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1.
Nutrient and trace element distributions were determined in the outflow region of the Mississippi and Atchafalaya rivers during high river discharge. This outflow region can be divided into two physiographic areas: the broad, shallow Louisiana Shelf off the Atchafalaya River and the narrow shelf off the Mississippi Delta. The physiographic differences between these two areas lead to observable differences in the chemical distributions. During high discharge conditions, nutrient depletion occurs at lower salinities on the Louisiana Shelf, relative to the delta outflow plume, and significant uptake of nickel and cadmium is apparent in these shelf waters, too. Important factors that appear to connect the physiographic to the chemical include the fate of the fluvial suspended load, rates of mixing, and the extent of productivity supported by recycled nutrients. The results suggest that the Mississippi-Atchafalaya outflow region may provide a natural laboratory for examining the possible effects of sea-level change on the biogeochemistry of estuarine and coastal environments.  相似文献   

2.
We synthesize and update the science supporting the Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001) with a focus on the spatial and temporal discharge and patterns of nutrient and organic carbon delivery to the northern Gulf of Mexico, including data through 2006. The discharge of the Mississippi River watershed over 200 years varies but is not demonstrably increasing or decreasing. About 30% of the Mississippi River was shunted westward to form the Atchafalaya River, which redistributed water and nutrient loads on the shelf. Data on nitrogen concentrations from the early 1900s demonstrate that the seasonal and annual concentrations in the lower river have increased considerably since then, including a higher spring loading, following the increase in fertilizer applications after World WarII. The loading of total nitrogen (TN) fell from 1990 to 2006, but the loading of total phosphorus (TP) has risen slightly, resulting in a decline in the TN:TP ratios. The present TN:TP ratios hover around an average indicative of potential nitrogen limitation on phytoplankton growth, or balanced growth limitation, but not phosphorus limitation. The dissolved nitrogen:dissolved silicate ratios are near the Redfield ratio indicative of growth limitations on diatoms. Although nutrient concentrations are relatively high compared to those in many other large rivers, the water quality in the Mississippi River is not unique in that nutrient loads can be described by a variety of land-use models. There is no net removal of nitrogen from water flowing through the Atchafalaya basin, but the concentrations of TP and suspended sediments are lower at the exit point (Morgan City, Louisiana) than in the water entering the Atchafalaya basin. The removal of nutrients entering offshore waters through diversion of river water into wetlands is presently less than 1% of the total loadings going directly offshore, and would be less than 8% if the 10,093 km2 of coastal wetlands were successfully engineered for that purpose. Wetland loss is an insignificant contribution to the carbon loading offshore, compared to in situ marine production. The science-based conclusions in the Action Plan about nutrient loads and sources to the hypoxic zone off Louisiana are sustained by research and monitoring occurring in the subsequent 10 years.  相似文献   

3.
To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (Pi), silicate (Si)), nutrient ratios (DIN/Pi), alkaline phosphatase activity, chlorophyll and 14C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed Pi limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/Pi and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33?C7.01 g C m?2 d?1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 ??M DIN and Pi). The results of this study indicate that Pi limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.  相似文献   

4.
This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ωaragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO2 fugacity (fCO2), total alkalinity (TA) and Ωaragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity. However, inner shelf waters (<50 m depth) exhibited large variations in fCO2, TA, and Ωaragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters were supersaturated with respect to aragonite. Offshore waters had an average Ωaragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ωaragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ωaragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological drawdown of CO2. A dominant factor controlling Ωaragonite distribution in offshore waters with relatively constant temperatures was fCO2, with higher supersaturation occurring in areas with low fCO2.  相似文献   

