Scientists say this year that the "dead zone" area that forms every summer in the Gulf of Mexico is one of the largest ever measured.
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 (20,000 square kilometres). The largest ever measured was just over 8,000 square miles (20,700 square kilometres) in 2001.
Pollution, such as runoff from farms along the Mississippi River, cause regions of low oxygen content or hypoxia in the Gulf through the introduction of nitrogen and phosphorus into the water system.
The nitrogen and phosphorus then stimulate the growth of algae blooms, which use up oxygen. The algae then die and sink to the bottom of the ocean, where bacteria feed on them and then further deplete the water of oxygen.