxygen in the water. But the researchers who measure the dead zone couldn't say there is a connection between the spill and the dead zone's size.
They say the dead zone is at least 7,722 square miles. The largest ever measured was just over 8,000 square miles in 2001.
The dead zone forms every year when bacteria feeds on algae blooms and uses up oxygen. The blooms are caused by the nutrient-rich waters from rivers that carry farm and urban runoff into the Gulf.