5.
Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities.  相似文献   

6.
Oxygen depletion is a seasonally dominant feature of the lower water column on the highly-stratified, riverine-influenced continental shelf of Louisiana. The areal extent of hypoxia (bottom waters ≤2 mg l?1 dissolved oxygen) in mid-summer may encompass up to 9,500 km2, from the Mississippi River delta to the upper Texas coast, with the spatial configuration of the zone varying interannually. We placed two continuously recording oxygen meters (Endeco 1184) within 1 m of the seabed in 20-m water depth at two locations 77 km apart where we previously documented midsummer bottom water hypoxia. The oxygen meters recorded considerably different oxygen conditions for a 4-mo deployment from mid-June through mid-October. At the station off Terrebonne Bay (C6A), bottom waters were severely depleted in dissolved oxygen and often anoxic for most of the record from mid-June through mid-August, and there were no strong diurnal or diel patterns. At the station 77 km to the east and closer to the Mississippi River delta (WD32E), hypoxia occurred for only 50% of the record, and there was a strong diurnal pattern in the oxygen time-series data. There was no statistically significant coherence between the oxygen time-series at the two stations. Coherence of the oxygen records with wind records was weak. The dominant coherence identified was between the diurnal peaks in the WD32E oxygen record and the bottom pressure record from a gauge located at the mouth of Terrebonne Bay, suggesting that the dissolved oxygen signal at WD32E was due principally to advection by tidal currents. Although the oxygen time-series were considerably different, they were consistent with the physical and biological processes that affect hypoxia on the Louisiana shelf. Differences in the time-series were most intimately tied to the topographic cross-shelf gradients in the two locations, that is, station C6A off Terrebonne Bay was in the middle of a broad, gradually sloping shelf and station WD32E in the Mississippi River Delta Bight was in an area with a steeper cross-shelf depth gradient and likely situated near the edge of a hypoxic water mass that was tidally advected across the study site.  相似文献   

7.
Hypoxic conditions in the coastal waters off Texas (USA) were observed since the late 1970s, but little is known about the causes of stratification that contribute to hypoxia formation. Typically, this hypoxia is attributed to downcoast (southwestward) advection of waters from the Mississippi–Atchafalaya River system. Here, we present evidence for a hypoxic event on the inner shelf of Texas coincident with the presence of freshwater linked to high flow of the Brazos River in Texas. These conclusions are based on hydrographic observations and isotopic measurements of waters on the inner shelf near the Brazos River mouth. These data characterize the development, breakdown, and dispersal of a hypoxic event lasting from June through September 2007 off the Texas coast. Oxygen isotope compositions of shelf water indicate that (1) discharge from the Brazos River was the principal source of freshwater and water column stratification during the 2007 event, and (2) during low Brazos River discharge in 2008, freshwater on the Texas shelf was derived mainly from the Mississippi–Atchafalaya River System. Based on these findings, we conclude that the Mississippi–Atchafalaya River System is not the sole cause of hypoxia in the northern Gulf of Mexico; however, more data are needed to determine the relative influence of the Texas versus Mississippi rivers during normal and low flow conditions of Texas rivers.  相似文献   

8.
The objective of this study was to examine the interaction between the Atchafalaya River and the Atchafalaya Delta estuarine complex. Measurements of suspended sediments, inorganic nutrients (NO3 ?, NH4 +, PO4 3?), chlorophylla (chla), and-salinity were taken monthly from December 1996 to January 1998. These data were compiled by season, and the Atchafalaya River plume data were also analyzed using the Generalized Additive Model technique. There were significant decreases in NO3 ? concentrations during summer, fall, and winter as river water passed through the estuary, that were attributable to chemical and biological processes rather than dilution with ambient water. In some regions there were higher chla concentrations during summer and fall compared to winter and spring, when river discharge and the introduction of inorganic nutrients were highest, suggesting biological processes were active during this study. The presence of NH4 +, as a percentage of available dissolved inorganic nitrogen, increased with distance from the Atchafalaya River, indicative of remineralization processes and NO3 ? reduction. Mean PO4 3? concentrations were often higher in the estuarine regions compared to the Atchafalaya River. During summer total suspended solid (TSS) concentrations increased with distance from the river mouth, suggesting a turbidity maximum. Highest chla concentrations were found in the bayous and shallow water bodies of the Terrebonne marshes, as were the lowest TSS concentrations. The low chla concentrations found in other areas of this study, despite high inorganic nutrient concentrations, suggest light limitation as the major control of phytoplankton growth. Salinity reached near seawater concentrations at the outer edge of the Atchafalaya River plume, but much lower salinities (<10 psu) were observed at all other regions. The Atchafalaya Delta estuarine complex buffers the impact of the Atchafalaya River on the Louisiana coastal shelf zone, with a 41% of 47% decrease in Atchafalaya River NO3 ? concentrations before reaching Gulf waters.  相似文献   

