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Fish-Free Zone

•  The Mississippi is the longest river in the United States, flowing more than 2,000 miles (3,200 kilometers) from Minnesota to Louisiana into the Gulf of Mexico.
•  Over 40 percent of the United States is drained by the Mississippi and its tributaries, including most of the states between the Rockies and the Appalachians.
•  Each year more 1.6 million metric tons of nitrogen from human activities in the Mississippi watershed wash into the Gulf of Mexico, setting off a chain of events each summer that creates a vast "dead zone."
•  The Mississippi's flow is restrained by 1,600 miles (2,500 kilometers) of flood-controlling levees, which separate the river from wetlands that filter nitrogen and other pollutants.

From the surface you would never know it's there. But beneath the waves of the Gulf of Mexico lurks the "dead zone," a vast area off the Louisiana-Texas coast where oxygen-depleted water collects every summer and suffocates sea life. Fish and shrimp swim away; starfish, sea anemones and other bottom-dwellers die. This barren area has grown as large as the state of New Jersey.

"When I'm diving in it, I see nothing," says Nancy N. Rabalais, a marine scientist with Louisiana Universities Marine Consortium, whose team has led research efforts on the Gulf's dead zone. "Maybe a few fish near the surface, lots of dead animals and stringy bacterial mats on the bottom."

The smothering layer lingers on the bottom from May through October, but is most severe in June, July and August, when the Gulf is its warmest and most calm. The zone has sprawled to nearly 8,000 square miles (20,000 square kilometers). Until winter's winds and water-churning cold break up the layer, fishermen have to travel much farther to find fish. Officials worry about disruption-even collapse-of the region's U.S. $26 billion-a-year fish and shellfish industry, including the oyster harvest in Texas, which usually peaks between Thanksgiving and Christmas.

Taking It Seriously

The Gulf's dead zone was discovered in 1974, but few people other than scientists took note until its size doubled following massive flooding that inundated the Midwest in 1993. Concern sharpened during subsequent years when the dead zone did not return to pre-flood size. By 1997, the White House was alarmed enough to launch the first large-scale study of the problem. Six teams of distinguished scientists, working together as the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, were asked to determine what causes the dead zone and what could be done about it.

The Task Force determined that the main cause is excess nutrients, especially nitrogen, washed into the Gulf from the Mississippi River. The annual discharge of the river, which drains 40 percent of the United States, includes 1.6 million metric tons of nitrogen, more than half of which enters the river in the form of fertilizer runoff, according to a study by the U.S. Geological Survey. The rest comes from other sources, including animal waste, industrial and sewage treatment wastewater, and atmospheric pollution.

Excess nitrogen feeds an explosion of algae, which eventually dies and sinks. As bacteria decompose the dead material, oxygen is depleted from bottom waters. When sea life can no longer survive, the condition is known as "hypoxia." At that level, says Rabalais, "trawlers catch little or nothing in their nets."

Barren areas, often called "dead zones," are becoming a problem in coastal areas around the world. Oxygen depletion occurs during the summer in more than half of U.S. estuaries. Serious conditions develop annually in Long Island Sound and the Chesapeake Bay, among other areas.

Hypoxia in coastal areas is exaggerated by changes on the land. Within the Mississippi watershed, millions of acres of wetlands have been drained for farmland and development, and a huge system of levees built to contain high water now separates the river from its natural floodplains of bottomland forests. Wetlands and bottomland forests naturally filter runoff, retaining nitrogen that is later released harmlessly into the atmosphere.

Studies show that the level of oxygen depletion in the Gulf has steadily worsened over the past two centuries, rapidly accelerating since the 1950s. Mud core samples taken from the seafloor where the dead zone regularly develops demonstrate an increase in deposits of algae and a corresponding decrease in sea life that requires higher levels of oxygen. Although the size of the dead zone varies from year to year, its average size has increased. According to assessments carried out since 1985, it doubled during the 1990s.

Shrinking the Dead Zone

Environmentalists have pressed for aggressive efforts to reduce the dead zone. Farmers countered that reductions in their use of fertilizer would be economically ruinous. Federal and state officials agreed in October on a compromise plan to reduce the dead zone to half its average size-from 3,800 square miles (10,000 square kilometers) to 1,900 square miles (5,000 square kilometers).

The plan is far from a reality. Rather than a federally imposed solution, the 31 states and 71 tribes in the Mississippi watershed would develop their own plans to reduce nutrients in steams and rivers, generally aiming at an overall 30-percent reduction in nitrogen discharged into the Gulf by 2015. Congress would have to appropriate approximately U.S. $1 billion for several years to help fund plans in each of the states to upgrade sewage treatment plants, install new wetlands and forest buffers along waterways, and develop programs to use fertilizer more efficiently.

"It's a jurisdictional nightmare," says Rabalais. "But the problem is not going to go away until we reduce nutrients in the river."


More Information
Hypoxia: Sometimes A Good Thing

Low oxygen, or hypoxia, threatens marine environments in the Gulf of Mexico and other coastal areas. Troublesome hypoxia is caused by excess nutrients resulting from human activities. But hypoxia and anoxia-no oxygen- occur naturally in some ocean environments.

Sea water that is both deep and still, such as at the bottoms of some fjords, tends to be hypoxic or anoxic. Because wood-boring sea creatures need oxygen to survive, these deep-sea environments are thought to be littered with the wooden remains of shipwrecks and other well-preserved evidence of early human activities.

This past September an expedition led by ocean explorer Robert Ballard deployed remotely operated vehicles (ROVs) to the naturally anoxic bottom of the Black Sea. The team, sponsored in part by the National Geographic Society, was searching for evidence of human habitation before a catastrophic flood-possibly Noah's Flood-inundated the area many thousands of years ago.

The mission was a success. Well below the so-called 'dead zone,' Ballard and his team discovered the well-preserved remains of a 1,500-year-old shipwreck. The condition of the wreck gives archaeologists hope that the anoxic layer of the Black Sea has preserved organic materials like clothing and food, which are quickly destroyed in oxygenated waters.