Mississippi River Water Research Articles

Selective Detection of Chemical and Biological Toxins Using Gold-Nanoparticle-Based Two-Photon Scattering Assay

Possible terrorist threats on water supplies are causes for concern given the easy availability of numerous biological and chemical toxins that could be used by a terrorist organization. In this article, we report gold-nanoparticle-based two-photon light-scattering (TPS) assay, for the label-free detection of arsenic and Salmonella bacteria separately, with excellent detection limit and selectivity over other analytes. Our experimental results show that arsenic can be detected quickly and accurately without any tagging, in 100-ppt level with excellent discrimination against other heavy metals. We have demonstrated that our TPS assay is capable of measuring the amount of arsenic in Bangladesh, West Bengal, and Nevada well water as well as in Mississippi river water. We have also shown that gold-nanoparticle-based TPS assay are capable for label-free detection of Salmonella typhimurium (S. typhimurium) with excellent detection limit (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> bacteria/mL) and high selectivity over other pathogens. The mechanism of TPS assay working principle has been discussed. Our results demonstrate the potential for a broad application of nanotechnology in practical defense applications.

Effects of freshwater input on nutrient loading, phytoplankton biomass, and cyanotoxin production in an oligohaline estuarine lake

Pulsed river water events can increase nutrient levels potentially translating into enhanced primary production, phytoplankton community shifts, and bloom formation. The Bonnet Carre Spillway is a managed river diversion which can be used to redirect a significant amount of Mississippi River water into Lake Pontchartrain, reducing the risks of flood in the downstream communities during runoff seasons. We investigated nutrient enrichment and consequent changes in phytoplankton biomass, including toxic species in Lake Pontchartrain during and after a 1-month Bonne Carre Spillway opening in 2008. Water samples were collected along a 30 km transect. A freshwater plume was found to have formed by the strong river input that had limited mixing with the lake during the opening. The plume and lake water gradually mixed together after the Spillway was closed, indicated by the reduction of the horizontal salinity gradient. The river pulse increased the lake nitrate and dissolved reactive phosphorus concentrations to more than five times the lake background in the plume stations. Nutrient concentrations decreased rapidly after the Spillway closure as the plume dissipated. Diatoms and chlorophytes dominated the system during the opening. After the Spillway closure, there was a shift over time from diatom dominance to toxic cyanobacteria dominance that corresponded to more stable, warmer, and nutrient-limited water conditions. Associated toxins were present and varied over time and space. Further research on the phytoplankton assemblages on the lake is needed in subsequent, non-Spillway opening years to evaluate the impact of river water pulses on the development of these toxic cyanobacterial blooms.

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH 3COO −) or strong (i.e., CO 3 2 - ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).

Denitrification Enzyme Activity as an Indicator of Nitrate Movement through a Diversion Wetland

The Davis Pond freshwater diversion is intended to help restore Louisiana's coastal wetlands by reintroducing Mississippi River water to Barataria Basin. We hypothesized that the high NO3− concentration (2.0 mg NO3–N L−1) of the Mississippi River water would control the rate of denitrification in the receiving marsh given that the soils are saturated, anaerobic, and contain high C. Therefore, areas of high denitrification enzyme activity (DEA) in the marsh would represent soils exposed to river NO3− and actively involved in denitrification. Data from 88 soil samples (0–10 cm) collected throughout the marsh revealed significantly higher rates of DEA in a 715‐ha area adjacent to the diversion inflow. This area of generally high DEA contained &gt;80% of all DEA observed while representing only 19% of the total marsh area at the low discharge rate of 39.5 m3 s−1 The area of high DEA coincided with the highest surface water NO3− and indicated that the marsh has a greater aerial capacity for NO3− removal than is utilized. A laboratory experiment suggested that soils loaded with external NO3− typically had higher DEA rates than soils receiving no added NO3− The DEA was strongly dependent on soil depth (92% of DEA occurred at 0–5 cm) and internal N cycling was substantial in this wetland soils. This study demonstrates the applicability of using soil DEA to map where denitrification activity is greatest, the aerial extent of soils involved in denitrification, and the general flow path of introduced nutrients in large wetlands where NO3− is the limiting factor for denitrification.

