A large zone of oxygen-depleted water extends across the Louisiana continental shelf and on to the Texas coast most summers. The Gulf of Mexico hypoxic zone is the largest such zone in coastal waters of the Western Hemisphere. The occurrence of severe oxygen depletion, either hypoxia (< 2 mg/l, or < 3 mg/l in some systems) or anoxia (0 mg/l), is a growing concern for U.S. estuarine and coastal waters. Many hypoxic zones elsewhere in the world have been caused by excess nutrients exported from rivers, resulting in reduced commercial and recreational fisheries. Prolonged oxygen depletion not only disrupts benthic and demersal communities but can cause mass mortalities of aquatic life (Diaz and Rosenberg, 1995). Among other problems, the consequences to coastal commercial fisheries can be disastrous (Baden et al., 1990; Zaitsev, 1991, 1993). Hypoxic zones are now one of the most widespread and accelerating human-induced deleterious impacts on the world's marine environments.

Oxygen depletion results from the combination of several physical and biological processes. In the Gulf of Mexico, hypoxia results from the stratification of marine waters due to Mississippi River system freshwater inflow and the decomposition of organic matter stimulated by Mississippi River nutrients. As a general rule, the nutrients delivered to estuarine and coastal systems support biological productivity. Excessive levels of nutrients, however, can cause intense biological productivity that depletes oxygen. The remains of algal blooms and zooplankton fecal pellets sink to the lower water column and seabed. The rate of depletion of oxygen during processes that decompose the fluxed organic matter exceeds the rate of production and resupply from the surface waters, especially when waters are stratified. Stratification in the northern Gulf of Mexico is most influenced by salinity differences year-round, but is accentuated in the summer due to solar warming of surface waters and calming winds. Following a fairly predictable annual cycle beginning in the spring, oxygen depletion becomes most widespread, persistent and severe during the summer months.

Figure 1. Histogram of estimated areal extent of bottom water hypoxia (< 2 mg/l) for mid-summer cruises in 1985-1999 (modified from Rabalais et al., 1998).

Midsummer coastal hypoxia in the northern Gulf of Mexico was first recorded in the early 1970s. In recent years (1993-1999), the extent of bottom-water hypoxia (16,000 to 20,000 km², see Figure 1) has been greater than twice the surface area of the Chesapeake Bay, rivaling extensive hypoxic/anoxic regions of the Baltic and Black Seas. Prior to 1993, the hypoxic zone averaged 8,000 to 9,000 km² (1985-1992) (Rabalais et al., 1998, 1999). The hypoxic area covered 12,400 km² in 1998, about the size of Connecticut.

Figure 2. The area of bottom water hypoxia from shelfwide mid-summer cruises is shown for 1993-1995.

Maps of the extent of bottom-water midsummer hypoxia (see examples in Figure 2) provide a benchmark for year-to-year comparisons, but should not be overinterpreted (Rabalais et al., 1999). The difference in hypoxia in a drought (1988) versus a flood (1993) year suggests a relationship between river discharge and the extent of hypoxia. A simple linear regression of midsummer area and mean Mississippi River discharge for the preceding year produced an R² of 0.934 for nine years of data from 1985-1993 (Wiseman et al., 1997). This relationship, however, fails to hold for the additional years of 1994-1998. Thus, a comparison of midsummer area (a minimal and rough estimate) versus discharge is not entirely satisfactory. There is evidence, for example, that carbon burial in 1993 was sufficient to support the extensive 1994 hypoxic zone despite "normal" flow conditions (Justic et al., 1997). Examples of variability in the midsummer extent of hypoxia due to physical conditions at the time of sampling include the reduced midsummer size in 1997 and 1998 due to the passage of a hurricane on the southeastern portion of the study area, and the current regime, respectively. Stronger relationships are evident for hypoxia versus river discharge and nutrient flux given time and spatial lags.

Figure 3. Comparison of bottom water oxygen less than 2 mg/l for three cruises on the Louisiana/Texas shelf in July 1993. (Top panel, Bratkovich et al., unpublished data; middle panel, Rabalais, 1998b; lower panel, Rabalais et al., 1998).

The persistence of the extensive midsummer areas is known from a few back-to-back shelfwide cruises in 1993 and 1994 (see Figure 3). These cruises found that the large size of the hypoxic zone persisted over 2- to 3-week periods, but varied somewhat in configuration.

Hypoxia occurs from late February through early October, nearly continuously from mid-May through mid-September, and is most widespread, persistent, and severe in June, July, and August. Hypoxic waters can include 20 to 80% of the lower water profile between 5 and 30 m water depth, and can extend as far as 130 km offshore. Throughout its distribution, the impact of hypoxic bottom waters is exacerbated by the release of toxic hydrogen sulfide from sediments (Harper et al., 1981, 1991).

