U. MICHIGAN (US)—Scientists predict this year’s seasonal drop in oxygen levels in Gulf of Mexico waters will be the largest on record, creating a so-called “dead zone” that threatens the health of a half-billion dollar fishery.

Donald Scavia, an aquatic ecologist and director of the Graham Environmental Sustainability Institute at the University of Michigan, expects the dead zone, or hypoxic zone, will be roughly the size of New Jersey—between 7,450 and 8,456 square miles.

That would put the years 2009, 2008, and 2001 in a virtual tie for second place on the list of the largest Gulf dead zones and would also mean that the five largest Gulf dead zones on record have occurred since 2001.

“The growth of these dead zones is an ecological time bomb,” says Scavia. “Without determined local, regional, and national efforts to control them, we are putting major fisheries at risk.”

This year’s hypoxic zone is the result of above-normal flows in the Mississippi and Atchafalaya rivers this spring, which delivered large amounts of nutrient nitrogen. In April and May, flows in the two rivers were 11 percent above average. Additional flooding since May, could result in a dead zone that exceeds the upper limit of the forecast, scientists say.

Farmland runoff containing fertilizers and livestock waste—some of it from as far away as the Corn Belt—is the main source of the nitrogen and phosphorus that cause the Gulf of Mexico dead zone. Each year in late spring and summer, these nutrients make their way down the Mississippi River and into the Gulf, causing large amounts of algae to bloom. When the algae die and sink, bottom-dwelling bacteria decompose the organic matter, consuming oxygen in the process. The result is an oxygen-starved region in bottom and near-bottom waters.

“The high water-volume flows, coupled with nearly triple the nitrogen concentrations in these rivers over the past 50 years from human activities, has led to a dramatic increase in the size of the dead zone,” says Gene Turner, a lead forecast modeler at Louisiana State University.

Northeast of the Gulf, low water flows into the Chesapeake Bay, where nutrients in the Susquehanna River trigger the event. In both the Gulf and the Bay, fish, shrimp, and crabs are forced to leave the hypoxic zone and animals that cannot move perish.

On a positive note, this year the oxygen-starved zone in the Chesapeake Bay is expected to shrink to between 0.7 and 1.8 cubic miles, with a “most likely” volume of 1.2 cubic miles—the lowest level since 2001 and third-lowest on record. The drop is largely due to a regional dry spell that lasted from January through April, Scavia says. Continued high flows in June, beyond the period used for the forecasts, suggest the actual size may be near the higher end of the forecast range.

“While it’s encouraging to see that this year’s Chesapeake Bay forecast calls for a significant drop in the extent of the dead zone, we must keep in mind that the anticipated reduction is due mainly to decreased precipitation and water runoff into the Bay,” rather than in cutbacks in the use of nitrogen, he notes.

The annual hypoxia forecasts are designed to help coastal managers, policymakers, and the public better understand what causes dead zones. The models that generate the forecasts have been used to determine the nutrient-reduction targets required to reduce the size of the dead zone.

“As with weather forecasts, the Gulf forecast uses multiple models to predict the range of the expected size of the dead zone. The strong track record of these models reinforces our confidence in the link between excess nutrients from the Mississippi River and the dead zone,” says Robert Magnien, director of NOAA’s Center for Sponsored Coastal Ocean Research.

U.S. Geological Survey data on spring river flow and nutrient concentrations inform the computer models that produce the hypoxia forecasts.

The official size of the 2009 hypoxic zone will be announced following a NOAA-supported monitoring survey in July. In addition, NOAA’s Southeast Area Monitoring and Assessment Program (SEAMAP) is currently providing near real-time data on the hypoxic zone during a five-week summer fish survey in the northern Gulf of Mexico.

The Gulf hypoxia research team is supported by the U.S. National Oceanic and Atmospheric Administration’s Center for Sponsored Coastal Ocean Research and includes scientists from Louisiana State University and the Louisiana Universities Marine Consortium.

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