Temperature Control on Soluble Reactive Phosphorus in the Lower Mississippi River?

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.

Simultaneous Screening of Herbicide Degradation Byproducts in Water Treatment Plants Using High Performance Liquid Chromatography−Tandem Mass Spectrometry

Currently, herbicides are widely used in various combinations at many stages of cultivation and during postharvest storage. There are increasing concerns about the public health impact of herbicide degradation byproducts that may be present in water bodies used either as drinking water or for recreational purposes. This work investigated the sulfonic acid and oxanilic acid degradation products of metolachlor, alachlor, acetochlor, and propachlor in a variety of water bodies. The objective was to develop a fast, accurate, and easy method for quantitative analysis of herbicide degradation products using liquid chromatography with tandem mass spectrometry without solid phase extraction, but performing levels of detection lower than those obtained in previous studies with solid phase extraction. This research also screened 68 water samples, both untreated source water and treated water, from 34 water treatment plants in Missouri. Finally, it examined seasonal trends in levels of those degradation products by collecting and testing samples monthly. This highly sensitive method can analyze these degradation products to low ng/L levels. The method limit of quantification ranges from 0.04 to 0.05 ppb for each analyte; and quantitative analyses show a precision with RSDs of around 0.6% to 3% in treated water and 2% to 19% in untreated source water. Concentrations of alachlor ESA, acetochlor OA, metolachlor OA, and metolachlor ESA were detected from the Missouri River and the Mississippi River water bodies in summer time. Occurrences of these compounds in treated water samples are all lower than those in the untreated source water samples.

Aquatic photochemistry of chlorinated triclosan derivatives: Potential source of polychlorodibenzo‐P‐dioxins

Triclosan (TCS; 5-chloro-2-(2,4-dichlorophenoxy)phenol), a common antimicrobial agent, may react with residual chlorine in tap water during transport to wastewater treatment plants or during chlorine disinfection of wastewater, generating chlorinated TCS derivatives (CTDs): 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol (4-Cl-TCS), 5,6-dichloro-2-(2,4-dichlorophenoxy)phenol (6-Cl-TCS), and 4,5,6-trichloro-2-(2,4-dichlorophenoxy)phenol (4,6-Cl-TCS). The photochemistry of CTDs was investigated due to the potential formation of polychlorodibenzo-p-dioxin (PCDD) photoproducts. Photolysis rates were highly dependent upon CTD speciation, because the phenolate species degraded 44 to 586 times faster than the phenol forms. Photolysis quantum yield values for TCS, 4-Cl-TCS, 6-Cl-TCS, and 4,6-Cl-TCS of 0.39, 0.07, 0.29, and 0.05, respectively, were determined for the phenolate species. Photolyses performed in Mississippi River and Lake Josephine (USA) waters gave similar quantum yields as buffered, pure water at the same pH, indicating that indirect photolysis processes involving photosensitization of dissolved organic matter are not competitive with direct photolysis. The photochemical conversion of the three CTDs to PCDDs under solar irradiation was confirmed in natural and buffered, pure water at yields of 0.5 to 2.5%. The CTD-derived PCDDs possess higher toxicities than 2,8-dichlorodibenzo-p-dioxin, a previously identified photoproduct of TCS, due to their higher chlorine substitution in the lateral positions. The load of TCS- and CTD-derived PCDDs to United States surface waters is estimated to be between 46 and 92 g toxicity equivalent units per year. Other identified photoproducts of each CTD were 2,4-dichlorophenol and reductive dechlorination products.

Planning for the Future of the Pontchartrain Coast

Abstract There have been many plans developed by governmental agencies to restore the coastal landscape in the Pontchartrain Basin. This article traces the development of coastal restoration planning in Louisiana and highlights the elements of three state and federal plans that have been developed in the last decade. While some specifics have changed, the fundamental approaches on which the plans are based largely remain the same: diversions of Mississippi River water and sediment, marsh creation using material dredged from another location, and shoreline protection. All plans accept that change is inevitable at the coast but more recent plans are less constrained by the impacts of these changes on local populations. Rather they anticipate adjustment and change within communities as well. Questions remain as to whether these measures will be enough in the face of the great uncertainty the twenty-first century holds and how science will be applied to support implementation.