The timing and location of low dissolved oxygen conditions in coastal waters is now fairly well documented, and there are studies that link the frequency and volume of summer oxygen depletion to increased nutrient inputs (Officer et al., 1984; Larsson et al., 1985; Tolmazin, 1985; Andersson and Rydberg, 1987; Justic et al., 1987; Cooper and Brush, 1991, 1993; Diaz and Rosenberg, 1995; Rabalais et al., 1996). Growth in population, changes in land cover, increase in agricultural acreage, and increases in fertilizer use and animal husbandry have resulted in two- to tenfold increases in the level of nutrient inputs during this century, with particularly dramatic increases since the 1950s (Turner and Rabalais, 1991; Justic et al., 1995a, 1995b; Howarth et al., 1996; Nixon, 1997, Goolsby et al., 1999).

Studies in the northern Gulf of Mexico, among other systems, provide evidence that the timing, spatial extent, and severity of oxygen depletion, as well as changes over time, are linked to freshwater discharge and nutrient flux (Justic et al., 1993, 1996, 1997; Rabalais et al., 1996). The severity and duration of oxygen depletion in the northern Gulf of Mexico depend, at least partially, on the amplitude and phasing of discharge from the Mississippi and Atchafalaya Rivers (Justic et al., 1993; Rabalais et al., 1996, 1998, 1999). Figure 3 illustrates the high variability of hypoxia possible during a one month time frame. The timing of Mississippi River discharge and nutrient flux, surface water phytoplankton production, and oxygen depletion are strongly correlated on the southeastern Louisiana shelf (Lohrenz et al., 1997, Justic et al., 1997). The highest surface water productivity occurs one month after the maximum river discharge, and oxygen depletion in bottom waters occurs two months after the highest river discharge (Justic et al., 1993). Figure 4 illustrates the increased frequency of hypoxia down current from the discharges of the Mississippi and Atchafalaya Rivers.

Figure 4. Distribution of frequency of occurrence of mid-summer hypoxia over the 60- to 80- station grid from 1985-1999 (data from Rabalias, Turner, and Wiseman hypoxia monitoring cruises).

Mississippi River nutrient concentrations and loadings to the adjacent continental shelf changed dramatically during this century, with an acceleration of these changes since the 1950s (Turner and Rabalais, 1991; Justic et al., 1995a, 1995b). Nitrogen is the principal nutrient yielding excess organic matter sedimentation to the Gulf hypoxic zone. Nitrogen export from the Mississippi River system has increased two- to sevenfold over the last century (Turner and Rabalais, 1991; Goolsby et al., 1999; Howarth et al., 1996). The majority of Mississippi River nitrogen originates from agricultural practice, while smaller fractions arise from human sewage, nonagricultural fertilizer use, and precipitation (Howarth et al., 1996; Goolsby et al., 1999; Downing et al., 1999). Silica and phosphorus also play a role, and the changing balance of nitrogen, silica, and phosphorus can affect marine food webs (Turner et al., 1998).

Hypoxia was first recorded on the continental shelf of the northern Gulf of Mexico in the early 1970s. A few directed studies in 1975-1976 (Ragan et al., 1978; Turner and Allen, 1982) and a series of environmental assessments revealed midsummer hypoxia in many inner shelf areas. Consistent data collection on the distribution and dynamics of hypoxia began in 1985 (Rabalais et al., 1991). Prior to the 1970s, scattered anecdotal data from shrimp trawlers in the 1950s to 1960s reported low or no catches, which was considered consistent with hypoxia but may also reflect other causes. Analysis of the sediment record, however, shows that severe hypoxia is a recent development in Gulf waters.

The sedimentary record supports the inference of increased eutrophication (increased rate of primary production) and hypoxia in the Mississippi River delta bight, primarily because of changes in nitrogen loading (Turner and Rabalais, 1994; Eadie et al., 1994; Nelsen et al., 1994; Sen Gupta et al., 1996; Rabalais et al., 1996). Changes in the productivity of surface waters and oxygen deficiency over time can be reconstructed from chemical and biological analyses of bottom sediments. In several U.S. estuaries and coastal waters, there is evidence of decade- and century-long increases in nutrient enrichment, hypoxia, and eutrophication. In the northern Gulf of Mexico, for example, sediment analysis shows clearly that the productivity of surface waters adjacent to the Mississippi River has risen as the nitrogen level in the river has risen (Eadie et al., 1994; Turner and Rabalais, 1994). Higher surface water production results in larger carbon fluxes to the bottom, thus increasing the severity and spatial extent of hypoxia (Qureshi, 1995; Rabalais et al., 1998). Analysis of benthic organisms, foraminifera, indicates that oxygen stress has worsened this century and accelerated since the 1950s when nitrogen flux in the river began to increase (Nelsen et al., 1994; Rabalais et al., 1996; Sen Gupta et al., 1996). These results are consistent with similar long-term evidence of eutrophication and worsening oxygen stress worldwide. The problems the Gulf of Mexico faces from hypoxia are not unique. Hypoxia related to human activities currently threatens many of the major coastal embayments and estuaries in the world (Diaz and Rosenberg, 1995).

