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Dead Zone

Task force seeks advice on how to reduce and control Gulf of Mexico's low-oxygen zone

by Cheryl Hogue
October 2, 2006 | A version of this story appeared in Volume 84, Issue 40

Credit: NOAA
Low-oxygen concentration in areas of the northern Gulf of Mexico, shown in red and orange, were measured by National Oceanic & Atmospheric Administration ships.
Credit: NOAA
Low-oxygen concentration in areas of the northern Gulf of Mexico, shown in red and orange, were measured by National Oceanic & Atmospheric Administration ships.

Each year, a vast area of water off the Gulf of Mexico's northern shore turns from a viable habitat for fish and other marine animals into a biological dead zone characterized by low oxygen, or hypoxia. A federal-state task force is seeking scientific input from the Environmental Protection Agency to guide its revision of the action plan to control and reduce this dead zone.

The size of the zone has increased since the 1980s, averaging in recent years about 15,000 km2, larger than Connecticut. Scientists have identified plant nutrients—primarily nitrate—in runoff carried by the Mississippi River as the major contributors to the formation of the hypoxic zone in the Gulf. Some 74% of the nitrate flushed into the Gulf each year washes off farm fields in the Mississippi River Basin. Another 10% comes from wastewater treatment plants, and rest is from other runoff.

In the spring, when nutrient runoff is at its peak, phytoplankton in the Gulf feast on the nutrients discharged by the river. The microscopic plants grow in massive numbers and then eventually die. Their tiny carcasses drift from near the surface into the colder, saltier, and denser lower layer of water below. There, bacteria consume the dead phytoplankton along with much of the oxygen in the water, leaving less than 2 ppm of the dissolved gas. A healthy concentration of dissolved oyygen is around 5 ppm. The oxygen is not replenished because the heavier layer of water at the bottom does not mix with the lighter, fresher, warmer, and more oxygenated water nearer to the surface. Fish are able to swim away from the hypoxic zone to waters richer in oxygen, but many bottom-dwelling creatures, such as crabs, die because they cannot move as fast.

The Gulf's dead zone, which is prevalent from late spring to early fall, reaches its maximum size each July.

Controlling the Gulf's dead zone remains a challenge. Last month, EPA's Science Advisory Board (SAB) began evaluating the complex scientific and technical issues that affect the causes, location, magnitude, and duration of hypoxia in the northern Gulf of Mexico. The report SAB will produce likely will have ramifications for agriculture in the Midwest—the source of much of the nitrate linked to the growing dead zone—as well as sales of fertilizer. The report also could impact wastewater treatment plants serving communities or industrial facilities, although these are tightly regulated under pollution control laws.

SAB's findings will influence an upcoming action plan from the federal-state Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. Formed in 1997 to address Gulf hypoxia, the task force consists of a swath of federal agencies including EPA, the Army Corps of Engineers, the National Oceanic & Atmospheric Administration, the U.S. Geological Survey, and the Department of Agriculture, as well as the White House Office of Science & Technology Policy. Other members are state agencies from Arkansas, Illinois, Iowa, Louisiana, Minnesota, Mississippi, Ohio, Tennessee, and Wisconsin.

The Atchafalaya River, which carries some of the Mississippi River's waters to the Gulf of Mexico, discharges near the continental shelf where the dead zone forms. Much of the water from the main stem of the Mississippi is diverted offshore.
The Atchafalaya River, which carries some of the Mississippi River's waters to the Gulf of Mexico, discharges near the continental shelf where the dead zone forms. Much of the water from the main stem of the Mississippi is diverted offshore.

The task force is now revising its 2001 action plan to reduce and control hypoxia in the Gulf of Mexico. The original plan relied on voluntary efforts to reduce the amount of nutrients that run off farm fields or are discharged into waterways and end up in the Gulf. The task force's goal is to complete its reassessment by 2008, according to Diane Regas, director of EPA's Office of Wetlands, Oceans & Watersheds. The new action plan will update the scientific understanding of the natural and human-induced causes of hypoxia in the Gulf and how to manage it.

At the behest of the task force, SAB is reviewing current data on the Gulf's dead zone. SAB's Hypoxia Advisory Panel, boasting a spectrum of experts, including biogeochemists, ecologists, oceanographers, a soil chemist, and economists, held its first meeting on Sept. 6-7.

The panel is charged with examining three related matters: the chemical, physical, and biological causes of hypoxia; the sources, transport, and fate of nutrients that are believed to encourage formation of the low-oxygen zone; and the scientific bases for policy options to manage hypoxia.

