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Updated 12 October, 2003
Impact, and Implications of the "Dead Zone" in the Gulf of
USGCRP Seminar, 13 July 1999
Dr. Charles (Chip) Groat
Dr. Nancy Rabalais
Dr. Donald Scavia
Every spring the process begins and culminates in a vast region of oxygen-starved ocean bottom that stretches along the Louisiana and Texas coasts. The phenomenon is known as hypoxia, but has been dubbed the "dead zone" by environmentalists and fishermen. Hypoxia is defined by a dissolved oxygen concentration in seawater of no more than 2 ppm. At oxygen concentrations below this level, fish trawlers are unable to find or capture any live shrimp or bottom-dwelling fish in their nets. The low oxygen levels drive away fish, while bottom-dwellers such as shrimp, crabs, snails, clams, starfish, and worms eventually suffocate. These features of an otherwise highly productive coastal ecosystem are cause for concern since fisheries are a vital renewable resource of the northern Gulf of Mexico.
Hypoxia results from a combination of natural and human-influenced factors. The Mississippi River Basin is the third largest in the world, after the Amazon and the Congo river basins, and drains about 41% of the conterminous United States--a total of over 3,200,000 km2--delivering freshwater, sediments, and nutrients to the Gulf of Mexico. This drainage basin includes all or part of 30 states, home to about 70 million people and one of the most productive agricultural regions in the country with over half (approximately 58%) of the total land area in the basin being devoted to cropland. The main stem of the Mississippi originates in northern Minnesota and flows southward for more than 3700 km to the Gulf of Mexico; en route the river is joined by the Missouri, Illinois, Ohio, Arkansas, and White rivers.
Upon entering the Gulf of Mexico, freshwater from the Mississippi River drainage basin floats over the saltier, denser water of the Gulf, resulting in a stratification of the water column. This stratification intensifies in the summer and prevents any oxygen housed within the upper layers of the Gulf of Mexico from being introduced to the bottom. In addition, the Mississippi River discharge contains high levels of nutrients, such as nitrogen, phosphorus, and silica, some of which is natural but much of which is derived from the widespread application of fertilizers on farmlands that drain into the Mississippi River and ultimately into the Gulf of Mexico. As in the case of fertilizers applied to grasses or crops, these nutrients stimulate the growth of phytoplankton (microscopic plants or algae) in the surface waters of the Gulf. These microscopic plants, in turn, support the rest of the marine food web. However, as these plants die and sink to the bottom, the natural decomposition of this dead plant material depletes the deeper waters of the Gulf of Mexico of what little oxygen it may contain.
Over the last 4 decades the amount of nitrogen delivered by the Mississippi River basin has tripled. More carbon is now being produced by algae than was the case historically, and conditions of oxygen stress have worsened. This same process of hypoxia occurs elsewhere in the world where humans have altered river chemistry. Notable examples are the Black Sea, Baltic Sea, Adriatic Sea, Chesapeake Bay, Long Island Sound, and the Pamlico-Albemarle Sound. The hypoxic zone in the northern Gulf of Mexico is the third largest in the world and covers an area of ocean bottom 3,000 to 4,000 mi2 in mid-summer, an area equal in size to the state of New Jersey.
Given the natural conditions of the northern Gulf of Mexico as the recipient of large quantities of freshwater and nutrients, it is logical to assume that hypoxia has always occurred in the Gulf. However, the first documented hypoxic condition along the Louisiana coast was in 1972. Thereafter, systematic sampling of the waters began in 1985. Thus, the modern observational database from which to determine long-term trends is minimal. However, the accumulation of centuries of sediment that now make up the Mississippi River delta can be used to reconstruct a reliable suite of indirect measures of historical environmental changes in the Gulf region.
Analyses of these sediments indicate that carbon production has increased, the productivity of phytoplankton, and, in particular, diatoms (microscopic marine plants whose skeletons are composed of natural glass) has increased, and oxygen conditions have worsened. Some of the changes date back to the turn of the century, but the problems have noticeably worsened and accelerated since the late 1940s and early 1950s.
This scenario of worsening oxygen conditions in coastal waters of the Gulf of Mexico adjacent to the nitrogen-enriched effluent of the Mississippi River has obvious consequences to living resources and humans. Similar situations now exist throughout the world's coastal ocean regions where environmental disruption caused by a planetary overload of nitrogen is emerging as a new global concern. Smaller ecosystems with nutrient enrichment problems have been able to rebound when management interventions have resulted in a reduction in the amount of nitrogen entering the systems in question. The scale of the Mississippi River watershed and the size of the hypoxia zone in the Gulf, however, are daunting barriers to success which is likely to come slowly.
Since the early 1900s, the hydrology of the Mississippi River system has been altered by levies, locks, dams, and reservoirs that have in turn led to dramatic changes in the transport of water, sediments, and nutrients from throughout the basin into the Gulf. Changes in agricultural practices over time--such as the use of tile-drains agricultural lands, ditches, and other means to lower the water table and increase the efficiency of farming methods--have hastened the transport of water from the landscape to the river system and subsequently to the Gulf. Nitrate, which is the most soluble and mobile form of nitrogen, is easily leached from the soils into these efficient drainage systems and is subsequently delivered much more rapidly from the land to the Mississippi River system, which has led to a larger nutrient flux to rivers in the basin and ultimately into the Gulf of Mexico over time. While long-term historical data on nitrogen concentrations in the basin are spotty, recent data suggest that average influx of nitrate to the Gulf of Mexico has nearly tripled in the last 4 decades resulting in a mean annual influx of 1.6 million tonnes of nitrogen per year. Models suggest that fertilizer and the soil inorganic nitrogen pool are the largest source of this nitrogen, contributing approximately half of the annual total nitrogen flux from the basin to the Gulf.
