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ARTICLE
SYMPOSIUM PAPER

Hypoxia in the Gulf of Mexico

^a Louisiana Universities Marine Consortium, 8124 Hwy. 56, Chauvin, LA 70344
^b Coastal Ecology Institute and Dep. of Oceanography and Coastal Sciences, Louisiana State Univ., Baton Rouge, LA 70803
^c Coastal Studies Institute and Dep. of Oceanography and Coastal Sciences, Louisiana State Univ., Baton Rouge, LA 70803

Corresponding author (nrabalais{at}lumcon.edu)

Received for publication July 14, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
Seasonally severe and persistent hypoxia, or low dissolved oxygen concentration, occurs on the inner- to mid-Louisiana continental shelf to the west of the Mississippi River and Atchafalaya River deltas. The estimated areal extent of bottom dissolved oxygen concentration less than 2 mg L^-1 during mid-summer surveys of 1993–2000 reached as high as 16000 to 20000 km². The distribution for a similar mapping grid for 1985 to 1992 averaged 8000 to 9000 km². Hypoxia occurs below the pycnocline from as early as late February through early October, but is most widespread, persistent, and severe in June, July, and August. Spatial and temporal variability in the distribution of hypoxia exists and is, at least partially, related to the amplitude and phasing of the Mississippi and Atchafalaya discharges and their nutrient flux. Mississippi River nutrient concentrations and loadings to the adjacent continental shelf have changed dramatically this century, with an acceleration of these changes since the 1950s to 1960s. An analysis of diatoms, foraminiferans, and carbon accumulation in the sedimentary record provides evidence of increased eutrophication and hypoxia in the Mississippi River delta bight coincident with changes in nitrogen loading.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
THE inner- to mid-continental shelf (depths of 5 to 60 m) of the northern Gulf of Mexico, from the Mississippi River birdfoot delta westward to the upper Texas coast, is the site of the largest zone of hypoxic bottom water in the western Atlantic Ocean coastal zone (values in Rabalais et al., 1999; Rabalais and Turner, 2001; cf. Boesch and Rabalais, 1991). Hypoxia is operationally defined as dissolved oxygen levels less than 2 mg L^-1, or ppm, for the northern Gulf of Mexico, because that is the level below which trawlers usually do not capture any shrimp or demersal fish in their nets (Pavela et al., 1983; Leming and Stuntz, 1984; Renaud, 1986). The areal extent of this zone, with estimates up to 20000 km² of near-bottom waters with dissolved oxygen levels <2 mg L^-1, rivals the largest hypoxic areas elsewhere in the world's coastal waters, namely the Baltic Sea and the northwestern shelf of the Black Sea. The northern Gulf of Mexico is strongly influenced by the Mississippi and Atchafalaya Rivers, whose combined discharges account for 80% of the total freshwater input (calculated from U.S. Geological Survey streamflow data for 37 U.S. streams discharging into the Gulf of Mexico; Dunn, 1996). Spatial and temporal variability in the distribution of hypoxia exists and is at least partially related to the amplitude and phasing of the Mississippi River discharge and nutrient fluxes (Pokryfki and Randall, 1987; Justi et al., 1993; Rabalais et al., 1996; Wiseman et al., 1997). The freshwater fluxes dictate, along with climate, a strong seasonal pycnocline. Nutrients delivered by the rivers support high primary production (Sklar and Turner, 1981; Lohrenz et al., 1990, 1994, 1997), of which approximately 50% fluxes to bottom waters and the seabed (Lohrenz et al., 1994; Qureshi, 1995). The high particulate organic carbon flux fuels hypoxia in the bottom waters below the seasonal pycnocline (Qureshi, 1995; Justi et al., 1996). Significant increases in riverine nutrient concentrations and loadings of nitrate and phosphorus and decreases in silicate have occurred this century (Turner and Rabalais, 1991; Goolsby et al., 1999). Sedimentary indicators of eutrophication and oxygen deficiency stress have increased this century with an acceleration since the 1950s to 1960s (Turner and Rabalais, 1994a; Rabalais et al., 1996; Sen Gupta et al., 1996). The changes since the 1950s are coincident with increased nitrate loading from the Mississippi River system to the adjacent continental shelf. The biological effects of increased riverine nutrient loads on the continental shelf adjacent to the Mississippi and Atchafalaya Rivers are comparable with indicators of increased eutrophication observed worldwide, especially in areas receiving increased and/or altered nutrient fluxes.

