The low oxygen “dead zone” that will form in the Gulf of Mexico this summer could be the largest - since measurements were first taken in 1985 - due to the large volume of water carrying nitrogen down the Mississippi River.

The low-oxygen dead zone is expected to cover between 8,500 and 9,421 square miles this summer, according to a forecast by scientists from LSU, Louisiana Universities Marine Consortium and University of Michigan.

If the forecast proves correct, this year could see a dead zone form that is bigger than the largest previous dead zone measured in 2002 - when the low oxygen area was 8,400 square miles, according to the National Oceanic and Atmospheric Administration.

“I’m usually worried about overstating the size, but this time I’m worried I’ve underestimated,” said Eugene Turner, professor with the Department of Oceanography and Coastal Sciences at LSU.

The low oxygen dead zone ? also known as hypoxia ? forms in the Gulf of Mexico when nutrients from agriculture and urban runoff flow into the Gulf. These additional nutrients help feed microscopic organisms that use oxygen when they die and decompose on the bottom of the Gulf.

During summer months, the upper layer of freshwater from the Mississippi River doesn’t mix with the saltier layer of water underneath where the organisms are using up the oxygen.

That process results in a bottom layer of water where the oxygen level falls to a point that can’t sustain marine life.

The actual size of the 2011 hypoxic zone will be released later this summer after a NOAA-funded survey done by LUMCON is conducted between July 25 and Aug. 6.