Geochemistry of Surficial Sediments from Lake Pontchartrain Resulting from the 1997 Opening of the Bonnet Carré Spillway

Abstract The Bonnet Carre Spillway is a flood-control structure that diverts Mississippi River water into Lake Pontchartrain during exceptionally high river stages. Because of elevated water levels in the Mississippi River in the spring of 1997, the Bonnet Carre Spillway was opened on March 17 and fully closed on April 18. The total volume of water discharged into Lake Pontchartrain was approximately 11.8 km3, or two times the volume of the lake, and the total mass of sediment discharged into the lake was approximately 7.1 × 108 kg (780,000 US tons). In 1996, 757 surface sediment samples were collected in Lake Pontchartrain and were analyzed by x-ray fluorescence spectroscopy for major cation constituents. These same sites were revisited following the 1997 Mississippi River discharge event. Analysis of the 1996 and 1997 lake-bed sediment samples was accomplished utilizing fundamental statistical and graphical methods. Element concentration contour maps and variograms for the major cations illustrate meani...

Water quality and phytoplankton communities in Lake Pontchartrain during and after the Bonnet Carré Spillway opening, April to October 2008, in Louisiana, USA

The Bonnet Carre Spillway, located 28 miles northwest of New Orleans, was constructed in the early 1930s as part of an integrated flood-control system for the lower Mississippi River system. From 11 April to 8 May 2008, Mississippi River water was diverted through the spillway into the 629-square-mile Lake Pontchartrain, which is hydraulically connected to the Gulf of Mexico. On 8 April, prior to the opening of the spillway, water-quality instruments were deployed and recorded hourly measurements of water temperature, dissolved oxygen, specific conductance, pH, and nitrate. Discrete water-quality and phytoplankton (algae) samples were collected in Lake Pontchartrain from 8 April to 3 October 2008 to assess the water-quality nutrient enrichment effects of the diversion on the lake. The maximum influence of river water in the southern portion of the lake was captured with continuous (hourly) monitoring of nitrate concentrations, and field measurements such as of specific conductance during the critical period in late April to early May. By late May, the deployed instruments had recorded the arrival, peak, and decline of selected constituents associated with the freshwater influx from the Mississippi River/Bonnet Carre Spillway diversion. The continuous monitoring data showed the short-term interactions of high-nitrate, low-specific conductance river water and low-nitrate, high-specific conductance lake water. The phytoplankton community composition, as an indicator of water quality, illustrated an extended response from the river water evident even after the continuous and discrete samples indicated that the lake had returned to pre-diversion conditions. The initial phytoplankton community response to nutrient increases was related to accumulations of diatoms. During periods of low nutrient concentrations, accumulations of blue-greens occurred by July and August. As blue-green algae cell densities and biovolumes increased in the summer, so did the species richness of blue-green algae, particularly the harmful algae bloom taxa. Cell densities and biovolume of the phytoplankton lake indicator taxa Skeletonema costatum, Anabaena sp., and Cylindrospermopsis raciborskii were highest and dominated the diatom and blue-green algae communities during the period of most river water influence on the lake and immediately following the freshwater inflows. The dominance and recession of these indictor taxa reflect the dramatic changes that occurred in the phytoplankton community in response to an increase in nutrient-rich freshwater from the diversion into the lake, and not normal seasonal phytoplankton compositional differences. Water-quality data indicated a gradual reversion to pre-diversion lake conditions by June to July, but shifts in the phytoplankton composition were still evident through August 2008. Observations from this study were similar to results from previous studies of Mississippi River/Bonnet Carre Spillway diversion opening in 1997.

