Resurrecting the Dead Zone

Reducing U.S. agribusiness' nitrogen runoff could reverse past damage to the Gulf of Mexico

• By Erica Pincus
• Sep 01, 2006

The Dead Zone -- sounds creepy doesn't it? But what is it? It's a crisis that's attacking oceans and bays throughout the world, and a reality more frightening than current governmental policies and actions have led the public to believe. Over the past 13 years, oxygen levels in the Gulf of Mexico have dropped to levels that endanger the health of aquatic animal and plant species and will soon no doubt impede the production of the coast's fisheries. Aside from these environmental and economic impacts, nearly 50 percent of the American population lives on coastal lands that are trapped in a cycle of pollution and health imperilment. The menace behind this situation is oxygen depletion down to levels of 2 milligrams per liter (mg/l) or less, a condition known as hypoxia. Under these conditions, aerobic species cannot survive, so they are forced to migrate further offshore or die.

The Louisiana Universities Marine Consortium discovered the hypoxic zone of the Gulf of Mexico in 1974, and an original member of this team, Nancy N. Rabalais, PhD, has annually monitored the location's size since 1985. Although scientists were aware of the situation, it went seemingly unnoticed until flooding of the Midwest in 1993 caused the Dead Zone to double in size. The Zone is seasonal and ceases in the winter months, but every April through September it returns to roughly the same size and has been slowly increasing since 1993. In 1993, Rabalais saw a hypoxic zone of 6,800 square miles, and by 1995, it had increased to 7,032 square miles. In 2001 the Dead Zone covered 7,722 square miles. In contrast, during the summer of 2005 the area was determined to cover 4,564 square miles, or roughly the size of Connecticut. Since monitoring began in 1985, the hypoxic zone has averaged 4,800 square miles, making it the second largest hypoxic zone in the world, right behind the Black Sea.

Causes of the Dead Zone
Nonpoint sources are the most pervasive contributors to the Gulf Coastal hypoxic zone. The increase of nitrogen loads, freshwater discharge, and sediment discharge throughout the area has resulted in unsustainable oxygen levels. The Mississippi River Basin encompasses 41 percent of the lower 48 states and all rivers in this region exit into the Gulf of Mexico through the Mississippi River or the Atchafalaya River. These two rivers are responsible for 80 percent of the total water discharge and 91 percent of the nitrogen load into the Gulf.

The most direct cause of hypoxia is nitrogen (more specifically nitrate) loads from fertilizer runoff, decomposed crop residue, human and animal waste, and atmospheric deposition. The breakdown of nitrogen in the Gulf of Mexico is as follows: 70 percent fertilizers, 11 percent municipal sewage, 12 percent animal waste, and 6 percent atmospheric deposition. When nitrogen enters the water system, it causes massive algal blooms, resulting in an influx of organic material. This substantial intake of oxygen depletes the water source and results in hypoxia. Loads from the Mississippi River have grown two- to seven-fold over the last century, and unmanageable, large-scale agricultural practices are to blame. The Midwest loads the 3 million acres of wetlands with an estimated 7.8 billion pounds of fertilizer each spring. Since nitrogen is stored in soils and groundwater, nitrates will continue to be loaded in the water supply even after fertilizer usage ceases.

The Environmental Working Group (EWG), a nonprofit environmental research organization based in Washington, D.C., issued a report in August 2005 explaining the effects of farming practices and government subsidies on the hypoxic zone. The report stated that, for every tax dollar spent on conservation, there are $500 tax dollars devoted to farm subsidies. One hundred and twenty four counties deemed as top polluting counties, in Illinois, western Iowa, western Indiana, northeastern Arkansas, and southeastern Missouri received $11.4 billion in subsidies from 1995 through 2002; during this same period, government spending on water quality payments was only $22.5 million.

