It can stretch for 7,000 square miles off the coast of Louisiana, a vast expanse of ocean devoid of the region's usual rich bounty of fish and shrimp, its bottom littered with the remains of crabs and worms unable to flee its suffocating grasp. This is the Gulf of Mexico's "dead zone," which last summer reached the size of the state of New Jersey.
Alarmed, the White House recently commissioned six teams of scientists to begin the first large-scale study of the area, hoping for a remission or cure.
The dead zone, researchers say, is emblematic of the growing ills suffered by the planet's seas. Earlier this month, hundreds of scientists, marking 1998 as the international Year of the Ocean, warned that unless action is taken, overfishing, coastal development, and pollution will multiply the kinds of problems that already plague the gulf.
The trouble with the dead zone is that it lacks oxygen, scientists say, apparently because of pollution in the form of excess nutrients flowing into the gulf from the Mississippi River. Animals in this smothering layer of water near the bottom of the sea must flee or perish.
"You can swim and swim and not see any fish," said Dr. Nancy Rabalais, a marine scientist at Louisiana Universities Marine Consortium who has dived in the zone. "Anything that can't move out eventually dies."
While scientists have yet to measure the impact of the zone on fishing yields, fishermen say they already feel its effects as they are forced to travel ever farther to escape the zone's barren limits.
"This is a very serious issue," said Jim Giattina, director of the Gulf of Mexico Program office at the Stennis Space Center in Mississippi. Giattina said the gulf boasts an annual catch of 1.7 billion pounds of fish and shellfish, worth $26 billion. "We've seen what can happen in other places in the world," he said. "We don't want to see a collapse of this fishery."
In fact, researchers say, the problem of rising nutrient loads and accompanying decreases in oxygen, known as hypoxia, is becoming ever more common in the coastal waters of the United States.
"Hypoxia in the gulf is a dramatic case," said Dr. Don Scavia, director of the National Oceanic and Atmospheric Administration's Coastal Ocean Program and overseer of the ongoing scientific assessment, "but it's symptomatic of what's happening coastally." More than half of the estuaries in the country experience oxygen depletion during the summer, he said, and a third experience a complete loss of oxygen.
Dr. Rabalais and her team have led the research efforts to date on the dead zone, also known as the hypoxic, or low-oxygen, zone. She and others involved in the new research initiative by the White House Office of Science and Technology Policy presented their latest findings in December at a meeting of the American Geophysical Union, in San Francisco.
The scientists trace the trouble to high levels of nutrients, in particular nitrogen, that flow out of the Mississippi and into the gulf. As in other coastal areas, these rich stores of nutrients feed algal populations which explode during the summer, producing oxygen, as all plants do. This oxygen stays near the gulf's surface. However, these blooms eventually fall to the ocean floor. When bacteria begin decomposing the dead algae, they deplete the oxygen from the ocean bottom, sometimes to the point where none is left.
At the same time, the lighter fresh water flowing in from the river forms a discrete layer on top of the heavier, salty gulf waters, keeping oxygen in the air from reaching and refreshing the hypoxic zone near the bottom of the sea.
Among the most compelling pieces of evidence are the maps researchers have made since 1985 of the hypoxic zone. Scientists measure the zone each summer, when it reaches its peak. Dr. Rabalais carries out the work along with her colleague and husband Dr. R. Eugene Turner, who is director of the Coastal Ecology Institute at Louisiana State University and who discovered the zone in 1974.
In 1993, the team witnessed a grand natural experiment as the American Midwest was deluged and the Mississippi flooded, pouring huge amounts of nutrient-rich runoff from waterlogged cities and agricultural lands into the gulf. That summer the hypoxic zone doubled in size.
In contrast, 1988 was the year of a great drought in the Midwest, Turner said, and "the hypoxic zone was almost absent," adding: "That clearly shows the influence of the river is dominant."
The team of researchers has gathered corroborative evidence from mud cores taken from the seabed of the hypoxic zone, studying algal and animal remains in the cores that are dated using radioisotopes. From these Turner and colleagues have been able to infer the relative levels of algae and oxygen in the gulf for the past 200 years. They found an increase in the amount of algae deposited, as well as a decrease in the animals that require high levels of oxygen and an increase in those that can tolerate low levels, such as microscopic, one-celled creatures known as foraminifera.