Nutrient Limitation on Phytoplankton Growth in the Upper Barataria Basin, Louisiana: Microcosm Bioassays

The Davis Pond Diversion (DPD) was constructed to divert Mississippi River (MR) water into the Barataria Basin to reduce the salinity in support of wetland restoration on the Louisiana coast. To assess the phytoplankton nutrient limitation in adjacent water systems and potential impacts of DPD, 12 seasonal nutrient-phytoplankton bioassay experiments were conducted from October 2003 to July 2004 using the natural phytoplankton assemblages from freshwater and brackish-water lakes, Cataouatche and Salvador, LA (USA), which receive Mississippi River water from the DPD, and from a nearby freshwater lake, Lac des Allemands, that does not. Dissolved inorganic nitrogen (N), phosphorus (P), and silicate (Si) were added with different combinations at Redfield ratios in 10-l microcosms. Nitrogen was found to be the sole or primary limiting nutrient in all 12 experiments. N and P colimitations were found in seven of 12 experiments, but N was always the stronger limiting factor. P limitation was never observed to be the sole limiting nutrient. The results showed that a low concentration of P and a relatively high concentration of N do not necessarily indicate only P limitation in these lakes. Lake Cataouatche and Lake Salvador were dominated by centric diatoms, and Anabaena spp. were detected at high levels, particularly in summer. Lac des Allemands was generally dominated by N-fixing Anabaena spp. and other cyanobacteria, and their biomass responded significantly to N addition but not to P addition, indicating that nitrogen fixation in Lac des Allemands may be inhibited by other factors such as iron. Our bioassay results demonstrate that whether a water body is N- or P-limited is the consequence of the nutrient status and not the salinity regime. The results suggest that the addition of nutrient-rich waters via diversions of Mississippi River water into these lakes might increase the frequency of algal blooms, including noxious and toxic freshwater cyanobacteria.

Denitrification Potential of Lake Sediment at the Davis Pond Mississippi River Freshwater Diversion Site, Louisiana, USA

ABSTRACT Denitrification potential was determined in surface sediment collected from Lake Cataouatche, the receiving basin for the Davis Pond Mississippi River freshwater diversion. Denitrification was measured in the laboratory using sediment microcosms flooded with lake water. Nitrate was added to the microcosms to achieve 1) an initial nitrate concentration similar to the mean concentration (1.4 mg NO3-N L−1) in Mississippi River water, and 2) a high nitrate concentration to elicit denitrification potential (50.0 mg NO3-N L−1). Mean denitrification rates in controls (no nitrate added) ranged from < 0.06 to 0.67 mg N m−2 d−1 and averaged about 0.22 mg N m−2 d−1. Denitrification potential measured at the primary sampling site (close proximity to outflow) during the most active period (days 3–10) ranged from 43.3 to 94.1 with a mean of 62.3±10.5 mg N m−2 d−1. Denitrification potential was about 18 times greater compared to the Mississippi River nitrate concentration denitrification rate (3.52 ± 0.42 mg N m−2 d−1). Mean denitrification potential of four secondary sites varied from 30.2 to 56.6 mg N m−2 d−1. Lake Cataouatche sediment has the capacity to remove nitrate from the water column via denitrification and could potentially remove a significant portion of nitrate inputs from the Davis Pond diversion.

Abundance and variation of colloidal organic phosphorus in riverine, estuarine, and coastal waters in the northern Gulf of Mexico

The abundance of colloidal organic phosphorus (COP) and colloidal inorganic phosphorus (CIP) was quantitatively determined using an ultrafiltration permeation model for riverine, estuarine, and coastal waters in the northern Gulf of Mexico. Dissolved inorganic phosphorus (DIP) was the dominant species in Mississippi and Pearl River waters, while dissolved organic phosphorus was dominant in marine environments. The abundance of COP was highest in the Pearl River (up to 88%), followed by the Mississippi Bight (~67%), and the Bay of St. Louis (~59%), but was lowest in the Mississippi River (41–50%). These variations highlight the roles of terrestrial inputs, autochthonous production, and anthropogenic activities in controlling the COP abundance in different aquatic environments. In the Gulf of Mexico, COP abundance generally decreased with increasing depth and coincided with chlorophyll a, reflecting the production of COP by phytoplankton and remineralization during downward transportation in the water column. The colloidal organic C: P molar ratios were substantially higher than the Redfield ratio but consistently lower than those of the bulk dissolved organic matter except for the Mississippi River, indicating a diagenetically fresher or younger COP pool. The percentage of CIP in the DIP pool was generally negligible or very low (≤3%) except in the Pearl River, where CIP abundance was as high as 27–47%, likely contributed from colloidal soils, minerals, and iron oxyhydroxides. We hypothesize that high COP abundance and seasonal P‐limitation play an important role in regulating the biogeochemical cycling of P and the development of hypoxia in the northern Gulf of Mexico.

