Wetland loss in the Northern Gulf of Mexico: Multiple working hypotheses

I examined four hypotheses about causes for the dramatically high coastal wetland losses (0.86% yr^?1) in the northern Gulf of Mexico: an extensive dredged canal and spoil bank network, a decline in sediments in the Mississippi River during the 1950s, Mississippi River navigation and flood protection levees, and salinity changes. Natural factors contributing to these habitat changes include eustatic sea-level rise and geological compaction, which appear to have remained relatively constant this century, although variation does occur. These four hypotheses were tested using data on land-to-water changes in 15-min quadrangle maps inventoried for four intervals between the 1930s and 1990. Land loss rates were directly proportional to changes in wefland hydrology in time and space. A linear regression of the direct losses due to dredging versus the losses due to all other factors (indirect losses) had a zero intercept and a slope that increased with time. The ratio indirect:direct land loss was highest nearest the estuarine entrance. The coastwide patterns of land loss do not appear to be affected by riverine sediment reductions over the last 60 yr. The effects of changes in wetland hydrology from dredging human-made channels and forming dredged spoil banks appear to be the most efficacious hypothesis explaning these dramatic losses. The effects of extensive human-induced changes on this coast have apparently overwhelmed the causal linkages identified in the historical re-constructionist view of deltaic gain and loss that emphasizes the role of mineral sediments. A paradigm shift is therefore proposed that emphasizes a broad ecological view as contrasted to a mostly physical view emphasizing the role of sediment supply in wetland maintenance. In this view, plants are not an ancillary consequence of strictly geological dynamics such as sediment supply but are dominant agents controlling factors relevant to coastal restoration and management efforts.     

Direct impacts of outer continental shelf activities on wetland loss in the central Gulf of Mexico

The direct impacts of outer continental shelf (OCS) development on recent wetland loss in the northern Gulf of Mexico were quantified using aerial imagery, field surveys, and literature review. The total direct impacts accounted for an estimated 25.6 percent of total net wetland loss within the Louisiana portion of the study area from 1955/56 to 1978. Of the total direct impacts of 73,905 ha, OCS-related activities accounted for 11,589-13,631 ha of the wetland loss during the same time interval. Although this is a substantial areal loss, it represents only 4.0-4.7 percent of the total Louisiana wetland loss from 1955/56 to 1978, and 15.7–18.4 percent of direct impacts.Direct impacts from OCS pipelines averaged 2.49 ha/km, lower than published guidelines, and totalled 12,012 ha. Lowest impacts are for backfilled pipelines in the Chenier Plain of western Louisiana and for small young pipelines built in clustered rights-of-way. Widening of OCS pipeline canals does not appear to be an important factor for total net wetland loss in the coastal zone because few pipelines are open to navigation and, for the examples found, the impact width was not significantly different than for open pipelines closed to navigation. Navigation channels account for a minimum of 16,902 ha of habitat change. Direct impacts per unit length of navigation channel average 20 times greater than pipelines.     

Characterization of nutrient, organic carbon, and sediment loads and concentrations from the Mississippi River into the northern Gulf of Mexico

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.     

Comparison of nutrient and trace element distributions in the delta and shelf outflow regions of the Mississippi and Atchafalaya Rivers

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.     

Accretion and canal impacts in a rapidly subsiding wetland II. Feldspar marker horizon technique

Recent (6–12 month) marsh sediment accretion and accumulation rates were measured with feldspar marker horizons in the vicinity of natural waterways and man-made canals with spoil banks in the rapidly subsiding environment of coastal Louisiana. Annual accretion rates in aSpartina alterniflora salt marsh in the Mississippi deltaic plain averaged 6 mm in marsh adjacent to canals compared to 10 mm in marsh adjacent to natural waterways. The rates, however, were not statistically significantly different. The average rate of sediment accretion in the same salt marsh region for a transect perpendicular to a canal (13 mm yr^?1) was significantly greater than the rate measured for a transect perpendicular to a natural waterway (7 mm yr^?1). Measurements of soil bulk density and organic matter content from the two transects were also different. This spatial variability in accretion rates is probably related to (1) spoil bank influences on local hydrology; and (2) a locally high rate of sediment input from lateral erosion associated with pond enlargement. In a brackishSpartina patens marsh on Louisiana’s Chenier plain, vertical accretion rates were the same along natural and canal waterways (3–4 mm yr^?1) in a hydrologically restricted marsh region. However, the accretion rates for both waterways were significantly lower than the rates along a nonhydrologically restricted natural waterway nearby (11 mm yr^?1). The vertical accretion of matter displayed semi-annual differences in the brackish marsh environment.     

