Gulf of Mexico Dead Zone Smaller Than Predicted, Still Harmful

By Environmental News Service
5 August 2011

CHAUVIN, Louisiana, August 5, 2011 (ENS) – This year’s dead zone in the Gulf of Mexico is roughly equal to the land area of the state of New Jersey, scientists said this week. At 6,765 square miles, this area of low oxygen is the 10th largest on record and is considered about average for the past five years.

Researchers had predicted the potential for a record-sized dead zone as large as 9,421 square miles due to the record spring flooding of the Mississippi River that sent large loads of fertilizer nutrients running off into the Gulf.

But strong winds and waves associated with Tropical Storm Don in late July mixed the layers of water, re-oxygenating the western portion of the dead zone.

"Although Tropical Storm Don disrupted part of the hypoxic zone, our monitoring over the past several months indicated the spring floods expanded the dead zone region," said Nancy Rabalais, PhD, executive director of the Louisiana Universities Marine Consortium, who led the NOAA-supported research team.

Nancy Rabalais (Photo courtesy LUMCON)

"However, sampling the hypoxic bottom layer on a ship rolling in 6-10 foot waves presented safety and sampling issues that interfered with precise measurements at some stations," Rabalais explained. "For these reasons, the size of the measured hypoxic zone was smaller than just before the storm, and is probably underestimated."

In addition to surveys in the traditional region of the dead zone, Rabalais’ research team documented a large area of hypoxia east of the Mississippi River in mid-July.

Rabalais said the team recorded severe areas of hypoxia offshore of Louisiana at Terrebonne Bay, Atchafalaya Bay and offshore of Barataria Bay – all areas just west of the Mississippi Delta that were affected by last year’s BP Deepwater Horizon oil spill.

However, she said, the low oxygen areas are probably unrelated to the BP oil spill. Scientists will not be able to say exactly what effect the oil spill has had, said Rabalais, because no sediments or water samples were tested for oil this year.

Brown runoff is visible at the mouth of the Atchafalaya River. (Photo courtesy U.S. Army Corps of Engineers)

"The samples we collected last year are still awaiting funding for analysis," she told reporters on a teleconference August 1.

When the oxygen level in the Gulf water becomes very low, sediments on the sea floor release hydrogen sulfide, a rotton-egg-smelling gas which is toxic to organisms.

Creatures that usually live in the sediments, such as eels and crabs, were observed swimming up at the surface off Grande Isle, Barataria Bay, and in the Port Fourchon area to escape the hydrogen sulfide, said Rabalais.

The dead zone is fueled by nutrient runoff from agricultural and other human activities in the Mississippi River watershed, which stimulates an overgrowth of algae that sinks, decomposes and consumes most of the life-giving oxygen supply in bottom waters.

The hypoxic zone off the coast of Louisiana and Texas forms each summer and threatens commercial and recreational Gulf fisheries.

"Despite fluctuations in size due to each year’s weather conditions, these chronic, recurring hypoxic zones every summer represent a significant threat to Gulf ecosystems," said Robert Magnien, PhD, director of NOAA’s Center for Sponsored Coastal Ocean Research.

"Until we achieve a substantial reduction in nutrient pollution from the Mississippi River watershed," said Magnien, "we will continue to experience extended periods of time each year when critically-needed habitat is unavailable for many marine organisms."

Rabalais says there was no dead zone in the Gulf of Mexico before the 1950s. Sediment analysis shows that the Gulf has not always had hypoxia, she said, and it has gotten worse over time.

"It is possible to reduce the size of the dead zone if the nutrient level is reduced," said Rabalais. "In other areas of the world, the problem has been alleviated."

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