Photochemical Formation of Halogenated Dioxins from Hydroxylated Polybrominated Diphenyl Ethers (OH-PBDEs) and Chlorinated Derivatives (OH-PBCDEs)

The potential photochemical formation of polybrominated and mixed halogenated dibenzo-p-dioxins (PBDDs and PXDDs) from hydroxylated polybrominated and polybrominated/ chlorinated diphenyl ethers (OH-PBDEs and OH-PBCDEs) in aqueous solution was studied. The ortho-hydroxylated BDE47 derivative 6-OH-BDE47, and chlorinated derivatives 3-Cl-6-OH-BDE47, 5-Cl-6-OH-BDE47, and 3,5-Cl-6-OH-BDE47 were photolyzed under sunlight at 45 degrees N latitude in buffered waters, Mississippi River water, Lake Josephine water, and ultrapure water adjusted to the pH of the natural waters. Chemical actinometry was used to determine reactant quantum yields which were calculated to be between 0.03 and 0.21, with lower yields for the chlorinated derivatives under all conditions. Quantum yields under natural water conditions were not significantly enhanced indicating that direct photolysis is the primary process of photochemical degradation. The formation of halogenated dioxins from the outdoor photolysis of the four OH-PBDEs/OH-PBCDEs under all conditions was confirmed. Dioxin yields of 0.7-3.6% were found, with higher yields for 6-OH-BDE47 under all conditions. This study suggests that photolysis of OH-PBDEs and OH-PBCDEs is a potential formation pathway of PBDDs and PXDDs in the environment.

Nutrient dynamics in the lower Mississippi River floodplain: Comparing present and historic hydrologic conditions

Alterations to the lower Mississippi River-floodplain ecosystem to facilitate commercial navigation and to reduce flooding of agricultural lands and communities in the historic floodplain have changed the hydrologic regime. As a result, the flood pulse usually has a lower water level, is of shorter duration, has colder water temperatures, and a smaller area of floodplain is inundated. Using average hydrologic conditions and water temperatures, we used established nitrogen and phosphorus processes in soils, an aquatic ecosystem model, and fish bioenergetic models to provide approximations of nitrogen and phosphorus flux in Mississippi River flood waters for the present conditions of a 2-month (mid-March to mid-May) flood pulse and for a 3-month (mid-March to mid-June), historic flood pulse. We estimated that the soils and aquatic biota can remove or sequester 542 and 976 kg nitrogen ha−1 during the present and historic hydrologic conditions, respectively. Phosphorus, on the other hand, will be added to the water largely as a result of anaerobic soil conditions but moderated by biological uptake by aquatic biota during both present and historic hydrologic conditions. The floodplain and associated water bodies may provide an important management opportunity for reducing downstream transport of nitrogen in Mississippi River waters.