	HISTORICAL BACKGROUND

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
The idea that low oxygen conditions have been reported from the northwestern Gulf shelf since the 1930s are clearly references to the oxygen minimum layer in deeper waters of the open Gulf of Mexico, which is disjunct from the continental shelf hypoxia that has been documented since the 1970s. The mention of low oxygen conditions from the mid-1930s in the Conseil Permanent International pour l'Exploration de la Mer Bulletin Hydrographique for 1935 were identified in Hedgpeth's Treatise on Marine Ecology and Paleoecology (Brongersma-Sanders, 1957; Richards, 1957) as records from the oxygen minimum zone of deeper waters (e.g., 400–500 m deep). Several authors (Pokryfki and Randall, 1987; Rabalais et al., 1991, 1999) have shown that there is no physical connection of the oxygen minimum zone with the hypoxia on the inner- to mid-continental shelf. In addition, the salinity, temperature, and respiration rates of water in the deep-water oxygen minimum zone in the Gulf of Mexico differ considerably from the continental shelf hypoxic waters. Furthermore, the oxygen consumption rates in the oxygen minimum layer are insufficient (by several orders of magnitude) to account for the observed seasonal decline in oxygen concentration on the shelf (Turner et al., 1997).

Documented shelf hypoxia dates back to 1972 (Ward et al., 1979). Following the initial documentation of hypoxia in 1972–1974, Ragan et al. (1978) and Turner and Allen (1982) followed up in 1975 and 1976 with broad spatial surveys. The 1975 and 1976 distribution maps were less extensive than those mapped since 1985 by Rabalais et al. (1991)(1998, 1999), but the study area of Turner and Allen (1982), at least, was smaller than the current 60- to 80-station grid (Fig. 1) . Environmental assessments for the U.S. Strategic Petroleum Reserve program brine disposal areas and further studies of oil and gas production areas by the U.S. Minerals Management Service revealed low oxygen conditions in most inner-shelf areas (<30 m deep) studied in mid-summer for the period 1978–1984 (Rabalais, 1992).

View larger version (16K): [in this window] [in a new window]   Fig. 1. Distribution of stations for mid-summer shelfwide surveys and more frequent sampling along transect C. Stations C6A and C6B are locations of moored instruments; station C6, not shown, is between C6A and C6B. C6* is composite data for stations C6A, C6B, and C6. Starred stations approximate the 20-m isobath

Prior to the 1970s, there is anecdotal information from shrimp trawlers in the 1950s–1960s of low or no catches, of "dead" or "red" water, but no systematic analysis of these records. The tendency has been to generalize that low oxygen conditions have always been a feature of the system; however, measurements do not exist to substantiate this statement and analyses of the sedimentary record (see below) show otherwise.

	TEMPORAL AND SPATIAL DISTRIBUTION OF HYPOXIA

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
General Characteristics
Oxygen-deficient waters occur below the pycnocline from as early as late February through early October and nearly continuously from mid-May through mid-September. Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) to as deep as 60 m. During shore-perpendicular wind conditions, upwelling-favorable conditions force hypoxic water masses onto barrier island shores, often causing massive fish kills. The more typical depth distribution of hypoxic bottom waters, however, is between 5 and 30 m. The distance offshore to which hypoxic water masses are found is contoured by the slope gradient of the continental shelf. On the southeastern Louisiana shelf, where the shelf slopes more steeply toward the Mississippi Canyon, hypoxia extends only 55 km from shore. On the central and southwestern Louisiana shelf, where the continental shelf is broader and the depth gradient is more gradual, hypoxic bottom waters may extend as far as 130 km offshore.

A typical cross-section through the zone of hypoxia on the southeastern shelf in the peak of mid-summer hypoxia is characterized by a distinct mid-depth pycnocline controlled primarily by salinity. A weaker secondary pycnocline controlled by temperature often dictates the morphology of the hypoxic layer (Wiseman et al., 1997). Hypoxia occurs not only at the bottom near the sediments, but well up into the water column. Depending on the depth of the water, hypoxia may encompass from 10 to more than 80% of the total water column. In cases of the higher percentages above, hypoxia may reach to within 2 m of the surface in a 10-m water column, or to within 6 m of the surface in a 20-m water column. Very often anoxic bottom waters occur, along with the release of hydrogen sulfide from the sediments.