Coincident with the change in nitrate flux has been changes in both the average streamflow and the interannual variability of streamflow throughout the Mississippi River basin. Over the last 100 years, annual precipitation in most sites in the Mississippi River basin has increased by 5 to 20%, coincident with a nationwide average increase in precipitation of 10 to 20%. Similarly, US Geological Survey data indicates that streamflow for the Mississippi was 30% higher during 1980-1996 than between 1955-1970. Streamflow also appears to have been more variable during the last 15 to 20 years, and this variability has been shown to be strongly correlated with nitrate flux. In general, during dry years there is little rainfall to transport nitrogen from the soil and unsaturated zones to streams, and nitrogen flux (particularly nitrate) is low. Nitrate levels have been demonstrated to build up in soils during dry years, largely as a result of reduced uptake by crops. By contrast, during periods of heavy precipitation, nitrate that has accumulated in the soil can be flushed into streams via agricultural drains, groundwater discharge, and overland flow at much higher rates than usual. Thus, wet years that follow dry years tend to produce the largest influx of nitrate from the basin to the Gulf. For example, data indicate that only a small area of hypoxic waters developed in the Gulf during the 1988 drought, but the massive amount of nitrogen introduced during the flood of 1993 caused the hypoxic zone to more than double in size. In fact, the drought of 1988 and the flooding of 1993 suggest that abrupt and short-term climate events such as the El Nino Southern Oscillation (ENSO) can greatly influence the development and extent of hypoxia in the Gulf.
The influx of nitrate to the Gulf of Mexico is likely to respond quickly and dramatically to future changes in precipitation patterns and the timing of precipitation. Currently, most general circulation models (GCMs) project that increases in temperature are likely to result in a more vigorous hydrological cycle. Recent observations on water vapor appear to validate this projection. However, the most significant projected impacts from GCMs, for both the Mississippi River basin and the Gulf of Mexico, are likely to result from changes in the frequency and intensity of extreme and short-lived events such as droughts, floods, hurricanes, and El Nino/La Nina phenomena.
In considering an array of potential options for mitigating or alleviating altogether the hypoxic condition in the Gulf, one of the most attractive and potentially effective options for reducing the amount of nitrogen and other nutrients coming into the Gulf might be to create and restore strategically placed wetlands and riparian zones where they can maximally intercept agricultural drainage, thus optimize nitrogen removal through plant uptake and denitrification. The construction and restoration of strategically placed wetlands would not only contribute to the reduction of nitrogen coming into the Gulf, thus decreasing the hypoxia, but would also serve as sinks for carbon dioxide. Other benefits would include improvements in stream and river water quality and drinking water protection, enhancing terrestrial wildlife in river corridors, and providing increased flood protection. The latter is especially significant. If future climate change increases the vulnerability of the Mississippi River basin to flood events, the combined effects of reducing nutrient loading while increasing flood protection would clearly benefit beleaguered ecosystems and people alike.
Dr. Nancy Rabalais is a Professor at the Louisiana Universities Marine Consortium (LUMCON). She teaches marine science courses at LUMCON and in the Department of Oceanography and Coastal Sciences at Louisiana State University. Dr. Rabalais' research interests include the dynamics of hypoxic (oxygen-deficient) environments, interactions of large rivers with the coastal ocean, estuarine and coastal eutrophication, benthic ecology, and environmental effects of habitat alterations and contaminants. Dr. Rabalais is a Fellow of the American Association for the Advancement of Science, President of the Estuarine Research Federation, and an Aldo Leopold Leadership Program Fellow. She has also recently been named as the recipient of the Blasker Award for Science and Engineering for her outstanding scientific work on identifying and understanding the linkages between the Mississippi River drainage basin and the Gulf of Mexico. Dr. Rabalais earned her Ph.D. in Zoology from the University of Texas at Austin in 1983, and her BS and MS degrees in Biology from Texas A&I University, Kingsville, in 1972 and 1975, respectively.
Dr. Donald Scavia is the Chief Scientist for NOAA's National Ocean Service and the Director of the National Centers for Coastal Ocean Science. Prior to taking on these new roles, Dr. Scavia was Director of NOAA's Coastal Ocean Program (COP). Before coming to the COP, Dr. Scavia was a research scientist at NOAA's Great Lakes Environmental Research Laboratory (GLERL) in Ann Arbor, Michigan. While at GLERL, he carried out a broad range of field and laboratory research and modeling studies on ecosystems of the Great Lakes, with particular emphasis on food-web dynamics and nutrient cycling. He has served on the Board of Directors of the International Association for Great Lakes Research and the American Society of Limnology and Oceanography, and is currently an Associate Editor for the journal "Estuaries". He has served in the following capacities under the President's Science Advisor: Chair of the Subcommittee on U.S. Coastal Ocean Science; Executive Secretary for the Subcommittee on Water Resources, Coastal and Marine Environments; co-chair of the Ecosystem Working Group; and co-chair of the Subcommittee on Ecological Systems. Dr. Scavia holds a Ph.D in Water Resources Engineering from the University of Michigan, and BS and MS degrees in Environmental Engineering from Rensselaer Polytechnic Institute.