Hypoxia affects living resources, biological diversity, and the capacity of aquatic systems to support biological populations. When oxygen levels fall below critical values (2 mg/l, for the northern Gulf of Mexico), those organisms capable of swimming (e.g., demersal fish, portunid crabs, and shrimp) evacuate the area (see Figure 5). The stress on less motile fauna caused by declining oxygen levels varies according to the oxygen requirements of the organism. Animals resident in the seabed are usually more resistant to low oxygen concentrations, but they also experience stress or die as oxygen concentrations decline from 1 mg/l to anoxia. Important fishery resources are variably affected by direct mortality, forced migration, reduction in suitable habitat, increased susceptibility to predation, changes in food resources and disruption of life cycles. Commercial and recreational fisheries in the Gulf generate $2.8 billion annually (Holiday and O'Bannon, 1997). Signs consistent with impact on Gulf of Mexico fisheries are (1)reduced food resources for fish and shrimp in hypoxic waters (Rabalais et al., 1995); (2) reduced abundance of fish and shrimp in hypoxic waters (Pavela et al., 1983; Leming and Stuntz, 1994; Renaud, 1986); (3) declinein shrimp catch and catch efficiency since hypoxia expanded (Zimmerman et al., 1997; Downing et al., 1999); and (4) loss of production potential due to the blocked migration of juvenile shrimp offshore by the presence of the hypoxic zone (Nance et al., 1994; Zimmerman et al., 1997; Downing et al., 1999).

Figure 5. Catch rates (kilograms per hour) are shown for shrimp and fish as a function of dissolved oxygen concentration. The catch drops to zero when oxygen is below 2 milligrams per liter (Leming and Stuntz, 1984).

The northern Gulf of Mexico adjacent to the discharge of the Mississippi River system is an example of a coastal ocean that has undergone eutrophication as a result of increasing nutrients and that has worsened century-long and accelerating recent decadal hypoxic conditions. Models that link Mississippi River discharge with Gulf of Mexico hypoxia demonstrate worsening hypoxia in bottom waters with increased freshwater discharge and even worse hypoxia with additional nitrogen accompanying the increased discharge (Bierman et al., 1994; Justic et al., 1996, 1997). Conversely, the models show that a reduction in oxygen demand in the lower water column will result from a reduction in nitrogen (and to a lesser degree the phosphorus) load to the surface waters (Limno-Tech, Inc., 1995). In other words, hypoxia in the northern Gulf of Mexico can be alleviated to some degree by a reduction in the nutrient loading. Whole system management of the entire watershed where most of the changes have occurred over the last several decades is a necessary step in alleviating the problems in the Gulf of Mexico. The ability to detect changes in the coastal system (given any nutrient reductions) will be complicated by an inherently variable biological system and extreme events. In addition, the eutrophic state may be persistent and recovery may be slow. Still, there are several success stories for improvement of estuarine and coastal ecosystems in response to nutrient abatement in the watershed or in direct discharges to the system, and similar activities on larger coastal systems with a much larger watershed, while daunting, are worthwhile and achievable.

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About the Author

Nancy N. Rabalais is a professor at the Louisiana Universities Marine Consortium, where she has been working since 1983. She earned a Ph.D. in zoology from The University of Texas at Austin in 1983, and her B.S. and M.S. from Texas A&I University, Kingsville, in 1972 and 1975. Before she joined LUMCON, Dr. Rabalais was a research associate, then graduate student, at the U.T. Marine Science Institute, Port Aransas Marine Laboratory. 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 environments, interactions of large rivers with the coastal ocean, estuarine and coastal eutrophication, benthic ecology, and the environmental effects of habitat alterations and contaminants. Dr. Rabalais is a Fellow of the American Association for the Advancement of Science, an Aldo Leopold Leadership Fellow, a co-recipient of the 1999 Blasker Award for Environmental Science and Engineering, a NOAA Environmental Hero, and a past-President of the Estuarine Research Federation, and a member of the Ocean Studies Board of the National Research Council (2000-2003).