9.
Seasonal phosphorus limitation occurs on the Louisiana continental shelf as a result of high nitrogen loads in the spring and early summer. Prior studies have assessed such nutrient limitation by laborious and time-consuming nutrient analyses, enzyme assays, and nutrient addition bioassays. We undertook surface (0.5–1 m) mapping of fast repetition rate fluorescence (FRRF) parameters to assess nutrient limitation in real time on the Louisiana continental shelf and Mississippi River plume from 29 June to 08 July, 2002 in an effort to further understand phytoplankton productivity in this region, as well as to better inform effective nutrient management strategies. Surface nutrient concentrations (NO3−, NO2−, NH4+, PO43−), chlorophyll a biomass, alkaline phosphatase (AP) activity, and four FRRF parameters: the maximum quantum yield of photochemistry (F v /F m ), the functional absorption cross section for PSII, the time constant for Q A reoxidation, and the connectivity factor, were measured during continuous underway mapping. Results from traditional methods to assess phytoplankton nutrient stress indicated widespread phosphorus limitation from the Mississippi River plume to the Atchafalaya River, manifested as high inorganic N/P ratios and elevated AP activities associated with phytoplankton biomass. The FRRF data indicated complex patterns of phytoplankton physiology that were likely driven by the rapidly changing conditions in local surface waters and heterogeneous phytoplankton community structure. Correlations of nutrient data and enzyme assays with FRRF parameters were significant but low, potentially due to differences in the manner and time scale with which nutrient limitation affects the different techniques used, indicating that further work is needed to interpret FRRF parameters in large, heterogeneous environments such as estuaries and continental shelves.  相似文献   

10.
The distributions of iodate and total inorganic iodine concentrations in the waters on the Texas?CLouisiana shelf in April, June, and August 2004 are described. Iodine?Csalinity graphs show three-end-member mixing involving onshore and offshore surface waters and deep offshore water. The April survey showed simple mixing on the surface, but in the later surveys, iodate concentrations were often much lower than predicted by the mixing curve while those for total inorganic iodine were higher. This demonstrated both iodate reduction in the water and iodide addition, although individual samples did not show equivalent speciation changes. Hydrographically, the system consists of the estuaries of the Mississippi and Atchafalaya rivers as they spill onto the shelf. The waters are stratified seasonally by a robust halocline, leading to hypoxia in the bottom waters from the combined effect of restricted downward diffusion of oxygen and the sinking of the luxuriant growth of phytoplankton induced by riverine nutrient supply. The distributions of iodate and total inorganic iodine are, therefore, interpreted in terms of water?Csediment interaction as the shelf shoals to the north.  相似文献   

11.
Salinization is a process impacting groundwater quality and availability across much of southern Louisiana, USA. However, a broad divergence of opinion exists regarding the causes of this elevated salinity: updip-migrating marine waters from the Gulf of Mexico, saline fluids migrating up fault planes, movement of water from salt domes, and/or remnant seawater from the last major marine transgression. The Mississippi River Alluvial Aquifer (MRAA) in south-central Louisiana is recharged by the Mississippi River, and there are discharge zones to the west and east. Recharge waters from the Mississippi River are fresh, but Cl− levels in the western portions of the aquifer are as high as 1000 mg/L. The aquifer is an important source of water for several municipalities and industries, but prior to this study the source(s) of the elevated salinity or whether the salinization can be remediated had not been determined.The low Br/Cl ratios in the groundwaters are consistent with a saline endmember produced by subsurface dissolution of salt domes, not a marine source. The H and O isotopic systematics of the aquifer waters indicate meteoric sources for the H2O, not marine waters or diagenetically-altered deep brines. The westward salinization of aquifer water represents a broad regional process, instead of contamination by point sources. Mapping of spatial variations in salinity has permitted the identification of specific salt domes whose subsurface dissolution is producing waters of elevated salinity in the aquifer. These include the Bayou Choctaw and St. Gabriel domes, and possibly the Bayou Blue dome. Salinization is a natural, on-going process, and the potential for remediation or control is slight, if not non-existent.  相似文献   

12.
Seasonal hypoxia [dissolved oxygen (DO)?≤?2 mg?l?1] occurs over large regions of the northwestern Gulf of Mexico continental shelf during the summer months (June–August) as a result of nutrient enrichment from the Mississippi–Atchafalaya River system. We characterized the community structure of mobile fishes and invertebrates (i.e., nekton) in and around the hypoxic zone using 3 years of bottom trawl and hydrographic data. Species richness and total abundance were lowest in anoxic waters (DO?≤?1 mg?l?1) and increased at intermediate DO levels (2–4 mg?l?1). Species were primarily structured as a benthic assemblage dominated by Atlantic croaker (Micropogonias undulatus) and sand and silver seatrout (Cynoscion spp.), and a pelagic assemblage dominated by Atlantic bumper (Chloroscombrus chrysurus). Of the environmental variables examined, bottom DO and distance to the edge of the hypoxic zone were most strongly correlated with assemblage structure, while temperature and depth were important in some years. Hypoxia altered the spatial distribution of both assemblages, but these effects were more severe for the benthic assemblage than for the pelagic assemblage. Brown shrimp, the primary target of the commercial shrimp trawl fishery during the summer, occurred in both assemblages, but was more abundant within the benthic assemblage. Given the similarity of the demersal nekton community described here to that taken as bycatch in the shrimp fishery, our results suggest that hypoxia-induced changes in spatial dynamics have the potential to influence harvest and bycatch interactions in and around the Gulf hypoxic zone.  相似文献   

