versão On-line ISSN 0717-6538
Gayana (Concepc.) v.70 supl.1 Concepción out. 2006
Suplemento Gayana 70: 73-78, 2006
Oxygen depletion in the gulf of Mexico adjacent to the Mississippi river
El agotamiento del oxígeno en la porción adyacente al río Misisipi en el golfo de México
Nancy N. Rabalais
Louisiana Universities Marine Consortium Chauvin, LA 70344 USA, email@example.com
The seasonal formation of a bottom-water layer severely depleted in dissolved oxygen has become a perennial occurrence on the Louisiana continental shelf adjacent to the Mississippi River system. Dramatic changes have occurred in this coastal ecosystem in the last half of the 20th century as the loads of dissolved inorganic nitrogen tripled. There are increases in primary production, shifts in phytoplankton community composition, changes in trophic interactions, and worsening severity of hypoxia. The river-influenced continental shelf is representative of similar ecosystems in which increased nutrient flux to the coastal ocean has resulted in eutrophication and subsequently hypoxia. The hypoxic conditions (dissolved oxygen less than 2 mg L-1) cover up to 22,000 km2 of the seabed in mid-summer. Dissolved oxygen concentrations seldom decrease to anoxia, but are often below 1 mg L-1 and down to 0.5 mg L-1. The biogeochemical processes of oxic versus suboxic conditions in the water column and sediments of the Louisiana shelf are similar to other areas of oxygen deficiency. However, the suboxic conditions in the Gulf of Mexico are less persistent in time and space, and anoxia at the seabed is not common or long-lasting.
Keywords: Anoxia, hypoxia, eutrophication, nutrients, nitrogen.
En el Golfo de México existe un área extensa donde el agua del fondo es deficiente en oxígeno durante ciertas estaciones del año. En esta área las descargas de nutrientes del Río Misisipi han aumentado desde los años 50. El fenómeno de hypoxia alcanza grandes extensiones y es persistente de mayo a septiembre en la mayoría de los años, pudiendo alcanzar un área de hasta 22,000 km2 a mediados del verano. Aunque la hypoxia es rara en este sistema, es común que ocurran extensos periodos en los que la concentración de oxígeno disuelto cae por debajo de 1 mg L-1, e incluso hasta 0.5 mg L-1, lo cual afecta los procesos bentónicos y a la diversidad.
Palabras Claves: Anoxia, hipoxia, eutroficación, nutrientes, nitrógeno.
Anoxic and suboxic conditions exist naturally in the world's oceans in fjords, deep basins, and oxygen minimum zones (Kamykowski & Zentara 1990, Helly & Levin 2004). Similar conditions frequently occur where upwelling systems associated with western boundary currents impinge on the continental shelf. Oxygen depletion in coastal waters not subject to upwelled nutrients results from eutrophication usually initiated and maintained by increased nutrient loads under stratified conditions. Nutrient load increases result from increasing human populations and their activitiesapplication of nitrogen and phosphorus fertilizers, planting of leguminous crops, atmospheric deposition of nutrients, and municipal and industrial wastewater. The negative effects of eutrophication may include increased noxious or harmful algal blooms, diminished water clarity, shifts in trophic interactions that do not result in the diatom-zooplankton-fish food web, loss of essential habitat and oxygen depletion. Over the last half of the 20th century, the impacts of eutrophication, including oxygen depletion, increased in frequency and severity and expanded geographically (Diaz & Rosenberg 1995). The altered northern Gulf of Mexico continental shelf ecosystem is one of several scenarios of changing nutrient loads and worsening hypoxia in the global coastal ocean.
The northern Gulf of Mexico receives the average annual Mississippi River discharge of 580 km3 through two main distributariesthe main birdfoot delta southeast of the city of New Orleans, Louisiana and the Atchafalaya River delta 200 km to the west that carries about one-third of the flow (Meade 1995). The fresh water, sediments, and dissolved and particulate materials are carried predominantly westward along the Louisiana/Texas inner to mid continental shelf, especially during peak spring discharge (Rabalais et al. 1996). Although the area of the discharge's influence is an open continental shelf, the magnitude of flow, annual current regime and average 75-day residence time for fresh water result in an unbounded estuary stratified for much of the year. The stratification is primarily due to salinity differences, and the stratification intensifies in summer with thermal warming of surface waters.
The annual discharge of the Mississippi River system contributes sediment yields of 210 x 106 t, 1.6 x 106 t nitrogen, of which 0.95 x 106 t is nitrate and 0.58 x 106 t is organic nitrogen, 0.1x106 t phosphorus and 2.1 x 106 t silica (Goolsby et al. 1999). The estimate of current river nitrogen export from the Mississippi River watershed over `pristine' river (pre-agricultural and pre-industrial condition) nitrogen export is a 2.5- to 7.4-fold increase (Howarth et al. 1996). The average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring; this is consistent with increased use of fertilizer in the watershed.
