Gulf of Mexico "Dead Zone" Is Size of New Jersey

John Roach
for National Geographic News
May 25, 2005
Each year a swath of the Gulf of Mexico becomes so devoid of shrimp,
fish, and other marine life that it is known as the dead zone.

Scientists have identified agricultural fertilizers as a primary culprit behind the phenomenon. Researchers are now focusing on shrinking the zone.

Dave Whitall is a coastal ecologist with the National Oceanic and Atmospheric Administration's Center for Coastal Monitoring and Assessment in Silver Spring, Maryland. He said the dead zone forms each April and lasts through the summer, adding that the zone "generally grows throughout the summer, reaching a peak in late July."

At its peak, the nearly lifeless water can span 5,000 to 8,000-plus square miles (13,000 to 21,000 square kilometers), an area almost the size of New Jersey.

The dead zone is the result of oxygen-depleted water. Fish, shrimp, and all other marine organisms that require oxygen to survive either flee the zone or die.

Whitall says the phenomenon is triggered by excess nutrients in the Mississippi and Atchafalaya River Basins. Streaming into the Gulf of Mexico along the Louisiana coast, the rivers drain about 40 percent of all U.S. land area and account for nearly 90 percent of the freshwater runoff into the Gulf.

Algal Blooms

The nutrients that flow into the Gulf of Mexico allow microscopic organisms called phytoplankton to bloom. When these algae die, they sink to the bottom of the ocean. There, they are decomposed by oxygen-consuming bacteria.

In the process, the bacteria consume most of the surrounding waterborne oxygen, leaving little oxygen for the other life-forms that depend on it.

While the excess nutrients that flow into the Gulf of Mexico come from a variety of sources, Whitall said, the main source is agricultural runoff, namely chemical fertilizers and animal manure.

"Nitrogen and phosphorus also come from human waste, via wastewater treatment plants and septic systems; domestic animal waste; and industrial sources," he said. Additional nutrient sources include power plants, cars, and agricultural and industrial ammonia emissions.

Dead zones are not unique to the Gulf of Mexico.

Robert Diaz is a researcher with the College of William and Mary's Virginia Institute of Marine Science in Gloucester Point, Virginia. He and his colleagues have found evidence for more than a hundred dead zones worldwide. They range in size from 0.4 to 27,000-plus square miles (1 to 70,000-plus square kilometers).

In the U.S. these oxygen-depleted, or hypoxic, zones develop annually in western Long Island Sound off New York and Connecticut, the Chesapeake Bay off Maryland and Virginia, and the Neuse River in North Carolina.

Outside the U.S., dead zones are found in the North, Adriatic, Baltic and Black Seas and Japan's Seto Inland Sea.

Zone Reduction

Scientists first noted the annual formation of the Gulf of Mexico dead zone in the 1980s.

In 2001 a multilevel government task force led by the U.S. Environmental Protection Agency set a goal of reducing the Gulf of Mexico dead zone to a five-year running average of less than 2,000 square miles (5,000 square kilometers).

Donald Scavia, a marine scientist at the University of Michigan in Ann Arbor who led the science team that advised the task force, said the figure "represents conditions that were likely to be typical in the early 1970s—and a significant reduction from the current conditions."

Whitall, the NOAA coastal ecologist, said, "To achieve this goal, a 30 percent reduction in nitrogen load from the Mississippi and Atchafalaya Rivers is being implemented."

The effort focuses on refining agricultural practices in the watershed. Such tactics include timing of fertilizer applications better, plowing more efficiently, restoring natural wetlands, and improving manure management.

Computer modeling by Scavia and colleagues suggest that, to reduce the Gulf of Mexico dead zone to 1970s levels, a 40 to 45 percent reduction in nutrient runoff may be necessary.

"This does not negate the original agreement," Scavia said. "In fact, it supports it. This basin is a massive, slow-moving system. And moving toward practices that produce [a] 30 percent or 40 percent [reduction] is appropriate."

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).


© 1996-2008 National Geographic Society. All rights reserved.