Dead zones

The term dead zone refers to those areas in aquatic environments where there is a reduction in the amount of dissolved oxygen in the water. The condition is more appropriately called hypoxia or hypoxic waters or zones. Hypoxia in marine environments is determined when the dissolved oxygen content is less than 2–3 milligrams/liter. Five to eight milligrams/liter of dissolved oxygen is generally accepted as the normal level for most marine life to survive and reproduce. Dead zones can not only reduce the numbers of marine animals, but they can also change the nature of the ecosystem within the hypoxic zone.

The main cause of oxygen depletion in aquatic environments is eutrophication. This process is a chain of events that begins with runoff rich in nitrogen and phosphorus that makes its way into rivers that eventually discharge into estuaries and river deltas. This nutrient-laden water, combined with sunlight, stimulates plant growth, specifically algae, seaweed, and phytoplankton . When these plants die and fall to the ocean floor, they are consumed by bacteria that use large amounts of oxygen, thereby depleting the environment of oxygen.

Dead zones can lead to significant shifts in species balances throughout an ecosystem. Species of aquatic life that can leave these zones—such as fish and shrimp—do so. Bottom-growing plants, shellfish, and others that cannot leave, die, creating an area devoid of aquatic life. This is the reason the term dead zone has been aptly used (especially by the media) to describe these areas.

Eutrophication of estuaries and enclosed coastal seas has often been a natural phenomenon when offshore winds and water currents force deep nutrient-laden waters to rise to the surface, stimulating algae bloom. The timing and duration of these conditions varies from year to year and within seasons. Climatic conditions and catastrophic weather events can influence the rapidity hypoxia can occur. In the past, these natural hypoxic areas were limited, and the marine environment could recover quickly.

Nutrient availability, temperature, energy supply (i.e., soluble carbon for most microorganisms or light and carbon dioxide for plants), and oxygen status all affect the growth and sustainability of aquatic plants and animals. One condition that perpetuates hypoxia is elevated temperatures because the ability of water to hold oxygen (i.e. water solubility) decreases with increasing temperature. Situations that promote consumption of oxygen such as plant and animal respiration and decomposition can also lead to hypoxic conditions in water. Therefore, situations that stimulate plant (i.e. phytoplankton, benthic algae and macroalgae) growth in water can lead indirectly to hypoxic conditions. Algal growth can be accelerated with elevated levels of carbon dioxide and certain nutrients (especially nitrogen and phosphorus), provided adequate sunlight is available. Low sediment loads also support increased algal growth because the water is less turbid allowing more light to penetrate to the bottom.

In the last two decades of the twentieth century, the increased incidence of hypoxia and the expanded size of dead zones have been the result of increased nutrients coming from human sources. Runoff from residential and agricultural activities are loaded with fertilizers, animal wastes, and sewage that have specific nutrients that can stimulate plant growth. In the United States, nutrient runoff has become a major concern for the entire interior watersheds of the Mississippi River Basin which drains into the Gulf of Mexico.

Hypoxic waters occur near the mouths of many large rivers around the world and in coastal estuaries and enclosed coastal seas. In fact, over 40 hypoxic zones have been identified throughout the world. Robert J. Diaz from the Virginia Institute of Marine Science has studied the global patterns of hypoxia and has concluded that the extent of these zones has increased over the past several decades. Hypoxic zones are occurring in the Baltic Sea, Kattegat, Skagerrak Dutch Wadden Sea, Chesapeake Bay , Long Island Sound, and northern Adriatic Sea, as well as the extensive one that occurs at the mouth of the Mississippi River in the Gulf of Mexico.

It is interesting to note that one of the largest hypoxic zones documented occurred in conjunction with the increase in ocean temperatures associated with El Ni ño . This weather event occurs periodically off the west coast of North and South America, and influences not only the winter weather in North America, but also affects the anchovy catch off Peru in the Pacific. This in turn, affects the worldwide price for protein meal and has a direct impact on soybean farmers (since this is another major source of protein for this product).

The large hypoxic zone in the northern Gulf of Mexico occurs where the Mississippi and Atchafalya rivers enter the ocean. The zone was first mapped in 1985, and has doubled in size since then. The Gulf of Mexico dead zone fluctuates in size depending on the amount of river flow entering the Gulf, and on the patterns of coastal winds that mix the oxygen-poor bottom waters of the Gulf with Mississippi River water. The zone in 1999 exceeded 8,006 mi² (20,720 km²) in area, making it one of the largest in the world. This zone fluctuates seasonally, as do others. It can form as early as February and last until October. The most widespread and persistent conditions exist from mid-May to mid-September.