Mid-Summer Extent
The 5-d mid-summer hypoxia survey cruise is similar from year to year, with 60 to 80 stations between the Mississippi River delta to the upper Texas coast (Fig. 1). On any cross-shelf transect, sampling is usually completed as soon as the 2 mg L^-1 isopleth is clearly delineated. Thus, the station configuration changes from year to year, but the same general area of the coast is surveyed.

During the last eight years (1993–2000), bottom water hypoxia has been extensive on the Louisiana shelf, with bottom horizontal areas estimated up to 16000 and 20000 km² (examples in Fig. 2) . However, under strong oceanic currents from west to east during survey periods and a drought in 2000, the size of the bottom-water hypoxic zone decreased to 12500 and 4400 km² in 1998 and 2000, respectively.

View larger version (56K): [in this window] [in a new window]   Fig. 2. Distribution of near-bottom water hypoxia (<2 mg L-1) in mid-summer for 1993, 1994, and 1995 (modified from Rabalais et al., 1998)

Justi et al., 1997

Prior to 1993, the average areal extent of bottom water hypoxia in mid-summer was 8000 to 9000 km². Distribution maps of mid-summer bottom water hypoxia from 1985–1992 often showed disjunct areas of low oxygen downfield of each of the river deltas. Other distributions were continuous from the Mississippi River delta to the upper Texas coast. When the 2 mg L^-1 isopleths were not continuous along the shelf, however, the areas between were still undersaturated in oxygen, with values usually below 4 mg L^-1 and mostly below 3 mg L^-1.

A 52-yr record low flow of the Mississippi River occurred in 1988. Discharge began at normal levels in 1988 and quickly dropped to some of the lowest levels on record during the summer months. In early June 1988, hydrographic conditions on the southeastern Louisiana shelf were similar to those observed in previous years, that is, a stratified water column and some areas of oxygen-deficient bottom waters (Rabalais et al., 1991). By mid-July during the shelfwide mapping cruise, few areas of lower surface salinities were apparent, there was little density stratification, and low oxygen conditions were virtually absent. Mid-summer surface to bottom density differences in 1988 were two to three times less than seen in previous years (Rabalais et al., 1991)—a situation that clearly identifies the need for density stratification in the maintenance of subpycnoclinal oxygen deficiency. Reduced summer flows in 1988 also resulted in reduced suspended sediment loads, reduced nutrient flux, and increased water clarity across the continental shelf. High percent saturation of oxygen in bottom waters compared with other years with 20% or less oxygen saturation indicated that the weak stratification was coupled with the photosynthetic production of oxygen in bottom waters, and a well-oxygenated water column resulted (Rabalais et al., 1991).

Temporal Variability
In March, April, and May, hypoxia tends to be patchy and ephemeral; it is most widespread, persistent, and severe in June, July, and August (Fig. 3) (Rabalais et al., 1991, 1999). The persistence of extensive and severe hypoxia into September and October depends primarily on the breakdown of the stratification structure by winds from either tropical storm activity or passage of cold fronts. While the areal extent of bottom water hypoxia is widespread, its permanence over such an extent is not known, except for consecutive shelfwide cruises (three in July 1993 and two in July 1994). These data show that the hypoxic water masses persist over two to three weeks or more and that the extensive areal distribution is not an ephemeral event. Severely reduced oxygen occurs over large areas of the Louisiana–Texas coast for extended periods.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Cross-shelf contours of near-bottom water hypoxia (dissolved oxygen <2 mg L-1 in stippled and <1 mg L-1 in black) for Transect C (Fig. 1) across the southeastern Louisiana shelf at monthly intervals in 1992 and 1993 for dates indicated. Data from hypoxia studies of N.N. Rabalais, R.E. Turner, and W.J. Wiseman, Jr