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P_i), silicate (Si)), nutrient ratios (DIN/P_i), 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 P_i 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/P_i 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 P_i 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.     

Effect of hydrologic management on marsh surface sediment deposition in coastal Louisiana

High rates of coastal land loss in Louisiana have prompted efforts to maintain or restore coastal wetland habitats, and structural management of marsh hydrology is one of a number of approaches that has been adopted. The aim of this study was to determine the effect of hydrologic management measures on marsh-surface sediment deposition in the Mississippi deltaic plain. Four impoundments, ranging in size from 50 ha to 177 ha and similarly sized control sites were included in the study. At each site, marsh-surface sediment traps were collected approximately biweekly for 3.5 yr to measure changes in sediment deposition. There was no significant difference in sediment deposition between any of the impoundment and control sites during the premanagement period. The results show no significant difference in marsh-surface sediment deposition between management year 1992–1993 and management year 1994–1995, but management year 1993–1994 had significantly lower sediment deposition than either the first or the last year of the study. Management year 1992–1993 showed the highest sediment deposition when it was assessed across all basins, areas, and sites, and this high rate of deposition is accounted for by the impact of Hurricane Andrew. For all management years there were significant differences between impoundment and control sites, with control sites showing higher rates of marsh surface sediment deposition than impoundments. No clear pattern was identified concerning the influence of various types of hydrologic management on rates of sediment deposition. This reduction in sediment deposition indicated that the areas under management in this study are receiving insufficient inorganic sediment deposition to keep pace with sea-level rise.     

Differential salinity response between two Mississippi River subdeltas: Implications for changes in plant composition

Many studies have determined the physical dynamics of salt-water movement into riverine systems, but few studies have elucidated the role of periodic salinity incursions in affecting primary succession in active deltaic wetlands. The emergence of the neighboring Atchafalaya and Wax Lake Deltas, since a record Mississippi River flood in 1973, has created a unique area of land building and primary succession within the deteriorating wetland landscape of Louisiana. The vegetation in these deltas experiences the extremes of allogenic (riverine and tidal flooding, sedimentation, storm disturbance) and autogenic (herbivory, plant competition) forces. The rapid decline of the dominant vegetation,Sagittaria latifolia Willd. (Broadleaf Arrowhead), in the Atchafalaya Delta, and the continued dominance of this species in the Wax Lake Delta prompted us to investigate whether periodic salinity incursions were an additional stress on this species. The objectives of our study were to determine if salinity differences occurred between these deltas, describe the forces that controlled salinity incursions, and the level of salt exposure that adversely affected the growth ofS. latifolia. Continuous measurements of salinity and water level at six sites, revealed that salinity pulses (up to 7.0‰) were common in the Atchafalaya Delta; the Wax Lake Delta was not prone to elevated salinities. Salinity incursions in the Atchafalaya Delta were related to water level set-up, which was forced by strong (>10 m s⁻¹) easterly and southerly winds prior to cold front passages. The movement of distant tropical storms in the Gulf of Mexico produced salinity incursions of longer duration than cold fronts. In a greenhouse experiment, the growth ofS. latifolia was impaired by salinity concentrations of 6.0‰ within 13 d; flooding treatments (20 cm) compounded the loss of aboveground tissue by the end of the experiment. Results from the field and greenhouse studies, compared with historic evidence of elevated salinities in the Atchafalaya delta, indicate that periodic salinity incursions, combined with additional stresses such as flooding and vertebrate herbivory, influence the distribution or presence ofS. latifolia in the Atchafalaya Delta.     

Relationships between wetland fragmentation and recent hydrologic changes in a Deltaic Coast

Patterns in coastal wetland loss in the northern Gulf of Mexico were examined using aerial imagery from 1955–56 and 1978. Five qualitative types of wetland changes are evident: (1) spoil bank-parallel pond formation, (2) pond formation with apparent random distribution for the smallest ponds, but very clumped distribution for larger ponds, (3) semi- or complete impoundment resulting in open water formation, (4) cutting off of stream channels upstream of where a spoil bank crosses a natural channel, and (5) erosion at the land-water interface. Only ponds <20 ha formed and disappeared in the interval, and it is clear that wetland breakup, not erosion at the pond-lake edge, is the dominant form of wetland-to-open water conversion. Canals and their spoil banks are spatially related to wetland-to-water conversion which is evident up to 2 km away from those man-made features. The indirect impacts of canals and spoil banks vary regionally, for example, with sediment compaction rates that increase with increasing sediment deposition. These results are consistent with the hypothesis that canals and spoil banks are a major factor driving wetland loss rates because they change wetland hydrology.     

Seasonal and spatial water quality changes in the outflow plume of the Atchafalaya River,Louisiana, USA

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.     