The concentrations and total loads of nitrogen, phosphorus and silica delivered to the coastal ocean influence the productivity of the phytoplankton community, the types of phytoplankton that are most likely to grow and the flux of phytoplankton-derived organic matter (Rabalais et al. 1996, Turner et al. 1998, Dortch et al. 2001). Phytoplankton not incorporated into the food web and fecal material generated via the food web sink into bottom waters where they are decomposed by aerobic bacteria. The source of the organic matter for this respiratory activity is mostly from marine phytoplankton growth stimulated by river-delivered nutrients, and not from the carbon in the Mississippi River (Eadie et al. 1994). The bacterial respiration in the lower water column and seabed results in a decline in oxygen concentration when oxygen is depleted faster than it can be replaced by vertical diffusion through the stratified water column.
Changes in the coastal ecosystem are directly linked to the changes within the watershed and nutrient loading to the continental shelf, especially of nitrate, which tripled in the last half of the 20th century (Rabalais et al. 1996, 2002a; Turner & Rabalais 2003). Evidence from long-term data sets and the sedimentary record demonstrate that indices of increased marine productivity and subsequent worsening of oxygen stress are highly correlated with historic increases in riverine dissolved inorganic nitrogen concentrations and loads over the last 50 years and implied nutrient load changes over the last 300 years. Evidence comes in long-term changes in water transparency and diatom productivity, increased accumulation of diatom remains and marine-origin carbon in sediments, and indicators of worsening oxygen conditions in the sediments. The sediment data suggest that hypoxia was not a feature of the continental shelf before 1900 and that hypoxia while it may have existed at some level before 1940_1950, it has worsened since then. Recent models of how the size and intensity of hypoxia are related to nitrate flux from the Mississippi River (Justiæ et al. 2002, Scavia et al. 2003, Turner et al. 2005, 2006) indicate that hypoxia as a widespread phenomenon was not likely on the Louisiana shelf before the early 1970s.
MATERIALS AND METHODS
Hypoxia is operationally defined as dissolved oxygen levels below 2 mg L-1 for the northern Gulf of Mexico. This is the level below which trawlers usually do not capture any shrimp or demersal fish (Renaud 1986). When dissolved oxygen values are below 2 mg l-1, they are often less than 1 mg L-1, a severe level that is stressful or lethal to benthic macroinfauna. Dissolved oxygen of 2 mg L-1 equates to 1.4 ml L-1, and approximates 20% oxygen saturation in northern Gulf of Mexico waters. Oxygen deficient (less than 100% oxygen saturation) waters are more widespread than indicated by the 2 mg L-1 cutoff, but for consistency a value of 2 mg L-1 is used throughout this synthesis.
Hypoxia studies of a consistent, long-term nature began in 1985 (Rabalais & Turner 2001). The initial goal of these studies was to document the location, duration and intensity of hypoxia on the Louisiana coast. It soon became clear that hypoxia was a dominant, perennial feature of the coastal ecosystem, and studies expanded to encompass many other aspects.
A regional survey is conducted once per year in mid summer along a series of transects from the Mississippi River delta westward along the Louisiana coast and onto the upper Texas coast. These data form the basis for a management goal of reducing the extent of hypoxia to 5,000 km2 over a five-year running average. A single cruise of this nature per mid-summer provides little information on the persistence of hypoxia over large areas, or the temporal sequence of physical and biological processes that preceded the cruise. Surveys along two transects one located 100 km west and down current of the Mississippi River delta (transect C) and one located offshore of the Atchafalaya River delta (transect F) are conducted monthly and bimonthly, respectively. These data provide a time series suitable for examining monthly- and seasonal-scale differences over a representative area of the Louisiana shelf and relationships with variability in Mississippi River discharge and nutrient flux. One station located in 20-m water depth on transect C is the site of more detailed data collection and experiments, for which the temporal resolution is best. These data can be extrapolated for a portion of the Louisiana shelf, but not the entire shelf. Technological improvements altered data acquisition through the years, but the data are consistent, precise and accurate.
Distribution of Hypoxia
The mid summer extent of bottom-water hypoxia on the Louisiana shelf averaged 12,700 km2 from 1985 to 2005 with the largest size mapped in 2002 at 22,000 km2 (Fig. 1, 10.5 mg L-1). 1960. The hypoxic water mass is distributed across the Louisiana shelf west of the Mississippi River and onto the upper Texas coast (Rabalais & Turner 2001, Rabalais et al. 2002a). Hypoxia extends from near shore to as much as 125 km offshore and in water depths extending from the shore up to 60 m.
Figure 1. Similar size and expanse of bottom water hypoxia in mid-July 2002 (shaded area) and in mid-July 2001 (outlined with dashed line) (N. Rabalais, LUMCON). Asterisk indicates location of high frequency temporal data.
Similar surveys in spring and fall provide information during the periods when hypoxia is likely to be developing or dissipating. A combination of March, April, May, October, November and December cruises indicate either a narrow band of hypoxia nearshore or the occurrence of hypoxia at shallow stations in the Mississippi River bight and off the Atchafalaya River in the months of March, April and May of various years. Otherwise, hypoxia occurs in isolated locations for short periods in October, November and February and has not been recorded in December and January.
