Dead Zone in Gulf Is Smaller Than Forecast but More Concentrated in Parts

July 28, 2009, The New York Times

Scientists said Monday that the region of oxygen-starved water in the northern Gulf of Mexico this summer was smaller than forecast, which means less disruption of shrimp, crabs and other marine species, and of the fisheries that depend on them. 

But researchers found that although the so-called dead zone along the Texas and Louisiana coasts was smaller — about 3,000 square miles compared with a prediction of about 8,000 square miles — the actual volume of low-oxygen, or hypoxic, water may be higher, as the layer is deeper and thicker in some parts of the gulf than normal. And the five-year average size of the dead zone is still considered far too big, about three times a target of 2,000 square miles set for 2015 by an intergovernmental task force. 

“It’s a smaller footprint,” said Nancy N. Rabalais, executive director of the Louisiana Universities Marine Consortium, at a telephone news conference announcing the finding. She said unusual winds and currents this spring had driven much of the hypoxic water to the east, reducing the size of the zone but concentrating it. “In actuality we found quite a severe area that was large in volume,” she said. “Organisms were obviously stressed.” 

Forecasts for hypoxic zones in the gulf are based on measurements of the nitrogen and phosphorus entering the water from agricultural runoff and other sources in the Mississippi River watershed. The forecast earlier this year was for a zone that would come near the record 8,500-square-mile-zone detected in 2002. 

“But the model is based on predictions of what the zone would look like in a normal physical environment,” said Donald Scavia of the University of Michigan, one of the forecast’s preparers. “But this year we didn’t have normal physical conditions.” 

When nitrogen and phosphorus enter the gulf, these nutrients cause an overabundance of algae — too much for other marine organisms to consume. Some of the algae die, sink to the bottom and decompose, and the bacteria that do the decomposing use up most of the oxygen in the water. 

Faced with depleted levels of oxygen, fish and other creatures that can swim will leave for other waters. Those that cannot leave often die or show signs of reproductive or other stress. Shrimp and other fisheries in the gulf can be affected for weeks or longer. 

In an interview, Dr. Rabalais, who has been mapping the gulf hypoxic zone during summer research cruises for 25 years, said that in the most affected areas, where levels of dissolved oxygen were near zero, she and her colleagues saw crabs, eels and brown shrimp swimming toward the surface, fleeing the low-oxygen water. Predator fish were obviously affected too, she said, as there were none around to eat the smaller escapees. “Any self-respecting fish would have eaten those brown shrimp,” she said. 

Jane Lubchenco, under secretary of commerce and administrator of the National Oceanic and Atmospheric Administration, said the gulf dead zone was the most “notorious” of about 250 such regions around the country. 

Agricultural runoff, she said, “continues to wreak havoc with life in the gulf.” Governments are working to promote programs to reduce nutrient runoff, like “engineered” wetlands that can remove nitrogen, but Dr. Lubchenco added, “Some progress is being made, but not enough.”