Gulf of Mexico Dead Zone Much Smaller Than Forecast

The dead zone is created as fertilizer-laden runoff from agricultural operations flows down the Mississippi River and its tributaries and the Atchafalaya River. These excess nutrients stimulate an overgrowth of algae in the Gulf that sinks, decomposes, and consumes most of the life-giving oxygen supply in the water, creating an area of hypoxia - a dead zone.

Nutrient-laden sediment washes down the delta of the Atchafalaya River on the Gulf of Mexico. (Photo courtesy U.S. Army Corps of Engineers)

There are other dead zones along the U.S. coastline but the Gulf of Mexico dead zone is of particular concern because it threatens commercial and recreational fisheries that generate about $2.8 billion annually.

Scientists from the Louisiana Universities Marine Consortium, LUMC, said their observations on the research cruise suggest that the smaller than expected dead zone is due to unusual weather patterns that re-oxygenated the waters, and does not indicate a trend.

"The winds and waves were high in the area to the west of the Atchafalaya River delta and likely mixed oxygen into these shallower waters prior to the cruise, thus reducing the area of the zone in that region," said LUMC scientist Nancy Rabalais, PhD.

"The variability we see within each summer highlights the continuing need for multiple surveys to measure the size of the dead zone in a more systematic fashion," Rabalais said.

Earlier this summer, NOAA-sponsored forecast models developed by R. Eugene Turner, PhD of Louisiana State University and Donald Scavia, PhD of the University of Michigan, predicted a larger than normal dead zone area of between 7,450 and 8,456 square miles.

The forecast was driven primarily by the high nitrate loads and high freshwater flows from the Mississippi and Atchafalaya rivers in spring 2009 as measured by the U.S. Geological Survey.

"The results of the 2009 cruise at first glance are hopeful, but the smaller than expected area of hypoxia appears to be related to short-term weather patterns before measurements were taken, not a reduction in the underlying cause, excessive nutrient runoff," said Robert Magnien, PhD, director of NOAA's Center for Sponsored Coastal Ocean Research.

Image of the 2009 Gulf of Mexico dead zone. Red areas indicate low oxygen levels. (Image courtesy NOAA)

"The smaller area measured by this one cruise, therefore, does not represent a trend and in no way diminishes the need for a harder look at efforts to reduce nutrient runoff," he said.

The average size of the dead zone over the past five years, including this cruise, is now 6,000 square miles.

The interagency Gulf of Mexico/Mississippi River Watershed Nutrient Task Force has a goal to reduce or make significant progress toward reducing this dead zone average to 2,000 square miles or less by 2015. The Task Force uses a five year average due to relatively high interannual variability.

The models used to forecast the area of the dead zone are constructed for understanding the important underlying causes to inform long-term management decisions, but they do not include short-term variability due to weather patterns.

Prior to the Louisiana consortium cruise, NOAA's Southeast Monitoring and Assessment Program found a similar sized dead zone during its annual five-week summer fish survey.

The Gulf of Mexico dead zone was severe where it did occur, the scientists said. Usually, the dead zone is limited to water just above the sea floor, but this year's low-oxygen area extends closer to the water surface than in most years.

Copyright Environment News Service, ENS, 2009. All rights reserved.