A compilation of 20 mid-summer shelfwide surveys (1985-2005, Fig. 2) shows that the frequency of occurrence of hypoxia is highest down current (west) from the freshwater and nutrient discharges from the Mississippi and Atchafalaya rivers. The mid-summer extent of hypoxia is strongly related to the discharge of freshwater and nutrients from the Mississippi and Atchafalaya rivers; however, variations may also be due to atypical oceanographic conditions (e.g. 1998) or disruption of stratification by tropical storms and hurricanes prior to the survey.
Figure 2. Distribution of frequency of occurrence of mid-summer bottom-water hypoxia over the 60- to 80-station grid from 1985-2005 (updated from Rabalais et al. 2002a).
More frequent sampling along a transect C indicates that critically low dissolved oxygen concentrations occur from as early as late February through early October and nearly continuously from mid-May through mid-September (Rabalais et al. 2002a). The monthly average value of bottom oxygen concentration along transect C illustrates the seasonal progression of worsening hypoxia across an increasingly greater area in May through August (Fig. 3). The persistence of extensive and severe hypoxia into September and October depends on the timing of the breakdown of vertical stratification from either tropical storms, passage of cold fronts or thermal turnover of the water column.
Figure 3. Seasonal progressions of water column parameters along transect C from station C1 inshore to C9 offshore, by month Jan through Dec 1985-1997. Modified from Rabalais et al. (2002b).
The occurrence of anoxia and production of H2S in bottom waters on this shelf are limited even though the continental shelf is seasonally hypoxic over a large area and oxygen concentrations are often below 0.5 mg L-1. While instrumentation may limit accuracy at levels below 0.1 mg L-1, the presence of H2S in bottom waters is a definitive indicator that the dissolved oxygen concentration is 0.0 mg L-1. H2S concentrations up to 50 mM have been measured in bottom water samples that emitted a strong odor of H2S (N.N. Rabalais et al. unpubl. data). H2S concentrations of 2-5 mM were chemically detected, when there was still a faint H2S odor. Thus, the `odor indicator' has been used to determine the occurrence of anoxia. In a mid summer survey of 80-90 stations, there are at most 10 stations in which the bottom water collections smelled of H2S (N.N. Rabalais et al. unpubl. data). For the nine-station transect C, up to two stations per month in June through September can have the H2S smell.
The presence of sulfur-oxidizing bacteria at the sediment-water interface on many occasions, observed both by divers and video surveillance from remotely operated vehicles (Rabalais et al. 2001a), indicates that extremely low oxygen concentrations, though not anoxic, commonly allow for the presence of these bacteria.
Consequences for Living Resources
The impacts of increased nutrient inputs and worsening hypoxia on overall system productivity are not well known for the Louisiana continental shelf food web. The hypoxic zone falls within an important commercial and recreational fishery zone that accounts for 25 to 30 percent of the annual coastal fisheries landings for the United States. It is well documented that the ability of organisms to reside, or even survive, either at the bottom or within the water column is affected as the depletion of oxygen progresses towards anoxia (Rabalais et al. 2001a). When oxygen levels fall below critical values, those organisms capable of swimming (e.g., demersal fish, portunid crabs and shrimp) evacuate the area. Less motile fauna experience stress or die as oxygen concentrations fall to zero. Larger, longer-lived burrowing infauna are replaced by short-lived, smaller surface deposit-feeding polychaetes, and several taxa are absent from the fauna, for example, pericaridean crustaceans, bivalves, gastropods, and ophiuroids (Rabalais et al. 2001b). These changes in benthic communities result in an impoverished diet for bottom-feeding fish and crustaceans and contribute, along with low dissolved oxygen, to altered sediment structure and sediment biogeochemical cycles.
Nutrients are essential for the support of productive coastal ecosystems, but excess nutrients lead to problems associated with eutrophication and hypoxia. There is a negative relationship between the catch of brown shrimp (the largest economic fishery in the northern Gulf of Mexico) and the size of the mid summer hypoxic zone (Zimmerman & Nance 2001). The decadal average catch per unit effort of brown shrimp declined during the last forty years in which hypoxia was known to expand (Downing et al. 1999). There are, however, changes in climate, river discharge, salinity of the estuary during critical development periods, acreage of nursery habitat and fishing effort that may also be implicated. The point on the continuum of increasing nutrients versus fishery yields remains vague as to where benefits are subsumed by environmental problems that lead to decreased landings or reduced quality of production. There are indications of a shift from a demersal dominated fish community to a pelagic dominated fish community over the last half of the 20th century (Chesney & Baltz 2001). The pelagic food web on the Mississippi River influenced shelf has changed in the last several decades to the point where it is now poised to switch between one with, and one largely without, the diatom-zooplankton-fish food web (Turner et al. 1998). There are also shifts in diatom community composition with implications for carbon flux (Dortch et al. 2001) and increased jellyfish abundance from 1987 to 1997 (Purcell et al. 2001). While there have been no catastrophic losses in fishery resources in the northern Gulf of Mexico, the potential impacts of increasing trophic state and worsening hypoxia on ecologically and commercially important species and altered ecological processes warrant attention.
The NOAA Center for Sponsored Coastal Ocean Research (CSCOR) sponsored the production of this manuscript (NOAA Grant No. NA06OP0528).
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