13.
Water column nutrients (nitrate, ammonium, soluble reactive phosphate, total Kjeldahl nitrogen, and total phosphorus) and suspended sediments (SS) were measured during one 44-h and two 28-h periods in March 1982 at two stations in Fourleague Bay, Louisiana, which is located at the mouth of the Atchafalaya River, a distributary of the Mississippi River. River water (a source of nitrate, total Kjeldahl nitrogen, total phosphorus, and suspended sediments to Fourleague Bay) flows into the upper reaches of the bay during high tide and frontal overrunning conditions with northerly and westerly winds. During one sampling period, decreasing wind speed and the rising tide resulted in Atchafalaya River water inundating the bay and nitrate concentrations in the upper bay increasing from 30–70 μM to 90–118 μM. Significant variations in nutrients associated with the movement of water masses from the river, marshes, and Gulf of Mexico occurred over several different time scales. Tidal transport occurred over 25-h periods, while frontal passages occurred at 3-d to 5-d intervals. Variability in nutrient and suspended sediment concentrations over these relatively short time scales can be as great as seasonal variability in the bay.  相似文献   

14.
The effects of discharge of municipal wastes on water quality within the lower Mississippi River below Old River have been reevaluated using published water quality data in the Louisiana reach of the river for the water years 1974–1984. A novel graphical technique has facilitated the evaluation of upriver controls on water quality and the identification of sources and sinks along the lower Mississippi. Comparison of calculated annual fluxes at different downstream monitoring stations has simplified some of the problems inherent in evaluating analyses of samples collected from different water masses during a typical sampling run. The absolute concentrations of chloride, nitrite plus nitrate, total phosphorous, dissolved oxygen, BOD, and COD are all strongly dependent on processes occurring upriver. Nonpoint influx of materials from agricultural wastes and natural plant debris may be the dominant upstream sources of N, P, BOD, and COD. Increases in chloride and phosphorous downstream within the Lower Mississippi appear to be caused by discharge of industrial wastes. Nitrogen fluxes decrease downriver, except where there is local discharge of high-N, high-P industrial waste water, possibly from fertilizer plants. Removal of N and increases in BOD may be due in part to biological uptake. High river discharge rates and efficient, natural processes of reaeration maintain high oxygen saturation levels. With the exception of an increase in bacterial count, the discharge of municipal waste into the Mississippi River in Louisiana appears to have had no significant effect on water quality, a finding consistent with the earlier U.S. Geological Survey study of Wells (1980). It would be highly desirable for future mass balance studies if existing water quality programs on the Mississippi River were to adopt a Lagrangian sampling approach.  相似文献   

15.
The Mississippi River system ranks among the world's top 10 rivers in freshwater and sediment inputs to the coastal ocean. The river contributes 90% of the freshwater loading to the Gulf of Mexico, and terminates amidst one of the United States' most productive fisheries regions and the location of the largest zone of hypoxia, in the western Atlantic Ocean. Significant increases in riverine nutrient concentrations and loadings of nitrate and phosphorus and decreases in silicate have occurred this century, and have accelerated since 1950. Consequently, major alterations have occurred in the probable nutrient limitation and overall stoichiometric nutrient balance in the adjacent continental shelf system. Changes in the nutrient balances and reduction in riverine silica loading to, the continental shelf appear to have led to phytoplankton species shifts offshore and to an increase in primary production. The phytoplankton community response, as indicated by long-term changes in biological uptake of silicate and accumulation of biologically bound silica in sediments, has shown how the system has responded to changes in riverine nutrient loadings. Indeed, the accumulation of biologically bound silica in sediments beneath the Mississippi River plume increased during the past two decades, presumably in response to, increased nitrogen loading. The duration, size, and severity of hypoxia has probably increased as a consequence of the increased primary production. Management alternatives directed at water pollution issues within the Mississippi River watershed may have unintended and contrasting impacts on the coastal waters of the northern Gulf of Mexico.  相似文献   