Denitrification potential and its relation to organic carbon quality in three coastal wetland soils

Capacity of a wetland to remove nitrate through denitrification is controlled by its physico-chemical and biological characteristics. Understanding these characteristics will help better to guide beneficial use of wetlands in processing nitrate. This study was conducted to determine the relationship between soil organic carbon (SOC) quality and denitrification rate in Louisiana coastal wetlands. Composite soil samples of different depths were collected from three different wetlands along a salinity gradient, namely, bottomland forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) located in the Barataria Basin estuary. Potential denitrification rate (PDR) was measured by acetylene inhibition method and distribution of carbon (C) moieties in organic C was determined by 13C solid-state NMR. Of the three wetlands, the FM soil profile exhibited the highest PDR on both unit weight and unit volume basis as compared to FS and SM. The FM also tended to yield higher amount of N 2O as compared to the FS and SM especially at earlier stages of denitrification, suggesting incomplete reduction of NO 3 − at FM and potential for emission of N 2O. Saline marsh soil profile had the lowest PDR on the unit volume basis. Increasing incubation concentration from 2 to 10 mg NO 3 −-N L − 1 increased PDR by 2 to 6 fold with the highest increase in the top horizons of FS and SM soils. Regression analysis showed that across these three wetland systems, organic C has significant effect in regulating PDR. Of the compositional C moieties, polysaccharides positively influenced denitrification rate whereas phenolics (likely phenolic adehydes and ketonics) negatively affected denitrification rate in these wetland soils. These results could have significant implication in integrated assessment and management of wetlands for treating nutrient-rich biosolids and wastewaters, non-point source agricultural runoff, and nitrate found in the diverted Mississippi River water used for coastal restoration.

Remotely sensed sea-surface chlorophyll and POC flux at Deep Gulf of Mexico Benthos sampling stations

Abstract Biweekly composite averages of the standing stock of sea-surface chlorophyll (SSC) were derived from SeaWiFS satellite ocean-color data at 44 benthic sampling stations occupied along the continental slope and rise by the Deep Gulf of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26°N and west of 91°W in the NW Gulf of Mexico, annual average SSC was 0.19 mg m −3 , ranging at most locations from annual highs of about 0.3 mg m −3 in November–February to lows of about 0.1 mg m −3 in May–August. Comparison of three years of SeaWiFS data (January 1998–December 2000) showed little inter-annual variation at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26°N and east of 91°W, SSC averaged 2.8 times higher than in the NW Gulf, showing also strong inter-annual variation. Maxima in the NE region occurred in November–February and also during summers. The summer maxima were associated with Mississippi River water transported offshore to the east and southward by anticyclonic eddies in the NE Gulf. The apparent increases in SSC in June–August at NE Gulf stations reached average monthly concentrations >50% greater than in November–February. Based on a primary productivity model and a vertical flux model, the calculated export of particulate organic carbon (POC flux reaching the seafloor) was estimated as ∼18 mg C m −2 day −1 at the 22 NE Gulf stations, and ∼9 mg C m −2 day −1 at the 22 NW Gulf stations. These estimates are comparable to fluxes measured by benthic lander by others in the DGoMB program, which may drive the differences in west versus east bathymetric zonation and community structure of macrobenthos that were sampled with large box corers by others in the DGoMB program.

Treated Wastewater Effluent Reduces Sperm Motility Along an Osmolality Gradient

Many toxic effects of treated wastewater effluent on organismal and reproductive health have been documented. However, the physicochemical environment of treated wastewater effluent frequently differs considerably from that of its receiving waters and may affect organismal function independently of toxic effects. Teleost sperm, for example, may be affected by the higher osmolality of treated wastewater, as this sperm is activated for a brief period of time following ejaculation due to the sudden decrease in osmolality of its surrounding environment. In this study, we examined the effects of treated wastewater effluent on sperm motility to test the hypothesis that the higher osmolality of effluent compared to river water will adversely affect sperm activation in a concentration-dependent relationship. Treated wastewater effluent was collected on 5 days from the outflow of the Metropolitan Wastewater Treatment Plant, St. Paul, Minnesota, and from an upstream site on the Mississippi River. Milt aliquots collected from goldfish were diluted in an isotonic extender solution and subsequently activated in either deionized water, 100%, 50%, or 10% effluent, a synthetic ion mixture, or river water. Sperm motility and velocity were assessed at 15-s intervals for 1 min using a computer assisted sperm analyzer. Significant differences in performance parameters were found only at 15 s, with sperm motility and velocity declining rapidly at later sampling times. Predictably, deionized water resulted in the greatest activation of sperm motility, while motility exhibited a concentration-dependent decline in 10%, 50%, and 100% treated wastewater effluent. Interestingly, Mississippi River water and a synthetic ion mixture with an osmolality comparable to 50% effluent both resulted in the least amount of sperm activation. However, sperm activation in river water varied between collection days during the study. River water and 100% effluent both had low sperm activation characteristics despite a 10-fold difference in osmolality between these two treatments (1 and 10 mOsmol kg(-1), respectively). Results of this study indicate a concentration-dependent decrease in sperm motility in treated wastewater effluent as well as significant fluctuations of sperm activation in Mississippi River water. This study illustrates the complexity of assessing the effects of treated wastewater effluents and the difficulty of determining appropriate reference sites for such studies.