A preliminary mass balance model of primary productivity and dissolved oxygen in the Mississippi River Plume/Inner Gulf Shelf Region

A deterministic, mass balance model for phytoplankton, nutrients, and dissolved oxygen was applied to the Mississippi River Plume/Inner Gulf Shelf (MRP/IGS) region. The model was calibrated to a comprehensive set of field data collected during July 1990 at over 200 sampling stations in the northern Gulf of Mexico. The spatial domain of the model is represented by a three-dimensional, 21-segment water-column grid extending from the Mississippi River Delta west to the Louisiana-Texas border, and from the shoreline seaward to the 30–60 m bathymetric contours. Diagnostic analyses and numerical experiments were conducted with the calibrated model to better understand the environmental processes controlling primary productivity and dissolved oxygen dynamics in the MRP/IGS region. Underwater light attenuation appears relatively more important than nutrient limitation in controlling rates of primary productivity. Chemical-biological processes appear relatively more important than advective-dispersive transport processes in controlling bottom-water dissolved oxygen dynamics. Oxidation of carbonaceous material in the water column, phytoplankton respiration, and sediment oxygen demand all appear to contribute significantly to total oxygen depletion rates in bottom waters. The estimated contribution of sediment oxygen demand to total oxygen-depletion rates in bottom waters ranges from 22% to 30%. Primary productivity appears to be an important source of dissolved oxygen to bottom waters in the region of the Atchafalaya River discharge and further west along the Louisiana Inner Shelf. Dissolved oxygen concentrations appear very sensitive to changes in underwater light attenuation due to strong coupling between dissolved oxygen and primary productivity in bottom waters. The Louisiana Inner Shelf in the area of the Atchafalaya River discharge and further west to the Texas border appears to be characterized by significantly different light attenuation-depth-primary productivity relationships than the area immediately west of the Mississippi Delta. Nutrient remineralization in the water column appears to contribute significantly to maintaining chlorophyll concentrations on the Louisiana Inner Shelf.     

Relationships among satellite chlorophylla, river inputs, and hypoxia on the Louisiana Continental shelf, Gulf of Mexico

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophylla (chla) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000–2003), interannual changes in shelf-wide chla concentrations averaged over January–July ranged from +57% to −33% of the 4-yr average, in accord with freshwater discharge changes of +20% to −29% and nitrate load changes of +20% to −35% from the Mississippi and Atchafalaya Rivers. Chla variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January–July mean chla concentrations (5.5 mg m⁻³ over 7,000 km²) occurred during 2000, the year of lowest freshwater discharge (16,136 m³ s⁻¹) and nitrate load (55,738 MT N d⁻¹) onto the shelf. Highest January–July mean chla concentrations (13 mg m⁻³ over 7,000 km²) were measured in 2002, when freshwater discharge (27,440 m³s⁻¹) and nitrate load (101,761 MT N d⁻¹) were highest and second highest, respectively. Positive correlations (R²=0.4–0.5) were found between chla and both fresh water and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chla blooms west of the Mississippi Delta 4–5 wk after peaks in river discharge. East of the delta, the chla concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chla and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior year's nitrate load and carbon accumulation at the seabed.     

Input of Nutritionally Rich Organic Matter from the Mississippi River to the Louisiana Coastal Zone

Isotopes have often been used to discern riverine subsidies to coastal food chains, but there are few direct measurements of nutritional quality of riverine particulates. We tested for nutritionally enriched organic matter in the Mississippi River suspended sediment and evidence for its delivery to Louisiana coastal sediments by measuring enzymatically hydrolysable amino acids (EHAA). Riverine suspended sediments contained EHAA concentrations of up to 5 mg g^?1, higher than reported in any coastal sediment. Pigment concentrations indicated that EHAA in some river samples were dominated by phytoplankton, but many samples contained significant non-algal EHAA. Coastal sediments showed EHAA concentrations lower than riverine sediments but still higher than most reported shelf values. Incubation of riverine sediment showed losses of 28–34% of their EHAA over 6 days, similar to differences found between riverine and coastal sediments. EHAA concentrations decreased more rapidly than total nitrogen, indicating the relative lability of this pool of material in the studied region. These EHAA-enriched materials provide fuel for various coastal biota whose composition likely depends on factors such as disturbance regimes.     