View larger version (29K): [in this window] [in a new window]   Fig. 4. Continuous dissolved oxygen data (15-min intervals) from 19 m depth in a 20-m water column at Station C6B (Fig. 1) on the southeastern Louisiana shelf off Terrebonne Bay for April through mid-November 1993. Data from hypoxia studies of N.N. Rabalais, R.E. Turner, and W.J. Wiseman, Jr

	IMPORTANCE OF PHYSICAL STRUCTURE OF WATER COLUMN

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

Water column stability throughout the year is clearly maintained by a strong haline stratification (Rabalais et al., 1991; Wiseman et al., 1997), although cooling of the surface waters in winter may tend to destabilize the water column. Seasonal thermal and haline variations are larger in the surface layer than in the near-bottom waters. Maximum water column stability occurs during spring when runoff is high and during summer when wind mixing is weak and solar heating is strong (Rabalais et al., 1991). The surface salinity signal on the southeastern shelf corresponds closely to the flow of the Mississippi River (Geyer, 1950; Justi et al., 1993; Wiseman et al., 1997) as does the signal on the southwestern Louisiana shelf for the Atchafalaya (Pokryfki and Randall, 1987). There is a significant correlation between runoff at Tarbert Landing and surface salinity at station C6* (Fig. 1), with the highest cross-correlation coefficient from monthly averages for the period 1985–1992 occurring at a time lag of two months from peak river flow (Justi et al., 1993). The same data set was analyzed by Wiseman et al. (1997) according to the methods of Dinnel and Wiseman (1986) to determine excess fresh water content at station C6*. They found that the fresh water content was well correlated with river discharge lagged by 15 d. These results suggest a surface water current of roughly 0.15 m s^-1 for the salinity signal to propagate from the delta region to the region of station C6* if the direction were due west. Low surface salinities on the southwestern shelf lagged one month behind peak Atchafalaya River flow (Pokryfki and Randall, 1987).

Measures of stratification (surface-to-bottom differences in sigma t; sigma t is a measure of density) are correlated in time and space with the intensity of hypoxia (Rabalais et al., 1991). The relationships between surface salinity and sigma t gradients are strong (r² = 0.69 for 1985 shelfwide data in Rabalais et al., 1991). Hypoxic bottom waters are greatest in areal extent when surface-to-bottom density differences are greatest (Rabalais et al., 1991). This relationship does not always hold, and the depth of the main pycnocline does not always track the depth of the oxycline. The height above the bottom of the 2 mg L^-1 oxygen isopleth is closely correlated with the height above bottom where the sigma t gradient first achieves a value of 0.01 m^-1 or more (Wiseman et al., 1997). Thus, the existence of a strong near-surface pycnocline is a necessary condition for the occurrence of hypoxia, while the weaker, seasonal pycnocline guides the morphology of the hypoxic domain.

Winds sufficient to cause vertical mixing within the water column will break down the density stratification as well as mix aerated waters from the surface layer with those of the bottom (Wiseman et al., 1992). Intense wind mixing due to cold air outbreaks and frontal passages is active from as early as late September to as late as June. Local squalls and thunderstorms, as well as tropical storms and hurricanes, are important during the summer.

	IMPORTANT BIOLOGICAL PROCESSES

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
High biological productivity in the immediate (320 g C m^-2 yr^-1) and extended plume (290 g C m^-2 yr^-1) of the Mississippi River (Lohrenz et al., 1990, 1994, 1997; Sklar and Turner, 1981; respectively) is mediated by high nutrient inputs and regeneration, temperature, and favorable light conditions. Small-scale and short-term variability in productivity are the consequence of various factors, such as nutrient concentrations, temperature, and salinity (Lohrenz et al., 1990, 1994). Spatial variation in primary production within a given period is related to salinity and the associated environmental and biological gradients (Lohrenz et al., 1990, 1994). Maximum values of biomass (Rabalais and Turner, 1998) and primary production (Lohrenz et al., 1990) are typically observed at intermediate salinities and coincide with nonconservative decreases in nutrients along the salinity gradient (i.e., biological uptake).