16.
In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model.  相似文献   

17.
Soluble reactive phosphorus (SRP) has recently been shown to be one of the limiting nutrients for the growth of phytoplankton in the northern Gulf of Mexico. We show here that during the past decade, SRP concentrations in the lower reaches of North America's largest river, the Mississippi River, were highest in summer and lowest in winter and positively correlated with water temperature. Upstream data showed this coupling to increase in a downstream trend in the Mississippi main stem. Water quality data analysis and phosphorus mass balances were conducted to examine the controls of this relationship. The results showed that the positive SRP–temperature correlation in the Mississippi River system was largely a result of gradual dilution of SRP-enriched upper Mississippi River waters, which contributed most to the Mississippi River during summer, by SRP-depleted waters from the Ohio and other tributaries. Particle buffering and organic matter mineralization might play a role in the observed SRP–temperature relationship, but their importance relative to tributary effects is not quantified. Future work on the seasonal dynamics of SRP in large river systems needs to consider the effects of both tributary dilution and in situ processes.  相似文献   

18.
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19.
The northern Gulf of Mexico (NGOMEX) was surveyed to examine the broad-scale spatial patterns and inter-relationships between hypoxia (<2?mg?L?1 dissolved oxygen) and zooplankton biovolume. We used an undulating towed body equipped with sensors for conductivity, temperature, depth, oxygen, fluorescence, and an optical plankton counter to sample water column structure, oxygen, and zooplankton at high spatial resolution (1?m??vertical; 0.25?C1?km??horizontal). We contrast the distribution of zooplankton during summer surveys with different freshwater input, stratification, and horizontal and vertical extent of bottom-water hypoxia. Bottom-water hypoxia did not appear to influence the total amount of zooplankton biomass present in the water column or the areal integration of zooplankton standing stock in the NGOMEX region surveyed. However, where there were hypoxic bottom waters, zooplankton shifted their vertical distribution to the upper water column during the day where they normally would reside in deeper and darker waters. When bottom waters were normoxic (>2?mg?L?1 dissolved oxygen), the daytime median depth of the water column zooplankton was on average 7?m deeper than the median depth of zooplankton in water columns with hypoxic bottom waters. A reduction in larger zooplankton when there were hypoxic bottom waters suggests that if zooplankton cannot migrate to deeper, darker water under hypoxic conditions, they may be more susceptible to size-selective predation by visual predators. Thus, habitat compression in the northern Gulf of Mexico due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey.  相似文献   

20.
Will lowering estuarine salinity increase Gulf of Mexico oyster landings?   总被引：1，自引：0，他引：1  
Previous studies provide conflicting opinions on whether lower than average salinities in Gulf of Mexico (GOM) estuaries are likely to increase or decrease oyster harvests (Crassostrea virginica), which represented 69% and 54% of the United States oyster landings by weight, and dockside value, respectively, in 2003. The present study examined a 54-yr record (1950–2003) of oyster harvests and river discharge in five major estuaries in GOM states (Florida, Alabama, Mississippi, Louisiana, and Texas). Oyster landings were inversely related to freshwater inflow. Peaks in landings, 21 of 23 in West Florida, Alabama, Mississippi, and Texas combined, were coincidental with lows in river discharge from the major rivers in the estuaries. Lows in landings in these states (17 of 19) coincided with peaks in discharge of the major rivers feeding their estuaries. Landings in Breton Sound, Louisiana, were also inversely related to river discharge. The only exception to this pattern was for landings in the Plaquemines Parish, Louisiana, part of the Breton Sound estuary, where there were higher landings following increased Mississippi River discharge. The Bonnet Carré spillway, completed in 1931, diverts flood waters from the Mississippi River to Lake Pontchartrain, and it has been opened to reduce flood heights in 1937, 1950, 1973, 1975, 1979, 1983, and 1997. Twenty-five of 28 times after the spillway was opened, oyster landings in Mississippi were lower than in the other four states. The inverse relationship between freshwater inflow and oyster landings suggests that the proposed Bonnet Carré Freshwater Project, designed to reduce estuarine salinity, cannot be justified on the basis of anticipated higher oyster yields in Mississippi or Louisiana. Manipulating estuarine salinity in the GOM should be done within the context of the whole estuary and not just part of the estuary.  相似文献   

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