Accretion and canal impacts in a rapidly subsiding wetland. I.137Cs and210Pb techniques

The influence of canals on vertical marsh accretion, including mineral sediment and organic matter accumulation, was evaluated at three locations along the Louisiana coast representing different geographic regions. The isotopes²¹⁰Pb and¹⁵⁷Cs were used to determine vertical accretion along transects representing a canal and a control site. Rapid rates of vertical accretion were measured at all sites and ranged from 0.47 cm yr^?1 to 0.90 cm yr^?1. Results indicated that there was no measurable effect of canals on marsh accretionary processes. In general, greater variation in vertical accretion, including mineral sediment deposition and organic matter accumulation, was observed between geographical regions than between canal and control sites within a region. Statistical analysis of data suggest that any difference between canal and control site would be less than 0.20 cm yr^?1. Such a change in marsh surface-water level relationships as a result of any canal influence on marsh accretionary processes would be less than reported eustatic sea-level rise for the Gulf of Mexico. Results suggest that any change in the marsh surface-water level relationship could be the influence of canals on local hydrology, resulting in increased water level rather than any appreciable reduction in accretionary processes. Such changes in hydrology under certain conditions could stress vegetation, resulting in marsh deterioration.     

A review of water column processes influencing hypoxia in the northern Gulf of Mexico

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.     

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.     

Shoreline erosion rates along barrier islands of the north central gulf of Mexico

Rates of shoreline change and overwash penetration distances were calculated for barrier islands along the Louisiana, Mississippi, and Alabama coasts with the orthogonal grid mapping system (OGMS). Average rates of shoreline change are exceptionally high in Louisiana, being of the order ?4.7 to ?7.4 m yr^?1. Mississippi and Alabama recession rates are lower and range from ?2.0 to ?3.1 m yr^?1 over the period of record. Erosion rates along the shorelines of these islands have remained relatively constant over the period of study with five exceptions in coastal Louisiana and the Chandeleur-Breton Islands Arc, and two exceptions along the Mississippi-Alabama barrier islands where they have accelerated. Mean overwash penetration is greatest along Dauphin Island, Alabama, and Cat Island, Mississippi: 207.6 and 197.9 m, respectively. The Chandeleur-Brenton Islands Arc range from 88.1 m at the central barrier to 180.4 along the flanks. The Mississippi islands range from 105.2 m on Ship Island to 200.5 m along central Horn Island. Mean overwash penetration along the Louisiana barriers is highly variable: 46.3 to 211.4 m.     

Water quality analysis of a freshwater diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km² of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summer water quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted Mississippi River water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite + nitrate (5.6–13.4 g m⁻² yr⁻¹), total nitrogen (8.9–23.4 g m⁻² yr⁻¹), and total phosphorus (0.9–2.0 g m⁻² yr⁻¹) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88–97%; total nitrogen 32–57%; total phosphorus 0–46%). The low impact of the diversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nitrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana.     

Control of the variability of nutrients and suspended sediments in a gulf coast estuary by climatic forcing and spring discharge of the Atchafalaya River

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.     

Nutrient inputs from the watershed and coastal eutrophication in the Florida keys

Widespread use of septic tanks in the Florida Keys increase the nutrient concentrations of limestone groundwaters that discharge into shallow nearshore waters, resulting in coastal eutrophication. This study characterizes watershed nutrient inputs, transformations, and effects along a land-sea gradient stratified into four ecosystems that occur with increasing distance from land: manmade canal systems (receiving waters of nutrient inputs), seagrass meadows, patch reefs, and offshore bank reefs. Soluble reactive phosphorus (SRP), the primary limiting nutrient, was significantly elevated in canal systems compared to the other ecosystems, while dissolved inorganic nitrogen (DIN; NH₄ ⁺ and NO₃ ^?) a secondary limiting nutrient, was elevated both in canal systems and seagrass meadows. SRP and NH₄ ⁺ concentrations decreased to low concentrations within approximately 1 km and 3 km from land, respectively. DIN and SRP accounted for their greatest contribution (up to 30%) of total N and P pools in canals, compared to dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) that dominated (up to 68%) the total N and P pools at the offshore bank reefs. Particulate N and P fractions were also elevated (up to 48%) in canals and nearshore seagrass meadows, indicating rapid biological uptake of DIN and SRP into organic particles. Chlorophylla and turbidity were also elevated in canal systems and seagrass meadows; chlorophylla was maximal during summer when maximum watershed nutrient input occurs, whereas turbidity was maximal during winter due to seasonally maximum wind conditions and sediment resuspension. DO was negatively correlated with NH₄ ⁺ and SRP; hypoxia (DO<2.5 mg l^?1) frequently occurred in nutrient-enriched canal systems and seagrass meadows, especially during the warm summer months. These findings correlate with recent (<5 years) observations of increasing algal blooms, seagrass epiphytization and die-off, and loss of coral cover on patch and bank reef ecosystems, suggesting that nearshore waters of the Florida Keys have entered a stage of critical eutrophication.