Patterns of nutrient depletion provide evidence that riverine inputs of dissolved inorganic nitrogen and its pattern of regeneration ultimately limit the extent of river-enhanced productivity and biomass. The nutrient most relevant to the amount of phytoplankton production in the broad region fueling hypoxia is nitrogen. Lohrenz et al. (1997) clearly demonstrated that primary production in shelf waters near the delta and to some distance from it was significantly correlated with nitrate and nitrite concentrations and fluxes over a 6-yr period from 1988 to 1994. Light limitation was probably an important factor during winter months, but a positive correlation was demonstrated between river inputs of nitrate and nitrite from other times of the year. The relationships between riverine flux and concentration for those stations on the western end of their study area (i.e., near the transect C, Fig. 1) were improved when the riverine input data were lagged one month. These results were consistent with those of Justi et al. (1993)(1997) for a one-month lag between net production in surface waters and river discharge and nitrate flux. Even stronger correlations were observed between the concentration of orthophosphate and primary production, but these were not significant because of a smaller sample size (Lohrenz et al., 1997). It follows, and is supported with evidence from long-term data sets (Turner and Rabalais, 1994b) and the sedimentary record (Turner and Rabalais, 1994a; Eadie et al., 1994), that increases in riverine dissolved inorganic nitrogen loads are correlated with indicators of increased productivity in the overlying water column (i.e., eutrophication of the continental shelf waters).

The availability of dissolved silicate and its ratio to total inorganic nitrogen are also important in controlling the extent of diatom production and the composition of the diatom community with implications to carbon flux and control of oxygen depletion (Dortch and Whitledge, 1992; Nelson and Dortch, 1996; Rabalais et al., 1996). It is possible that the lower concentration of silicate and a ratio of Si to N closer to the Redfield ratio would favor nonsiliceous forms of phytoplankton, such as dinoflagellates or cyanobacteria. On the other hand, it is plausible with the increase of N that larger, more heavily silicified diatoms that sink more readily and add to the oxygen demand of bottom waters would be competitively superior. Evidence supports both of these hypotheses in varying degrees.

Particulate organic carbon flux to the lower water column is high in the extended plume over the inner shelf (approximately 500 to 600 mg C m^-2 d^-1 in 15 m water depth; Qureshi, 1995; see also Redalje et al., 1994). The fraction of production exported from the surface waters is highly variable, ranging from 10 to 200% of the integrated primary productivity, but averaging about half, with statistically higher percentages in spring. A large proportion of the particulate organic carbon flux reaches the bottom incorporated in zooplankton fecal pellets (55%; Qureshi, 1995), but also as individual algal cells or in aggregates. In a particle trap study at station C6B (Fig. 1), the fluxes of fecal pellet carbon, organic carbon and nitrogen, and phytoplankton carbon varied similarly between seasons, with the highest sedimentation in spring and the lowest in summer (Qureshi, 1995). The fluxes of all components were greater in 1991 than in 1992. Seasonal variations in fecal pellet number and carbon fluxes were positively correlated with indicators of high surface water productivity in 1991, but not in 1992. A higher spring discharge of the Mississippi River in 1991 compared with 1992 corresponded to higher fluxes of total particulates, total carbon, and fecal pellet carbon in 1991. The carbon fluxed via fecal pellets in spring 1991 was sufficient to deplete the bottom water oxygen reserve in spring, thus creating hypoxic conditions that then prevailed through the stratified summer period. Fecal pellet carbon flux into the bottom trap was low in spring of 1992, and the oxygen depletion rate for this flux was close to the calculated oxygen depletion rate.

	TEMPORAL LINKAGES WITH MISSISSIPPI RIVER DISCHARGE

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
Seasonal variations in net productivity in the northern Gulf of Mexico correlate with the dynamics of freshwater discharge (Justi et al., 1993). The surface layer (0 to 0.5 m) shows an oxygen surplus (above saturation values) during February–July; the maximum occurs during April and May and coincides with the maximum flow of the Mississippi River. The bottom layer (approximately 20 m), on the contrary, exhibits an oxygen deficit (below saturation values) throughout the year, but peaks in July. The correlation between Mississippi River flow and surface oxygen surplus peaks at a time lag of one month, and the highest correlation for bottom oxygen deficit is for the time lag of two months (Justi et al., 1993). These findings suggest that the oxygen surplus in the surface layer following high flow depends on nutrients ultimately coming from the river but regenerated three to four times. This is an important finding, since a surplus of oxygen relative to the saturation value is an indicator of net productivity in the surface waters. An oxygen surplus also means that there is an excess of organic matter derived from primary production that can be redistributed within the system; some of this will eventually reach the sediments.

