The hypoxic area forms every year in the Gulf, caused by bacteria feeding on algae blooms from the flow of farming runoff and other nutrients from the Mississippi River and others.
This year's area covers 3,000 square miles, but is also unusually thick, stretching from the bottom nearly to the surface, according to Nancy Rabalais, a researcher who specializes in the problem for the Louisiana Universities Marine Consortium.
The 3,000 square miles is one of the smallest measurements of the zone since measurements began in 1985, according to a graph in a news release sent from a research vessel in the Gulf. Only those in 1987, 1988 and 2000 were smaller.
Other scientists had predicted that this year's dead zone would cover about 7,500 to 8,500 square miles.
Possible reasons for the difference include high winds and waves that helped mix more oxygen into some waters, she wrote.
"This was surprisingly small given the forecast to be among the largest ever and the expanse of the dead zone earlier this summer," she wrote.
Hypoxia occurs when algae blooms, fed by nitrates and phosphates in the water, die and fall to the bottom. At the same time, winds die down, meaning that fresh water coming out of the rivers doesn't get mixed into the denser salt water below it. Microbes feeding on the dead algae use up oxygen from the bottom up.
Rabalais said that in some areas where the oxygen was lowest, crabs, eels and shrimp
- creatures which usually live on the bottom - were seen swimming at the surface.
Other studies indicate that severely low oxygen levels in early July contributed to "jubilees"
- forced movement of fish, crabs and shrimp into shallow waters - off Grand Isle, she said.
