The low-oxygen “dead zone” expected to form in the Gulf of Mexico this summer could be the largest since measurements began in 1985 due to the large volume of water carrying nitrogen down the Mississippi River this spring, researchers said Tuesday.

The forecast by scientists from LSU, Louisiana Universities Marine Consortium and University of Michigan predicts that the low-oxygen area dead zone will cover between 8,500 and 9,421 square miles. That would make it bigger than the largest 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 actual size of the 2011 hypoxic zone will be released later this summer after a National Oceanic and Atmospheric Administration-funded survey by LUMCON is conducted July 25 to Aug. 6.

Turner said he has been working on dead zone forecasts for about 15 years. In that time, he has tried different inputs into the formula he developed, but found that the best indicator of how large the dead zone will be in the summer is to measure nitrogen loads in the river in May.

The formula uses water samples gathered by the U.S. Geological Survey to measure the concentration of nitrogen in the Mississippi and multiplies it by the amount of water being discharged from the river.

Turner said he also collects his own water samples in Baton Rouge to measure for nitrogen levels but that they’re very close to what USGS found.

This calculation gives the load of nitrogen the river is carrying into the Gulf and is put into an equation to help estimate the dead zone that will form, he said.

Every year, Turner said, the formula shows an increase in the amount of dead zone created based on the nitrogen. The reason is there is a “legacy” effect from previous summers, when carbon from the dead organisms has built up in sediment, he said.

In effect, instead of starting at “zero” in the Gulf, the Gulf flood starts with a load of oxygen-depleting material even as new nitrogen goes into the system, Turner said.

“The longer we wait to fix it, the harder it’s getting to fix,” he said about the dead zone.

The low-oxygen dead zone — also known as hypoxia — forms in the Gulf off Louisiana’s coast after nutrients from upriver 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 water bottom.

During the summer, 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 oxygen. That process results in a bottom layer of water where the oxygen levels fall to a level that it can’t sustain marine life.

Tropical storms and hurricanes can help in mixing the bottom layers into the more oxygen-rich water layers near the surface and that has reduced the measured size of dead zones in previous years.

The goal set by the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force, a collaboration of federal agencies and university scientists, is to reduce the dead zone to 1,900 square miles.

However, Turner said, reducing the dead zone means addressing water quality issues and agricultural demands for corn crops in the Midwest that include large applications of fertilizer, which eventually ends up in the Mississippi.

“We can’t get ours fixed without dealing with Midwest water quality issues,” Turner said. The entire Mississippi River basin needs to be treated as an entire watershed instead of just in pieces, he said.

Matt Rota, director of science and water policy at the Gulf Restoration Network, said the new forecast “is another harsh reminder that our country must work aggressively to clean up the Mississippi River.”

In a prepared statement, Rota said that the current voluntary actions available to reduce nitrogen input into the Mississippi are not enough.

“The EPA must use all of its available tools to reduce Dead Zone-causing pollution, which include setting numeric limits on the amount of nitrogen and phosphorus pollution allowed in the Mississippi Basin, and developing a cleanup plan that includes solid goals and deadlines,” he said.

Doug Daigle, coordinator of the Lower Mississippi River Sub-basin Committee on Gulf Hypoxia, said funding to address upriver water quality issues isn’t likely, given the federal budget climate.

“The hope is we can get some portion of the BP (oil spill) money,” Daigle said. Projects, such as wetland restoration upstream, can help remove nutrients before they get to the river, which would help reduce the dead zone, he said.