Hypoxia on the southwestern Louisiana shelf downfield from the plume of the Atchafalaya River was examined by Pokryfki and Randall (1987). They found a similar two-month lag of bottom water hypoxia on the southwestern Louisiana shelf following peak Atchafalaya River discharge, as did Justi et al. (1993) for the southeastern Louisiana shelf. Low surface salinities lagged one month behind peak Atchafalaya River flow. Pokryfki and Randall's model did not incorporate any biological processes, which with additional lags would increase the accuracy of their predicted low oxygen periods. The physics, geological setting, and important biological parameters, such as light fields and nutrient flux, differ on the southwestern Louisiana shelf from that of the southeastern shelf. It is also not clear how the effluents of the Mississippi River and the Atchafalaya River merge to produce the physical structure of the area. Wiseman and Kelly (1994) demonstrated that salinity signals from both river discharges were detectable off the Calcasieu estuary. However, similar biological processes occur on the southwestern shelf, so that a large area of hypoxic bottom waters to the west of the Atchafalaya River appears to form each season.

	HISTORICAL CHANGES IN EUTROPHICATION AND OXYGEN STRESS

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
Changes in Nutrient Loadings
Mississippi River nutrient concentrations and loadings to the adjacent continental shelf have changed dramatically this century, with an acceleration of these changes since the 1950s to 1960s (Turner and Rabalais, 1991, 1994a,b; Rabalais et al., 1996). The concentrations of dissolved N and P doubled and Si decreased by 50%, the dissolved Si to N ratio dropped from 4:1 to 1:1, and seasonal trends have changed. The resulting nutrient composition in the receiving Gulf waters became more balanced and shifted toward stoichiometric nutrient ratios closer to the Redfield ratio (Justi et al., 1995a,b). Nitrogen and phosphorus are indicated to be less limiting now for phytoplankton growth, while some increase in silica limitation is probable (Justi et al., 1994). The effects of these changes on the continental shelf have not been fully explored. An analysis of diatoms, foraminiferans, and carbon accumulation in the sedimentary records provides evidence of increased eutrophication and hypoxia in the Mississippi River delta bight, a temporal pattern that is paralleled by increased nitrogen loads to the continental shelf (Eadie et al., 1994; Turner and Rabalais, 1994a; Rabalais et al., 1996; Sen Gupta et al., 1996).

Biological Responses
In spite of a probable decrease in Si availability, the overall productivity of the ecosystem appears to have increased this century. This is evidenced by (i) equal or greater net silicate-based phytoplankton community uptake of silica in the mixing zone, compared with the 1950s (Turner and Rabalais, 1994b); (ii) greater accumulation rates of biogenic silica (BSi, the remnants of diatom cell walls) in sediments beneath the plume, but not further away, and in agreement with results found in freshwater systems (Turner and Rabalais, 1994a); and (iii) increased relative proportion of marine in situ produced carbon in sediments compared with terrestrially derived carbon (Eadie et al., 1994). The increased percent BSi in Mississippi River bight sediments parallels increased N loading to the system and is, therefore, direct evidence for the effects of eutrophication on the shelf adjacent to the Mississippi River. Individual phytoplankton species composition shifts (heavily silicified diatoms lightly silicified diatoms; diatom non-diatom) would indicate some population-level responses to reduced Si supplies and/or changes in nutrient ratios (Rabalais et al., 1996). Finally, an analysis of benthic foraminiferans indicates an increase in oxygen deficiency stress this century, with a dramatic increase since the 1950s (Rabalais et al., 1996; Sen Gupta et al., 1996). The presence of low oxygen–intolerant foraminiferans from 1700 to 1900 in sediments of the Mississippi River bight indicates that hypoxia was not a feature of the system at that time. Increased bottom-water hypoxia could result from increased organic loading to the seabed, shifts in material flux (quantity and quality) to the lower water column, increased freshwater flow and stratification, or combinations of these events. Evidence, however, points to a change in the quality of the river discharge, not a change in the quantity.