Having the best scientific information on hypoxia in the Gulf is essential to convincing upstream farmers and others to change their land management practices voluntarily, Regas told the SAB panel. But economic factors are key to implementing any successful changes in the current strategy. The goal of the task force is to manage the Gulf's low-oxygen zone in a way that protects the economies upstream, she said, and it is using a policy technique called "adaptive management." This means taking action, such as reducing nutrient flow into the Gulf, on the basis of the best available science data and revising the strategy over time in light of new information, she explained.

Regas encouraged the SAB panel to provide information to the task force such as how long it will take to see a change in the size of the hypoxic zone after cutting the nitrate runoff or discharges. Darrell Brown, leader of EPA's hypoxia team, said the task force also wants the panel to describe areas where scientific information is lacking and to recommend topics for further study that can be funded by federal agencies or states. Regas acknowledged that some of the monitoring and research projects recommended by the task force years ago were never funded.

Research has brought to light more information about factors that contribute to Gulf hypoxia since the task force's last scientific assessment in 2000, said Richard M. Greene, chief of the Ecosystem Dynamics & Effects Branch in the EPA Office of Research & Development's Gulf Ecology Division.

For instance, some scientists have concluded that water carried by the Atchafalaya River may be as large an influence on hypoxia as the discharges of the main stem of the Mississippi River, Greene said. The Atchafalaya begins where the Red River meets the Mississippi River near Simmesport, La. As a major distributary of the Mississippi, the Atchafalaya carries some of the mighty river's load of water, sediment, and nutrients to the Gulf. The Atchafalaya discharges into the Gulf south of Baton Rouge and well west of the main stem of the Mississippi, which flows into the Gulf southeast of New Orleans.

Once the Mississippi hits the Gulf, its freshwater with its nutrient runoff is directed away from the continental shelf by currents and a deep, underwater canyon just offshore of the river's mouth, Greene explained. The Atchafalaya, meanwhile, discharges its water and sediment load onto the continental shelf, which is where hypoxia develops.

The level of nutrients, primarily nitrate, carried in the Mississippi's waters is highest in spring and early summer, during the river's peak flow, Greene continued. Researchers have found that this increase in nitrogen has altered the nitrogen-to-phosphorus ratio in the northern Gulf waters so much that the availability of phosphorus has become the limiting factor for the size of phytoplankton blooms, he said.

In addition to scientific advice, the task force is seeking feedback from the SAB panel on a possible ecological goal for policies to manage hypoxia. John Wilson, a staff member in EPA's Coastal Management Branch, told the panel that researchers have estimated that the historic size of the Gulf hypoxic zone averaged about 5,000 km2, about a third of its current size. The task force is considering a return of the dead zone to this historic level as the goal for actions to reduce nutrient loads to the Gulf. If scientific data suggest a different size, Wilson said, the SAB panel should indicate so in its report.

This possible goal generated a number of comments from members of the SAB panel, including many who probed the rationale for policies to shrink the dead zone.

Thomas S. Bianchi, professor of oceanography at Texas A&M University, said researchers have not yet determined whether hypoxia in the Gulf of Mexico is harmful to the ecosystem. Hypoxia has been found to have negative effects in estuaries, like the Chesapeake Bay. But the Gulf is less like an estuary and more akin to an open coastal system, he said. Some paleoecological studies show that periodic low-oxygen conditions are the nature of delta ecosystems, such as those around the mouth of the Mississippi, Bianchi said.

Kenneth H. Reckhow, professor of water resources at Duke University, wondered whether EPA officials have specified just what valued resources are at risk from the hypoxic zone. Virginia Dale, the panel's chairwoman, noted that the Gulf Coast shrimp fishery is the most vulnerable fishery in the U.S. Dale is a corporate fellow in environmental sciences at Oak Ridge National Laboratory.

Some of the advisers suggested that the task force needs to gather more information before revising its strategy for controlling hypoxia. James J. Opaluch, professor of environmental and natural resource economics at the University of Rhode Island, said the SAB panel needs to have more data on the expected benefits of controlling hypoxia in the Gulf before it can give counsel on what the best policy choices might be for controlling it. "You can't ask people to pay for something you haven't quantified," commented Catherine L. Kling, an economics professor at Iowa State University.

Benjamin Grumbles, EPA assistant administrator for water, told the SAB panel that the dead zone in the Gulf of Mexico is "an important environmental and economic issue." He said the task force wants to "put an end to disputes, myths, by having the facts, the knowledge."

The SAB panel report on the Gulf dead zone is due to be completed by mid-2007. The task force will rely on that report as it updates the federal and state strategy for controlling hypoxia.


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