The timing of the changes, said Turner, matches well the times of known increases in nutrients in the river, with levels lowest early in the century and striking increases since the 1950s.
The timing also matches large increases in fertilizer use, suggesting farming as a key source of nitrogen in the river. In addition, a U.S. Geological Survey study estimated that more than half of the nitrogen reaching the gulf appears to come from agricultural sources.
But farm fertilizers are not the only likely culprit. A report released in December by the Senate Agriculture Committee estimated that 1.37 billion tons of manure was produced by livestock in the United States last year alone, much of it making its way to the sea.
Despite the evidence, scientists remain reluctant to blame the dead zone entirely on farmers.
"We're all fairly convinced that it's going to be agriculture that's going to have to kick in and change to some degree to make a big difference," said William Battaglin, hydrologist at the Geological Survey, in Denver, and part of a team tracking the sources of nitrogen in the Mississippi River Basin. "But we don't want to point the finger at the farmer unless we're absolutely sure. He's the one that's going to suffer."
Agriculture, researchers are quick to note, is not the only source of nitrogen in the river, which drains 31 states from Montana to New Mexico to New York, including nearly every state between the Rockies and the Appalachians. Sewage treatment waste water, industrial wastes, and atmospheric pollutants all contribute nitrogen to the Mississippi.
At the same time, difficult questions remain.
Despite the significantly decreased flow of the Mississippi since the great flood of 1993, the dead zone has grown to essentially the same size every summer. But Dr. Rabalais says there may be explanations for that. Theoretical models predicted a large dead zone in 1994, the first year after the flood, and every year since then the gulf has been hit with either floodwaters or pulses of water from the river at just the right time to boost growth of the algae, making it impossible to say with absolute certainty that the flood of 1993 caused the explosive growth of dead zone.
"I haven't had a normal year since 1994," Dr. Rabalais said. "The gulf is an uncontrolled experiment."
In addition, researchers say that while the growth of algae and the hypoxic zone appear to be controlled largely by nitrogen, complicating roles are now known to be played by silica from rocks and phosphorus from municipal waste waters and fertilizers. But their influence remain less well understood.
Another risk with increasing nutrient loads is an increase in harmful algal blooms, like those seen with Pfisteria and Pseudo-nitzschia, the algae causing amnesic shellfish poisoning, an illness that can result in permanent memory loss. Dr. Quay Dortch, an oceanographer at the Louisiana Universities Marine Consortium, said she has already found blooms of toxic Pseudo-nitzschia in the gulf.
"These are the highest concentrations of this organism registered anywhere," Dr. Dortch said of the Pseudo-nitzschia found in the plume of the Mississippi. She said they reach their peak when the river's flow into the gulf peaks. To her surprise, there has been no documented harm so far to humans from these blooms, leaving Pseudo-nitzschia in the gulf "a potential threat."
Though researchers agree that cutting the levels of nutrients in the river is the way to tame the hypoxic zone, the best method to do so remains unclear.
For example, Scavia said, though much of the nitrogen appears to be coming from the middle Mississippi, a region including Illinois and parts of Iowa, that region may not be the best one to try to control first. "It's pretty far from the gulf," he said. "It may be more appropriate and feasible to control lesser loads in other places."
Until researchers know more, Scavia and others say they are working to find changes that benefit farmers and the gulf. For example, Giattina said, farmers could turn riverside land into wetland reserves, receiving compensation at the same time that they create a buffer that reduces the nutrient load draining out of the area. Such federal and state programs to protect riverside land are now being considered in a number of areas that have water quality problems linked to agriculture.
In the meantime, each summer brings on a new dead zone that blots out vast stretches of ocean, driving away fish, shrimp, and the people searching for them.
Cynthia Sarthou, campaign director for the Gulf Restoration Network, in New Orleans, said: "If there was a dead zone 6 to 7,000 square miles in the middle of Iowa, people would sit up and take notice. This is a problem that needs to be solved."
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