Although they cover only 5 percent of the Mississippi River Basin, these government-funded farms account for 40 percent of nitrate fertilizer pollution in the Gulf. An Action Plan proposed by the National Oceanic and Atmospheric Administration (NOAA) in 1999 is mostly a call for voluntary efforts on the part of farmers and agribusinesses to use best management practices to prevent fertilizer runoff from farmed lands. Although the government is encouraging sustainable farming techniques, the practicality of that request is minimal. Farmers, in general, are more concerned with the rising price of energy than a remote body of water. The EWG report suggested that the Dead Zone could be reduced by shifting a portion of the government subsidies, which often go to the largest and most profitable agricultural operations, into programs that encourage more careful fertilizer use, wetland restoration, and the planting of streamside buffers of grass and trees to absorb runoff.

Freshwater discharge is also cause of concern for the Gulf Coast. High freshwater dischargers into the Gulf are not as salty as the existing water, and they float to the top. This prevents oxygen recharge to lower waters and depletes benthic oxygen at the lowest strata of the ocean. The Mississippi and Atchafalaya Rivers account for 80 percent of the total freshwater input to the Gulf. The U.S. Army Corps of Engineers began maintaining water delivery of the coast in 1977 at 30 percent, but the discharge from the Atchafalaya continues to increase. The increase in freshwater discharge has been associated with increased precipitation from global warming.

The greater amounts of total suspended solids (TSS) in a body of water, the murkier, or more turbid it is. Phytoplankton is the most typical source of turbidity. The direct correlation between nitrogen levels and phytoplankton populations prove that this is a treacherous cycle. Watershed development and poor land-use practices increase erosion, organic matter, and nutrients, which all cause an increase in suspended particulates and algae growth. High concentrations of particulate matter can modify light penetration and smother benthic communities.

As matter settles to the ocean floor it can suffocate newly hatched larvae and displace space that could have been used for habitat. After days of exposure to TSSs measuring over 100 nephelometric turbidity units (NTU), fish will abandon cover, show avoidance behavior, increase respiration, reduce feeding rates, and increase coughing rates. Weeks of high turbidity result in reduced growth rates, delayed hatching rates, and long-term reduction in feeding successes; and months of exposure will lead to death. Aside from the immediate effects of TSS, organic nitrogen desorbs from sediment particles in the freshwater/seawater mixing zone and contributes to the total nitrogen load of the Gulf of Mexico.

Effects on Aquatic Organisms
When waters reach the dangerously low oxygen levels of less than 2 mg/l, aerobic species are forced to migrate farther offshore or die. Species exposed to hypoxic conditions experience decreased size of reproductive organs, low egg counts, and a lack of spawning. It has been found that female fish living in hypoxic areas have one-seventh the number of eggs as fish living in normal conditions. The most common problems associated with hypoxia are avoidance of an area and migratory disturbances, which interferes with regular life processes.

Benthic species -- organisms that live in the bottom of water bodies -- are accustomed to cool temperatures, low oxygen levels, and little to no sunlight, yet these species are the first to feel the effects of hypoxia. Slow-moving clams, lobsters and oysters are unable to escape the hypoxic areas and die. Brown shrimp is a commercially important benthic species whose migratory patterns have been dramatically changed by depleted oxygen. Brown shrimp are affected more than similar species because they require offshore shelf habitat; thus, they inhabit a guaranteed hypoxic area. Limited migration has left fisheries with new problems. The brown shrimp catch in the Gulf declined from a record high in 1990 to below the historical average during 1992 to 1997, coinciding with years of high hypoxia.

Algal blooms associated with hypoxia pose an immediate threat to fish living in the Dead Zone. When excess nitrates enter the body of water, organic material responds with massive growth. Algal blooms are directly linked to fish kills, and, at the very least, fish habitats are restricted and the risk of predation mortality increases.

Impact on the Gulf Region's Economy
The Gulf of Mexico supports many local residents through the commercial fishing of red snapper, amberjack, tilefish, swordfish, shrimp, and crabs. As well, the region supports many industries, including shipping, petrochemical processing, paper manufacturing, and tourism. Residents of the region have strong economic ties to the Gulf of Mexico that are being threatened by the far-ranging impacts of hypoxia.