Changes in Freshwater Delivery
The importance of the water column physical structure to the development and persistence of hypoxia is clear, and the discharge of the Mississippi River (i.e., amount of flow) since the 1950s has been relatively constant aside from normal decadal scale variations in runoff. The allocation of flow between the Mississippi River proper and the Atchafalaya River has been maintained since 1977 by the U.S. Army Corps of Engineers according to Congressional mandate. The 1820 to present average discharge rate (decadal time scale) for the lower Mississippi River is remarkably stable near 14000 m³ s^-1. The riverine flow delivered to the shelf waters adjacent to Atchafalaya Bay has slowly increased (Bratkovich et al., 1994), and the combined flow delivered to the shelf region has also increased, more notably over the last two decades (Bratkovich et al., 1994). The results of these effects, however, would be on the southwestern shelf and not the southeastern. Also, the increased flow is primarily in the fall, at a time of year when biological processes and wind-mixing of the water column are not conducive to the development and maintenance of hypoxia. Damming and channelization of the river have undoubtedly had substantial implications in the fate of nutrients flowing down the river. In some cases, the effects of dams might decrease the flux of certain particles or particle-bound nutrients. On the other hand, channelizing the river and reducing the flood plain would probably reduce the in-stream loss of nutrients. Certainly, most of the channelization occurred prior to the 1950s and could not account for the doubling of nitrogen delivery over the 30-yr period following the 1950s and through the mid-1980s. Thus, the observed changes in biological responses in the Louisiana bight are probably not due to changes in amount or distribution of freshwater runoff and resultant stratification, but rather changes in water constituents.

	EFFECTS ON LIVING RESOURCES

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 
Several studies have documented the responses of benthic and demersal communities to varying levels of oxygen stress via benthic cores, remotely operated vehicle video taping, and diver observations (Rabalais et al., 2001a,b). A fairly predictable pattern in responses of components of the community follows a decrease in oxygen concentrations from 2 mg L^-1 to anoxia. Motile fish and crustaceans are generally absent from bottom habitats when the oxygen falls below 1.5 to 2 mg L^-1, less motile invertebrates die at oxygen below 1.5 mg L^-1, infaunal invertebrates display stress behavior below 1.0 mg L^-1, and a fairly linear decrease in benthic macroinfauna diversity and abundance occurs between 0.5 mg L^-1 and anoxia. Monthly macroinfaunal samples from an area with persistent and severe hypoxia versus an area with intermittent and less severe hypoxia indicate a much reduced abundance, species richness, and biomass under conditions of severe hypoxia and/or anoxia. The more stressed community is characterized by limited taxa (none with direct development, e.g., amphipods), characteristic resistant fauna (e.g., a few polychaetes and sipunculans), a reduced species richness, severely reduced abundance (but never azoic), low biomass, and limited recovery following abatement of oxygen stress. Oxygen stress overwhelms sedimentary characteristics in the structuring of benthic communities. Effects of hypoxia on fishery resources include direct mortality, altered migration, reduction in suitable habitat, increased susceptibility to predation (including by humans), changes in food resources, and disruption of life cycles.

Oxygen-depleted bottom waters in the coastal ocean are found worldwide (Stachowitsch, 1984; Faganeli et al., 1985; Tolmazin, 1985; Rosenberg, 1986; Westernhagen et al., 1986; Friligos and Zenetos, 1988). The incidence and extent of such areas in coastal waters is apparently increasing (Officer et al., 1984; Fransz and Verhagen, 1985; Larsson et al., 1985; Tolmazin, 1985; Andersson and Rydberg, 1987; Justi et al., 1987; Diaz and Rosenberg, 1995). The patterns of worsening water quality in coastal waters adjacent to the terminus of major rivers undergoing nutrient flux or water quality alterations are consistent with the conditions identified for the Mississippi River.

	REFERENCES

TOP ABSTRACT INTRODUCTION HISTORICAL BACKGROUND TEMPORAL AND SPATIAL... IMPORTANCE OF PHYSICAL STRUCTURE... IMPORTANT BIOLOGICAL PROCESSES TEMPORAL LINKAGES WITH... HISTORICAL CHANGES IN... EFFECTS ON LIVING RESOURCES REFERENCES

 

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