Recent research has shown that increased nitrogen concentrations in the river water, which act like fertilizer , stimulate massive phytoplankton blooms in the Gulf. Bacteria decomposing the dead phytoplankton consume nearly all of the available oxygen. This, combined with a seasonal layering of the fresh water from the river and salt water in the Gulf, results in the zone of low-dissolved oxygen. Within the zone there are very small fish and shellfish populations.

The effect of periodic events on the transport of nutrients to the Gulf, derived primarily from fertilizer in areas of intensive agriculture in the upper Mississippi River Basin, have been implicated as the most likely cause. The largest amount of nutrients is delivered each year after the spring thaw when streams fill and concentrations of nutrients, such as nitrogen, are highest. In addition, during extreme high-flow events, such as those that occurred during the floods of 1993, very high amounts of nutrients are transported to the Gulf. Levels were 100% higher in that year than in other years.

The nature of the hypoxia problem in the Gulf is complicated by the fact that some nutrient load from the Mississippi River is vital to maintain the productivity of the Gulf fisheries but in levels considerably lower than are now entering the marine system. Approximately 40% of the U.S. fisheries landings comes from this area, including a large amount of the shrimp harvest. In addition, the area also supports a valuable sport fishing industry. The concern is that the hypoxic zone has been increasing in size since the 1960s due to human activities in the Mississippi Watershed that have increased nitrogen loads to the Mississippi River. The impact of an expanding Gulf hypoxia include: large algal blooms that affect other aquatic organisms, altered ecosystems with changes in plant and fish populations (i.e., lower biodiversity ), reduced economic productivity in both commercial and recreational fisheries, and both direct and indirect impact on fisheries such as direct mortality and altered migration patterns which may lead to declines in populations.

Studies were conducted during the 1990s on the sources of increased nutrient concentrations within the Mississippi River. A significant amount of nutrients delivered to the Gulf come from the Upper Mississippi and Ohio River watersheds. The amount of dissolved nitrogen and phosphorus in the waters of the Mississippi has more than doubled since 1960. The principal areas contributing nutrients are streams draining the corn belt states, particularly Iowa, Illinois, Indiana, Ohio, and southern Minnesota. About 60% of the nitrate transported by the Mississippi River comes from a land area that occupies less than 20% of the basin. These watersheds are predominantly agricultural and contain some of the most productive farmland in the world. This area produces approximately 60% of the nation's corn. The U.S. Geological Survey has estimated that 56% of the nitrogen entering the Gulf hypoxic zone originates from fertilizer. Potential agricultural sources within these regions include runoff from cropland, animal grazing areas, animal waste facilities, and input from agricultural drainage systems. The contributions to nutrient input from sources such as atmospheric deposition , coastal upwelling, and industrial sources within the lower Mississippi Watershed are being evaluated also. It is unclear what effect the damming and channelization of the river for navigation has on nutrient delivery. The dead zone area in the Gulf will continue to be monitored to determine whether it continues to expand.

In the meantime, efforts to reduce nutrient loading are being undertaken in agricultural areas across the watershed. Several strategies to reduce nutrient loading have been drafted. They are: a reduction in nitrogen-based fertilizers and runoff from feedlots , planting alternative crops that do not require large amounts of fertilizers, removing nitrogen and phosphorus from wastewater , and restoring wetlands so that they can act as reservoirs and filters for nutrients. Depending on whether the zone continues to expand or decreases as nutrient levels diminish will determine whether it remains a significant environmental problem in the Gulf of Mexico. Knowledge gained from the study of changing nutrient loads in the Mississippi River will be useful in addressing similar problems in other parts of the world.

[James L. Anderson and Marie H. Bundy ]

RESOURCES

PERIODICALS

Rabalais, N. N., R. E. Turner, D. Justic, Q. Dortch, W. J. Wisenman, Jr., and B. K. Sen Gupta. "Nutrient changes in the Mississippi River and septum responses on the adjacent continental shelf."Estuaries 19, no. 2B (1996): 386-407.

Turner, R.E., and N.N. Rabalais. "Changes in Mississippi River water quality this century: Implications for coastal food webs."Bio Science 41 (1991): 140-147.