The Louisiana Department of Wildlife and Fisheries conducted a project from 1998 to 2002 titled "Effects of Hypoxia on Commercial Shrimp and Charter Boat Fisheries in Louisiana." This project was conducted along the Louisiana coast from Quatre Bayou to Belle Pass to determine the impacts of adverse environmental and climatological conditions on fishing patterns and subsequent income of commercial fishermen. Through a voluntary logbook by coastal fishermen and biweekly oxygen concentration measurements, the project concluded that environmental events are rarely isolated to a time frame suitable for analysis, and no correlation between hypoxia and lowered productivity could be established. It did note that fishermen showed a drastic drop in activity during the month of July when hypoxia is at its peak, so growth in productivity may be stifled by the condition.

Although commercial fishing in the Gulf Region has not yet suffered a steep decline as a result of hypoxia, this does not mean the economy will not be affected in the future. Based on other historical hypoxic events, the possible adverse impacts on the fisheries could progressively worsen as the level of hypoxia in the area increases. Additionally, some experts worry about the possible negative health effects on people who consume the fish, shellfish, and other animal species that are harvested from the Dead Zone area of the Gulf of Mexico.

Government in Action
In 1997, the White House Office of Science and Technology Policy commissioned the U.S. Department of Commerce, as well as the National Oceanic and Atmospheric Administration (NOAA), the National Ocean Service, the National Centers for Coastal Ocean Science, and the Center for Sponsored Coastal Ocean Research to put together a series of reports to explain the hypoxic situation of the coast and propose a plan of rehabilitation. Topics included a general explanation of hypoxia, the ecological and economic consequences of hypoxia, the flux and sources of nutrients in the river basin, the effects of reducing nutrient loads, and an evaluation of the costs and benefits of reducing nutrient loads in the hypoxic zone.

The reports forecasted that a 30 percent reduction in nitrogen loads will solve hypoxia in the Gulf of Mexico and improve water quality and habitat within the Mississippi River Basin. Furthermore, the report stated this goal was to be obtained by the Action Plan. This plan spelled out a voluntary, incentive-based strategy to implement best management practices on agricultural lands, wetland restoration and creation, river hydrology remediation, and stormwater and wastewater nutrient removal.

Solving the Dead Zone Problem
If the government's advice is properly executed and U.S. agribusinesses internalize the cost of managing their pollutants, a 20 percent decrease in nitrate loads is economically feasible. One way to do this is to cut down on fertilizer use and apply fertilizers at rates and times at which they can be absorbed and used most efficiently. According to NOAA reports, the demand for fertilizer is inelastic so a 500-percent tax would be necessary for a 45-percent nitrate decrease, resulting in an aggregate loss of $14.9 billion. This approach is unrealistic from an economic perspective; consequently, it appears that it would be far more effective to place some type of restriction on the amounts of fertilizer used by agricultural operations.

Another possible way to deal with the Dead Zone is to restore wetlands to intercept nitrogen and cause dentrification. By restoring 13 to 18 million acres of wetlands at $495 million to $32.4 billion, the total nitrate loads would decrease by 50 to 70 percent. Additionally, this initiative would preserve critically endangered areas. The least economically desirable approach is using riparian buffers to intercept nitrogen runoff and promote dentrification before water reaches streams and rivers. According to research, approximately 46 million to 68 million acres of buffers, which would cost at an estimated $46.3 billion, would be needed to accrue a 50- to 80-percent decrease in nitrate loads. The high opportunity cost of retiring fertile land compared to the relatively small environmental payoff makes this option unattractive.

The most feasible way to reach the goal of a 20-percent decrease in nitrate loads is to use both pre-emptive methods, like fertilizer restrictions and riparian buffers, and rehabilitative methods, including restoring wetlands. Achieving this goal would decrease the contamination of surface water, which, in turn, would reduce the cost of treating raw water in order to convert it into potable drinking water. This approach would also expand the area of recreational waters, reduce soil erosion, restore wetlands, and reduce nutrient contamination of drinking water.

This article originally appeared in the 09/01/2006 issue of Environmental Protection.