Through droughts and hurricanes: Tree mortality, forest structure, and biomass production in a coastal swamp targeted for restoration in the Mississippi River Deltaic Plain

Coastal swamps are among the rapidly vanishing wetland habitats in Louisiana due to accelerated sea-level rise and hydrological alterations that alter the natural flooding regime. In particular, the swamp forests of Lake Maurepas, Louisiana, have degraded considerably, and research regarding their condition might suggest approaches for their restoration. We measured forest structure, species composition, tree mortality, annual aboveground net primary production (ANPP) of woody species, and aboveground biomass allocation to leaf litter and wood, and soil strength at forty study plots within the Lake Maurepas basin over 5 years to evaluate the current condition of this coastal forested wetland. Local measures of salinity and regional measures of flooding were used to predict ANPP and aboveground biomass allocation. The 5-year study period included an intense drought as well as years characterized by hurricane-induced flooding. The forty study plots could be divided into four distinct habitat clusters based on standing biomass, structural variables, and salinity. The majority of the plots were co-dominated by Taxodium distichum and Nyssa aquatica. Acer rubrum var. drummondii and Fraxinus pennsylvanica were common mid-story species throughout the western and southern parts of the study area, while Salix nigra, Morella cerifera, and Triadica sebiferum were more important at the more degraded plots in the eastern part of the basin. Annual mean soil salinity reached unprecedented level (2–5 psu) during the drought and cumulative tree mortality reached up to 85% in areas characterized by frequent saltwater intrusions. The ANPP was higher during the drought period in 2000–2001 than during subsequent years, and was dominated by T. distichum. At most sampling plots, litter production exceeded wood production annually. A negative correlation between aboveground biomass allocation to litter and flooding indicated that biomass allocation shifted from litter toward wood during wet years. Overall, the majority of the plots sampled produced less than 400 g m −2 yr −1 of aboveground biomass annually due to the interacting negative effects of saltwater intrusion and prolonged flooding with nutrient-poor water. Reintroduction of Mississippi River water to the Maurepas system has the potential to benefit these swamps greatly by restoring a greater flow of nutrients, sediments, and fresh water through the wetlands. The historically slow (i.e., multi-decadal) process of swamp deterioration was greatly sped by low salinity (i.e., 2–5 psu) saltwater intrusions during a drought in 1999–2000. The majority of the coastal swamps in the Pontchartrain Basin are deteriorating, and most of this swamp area will be lost to open water in the foreseeable future if no restoration action is taken.

Influence of salinity level on sediment denitrification in a Louisiana estuary receiving diverted Mississippi River water

The Mississippi River water containing elevated nitrate is being diverted into Louisiana coastal estuaries to abate wetland deterioration attributed to lack of sediment and nutrients, rapid subsidence and accompanying salt water intrusion. In this study effect of salinity change on sediment denitrification at a Mississippi River freshwater diversion site (Davis Pond, Louisiana) was determined. Results show that the denitrification potential of the sediment was highest under fresh water condition (salinity close to 0‰). Addition of sea water immediately inhibited the denitrification activity of the sediment. Further analysis, by separate treatment of NaCl and K2SO4 addition, revealed that inhibition of the denitrification of the sediment by sea water was mainly caused by NaCl content in sea water. Denitrification activity of the sediment was not significantly affected by the sulfate content in sea water. Salinity increase seems a primary reason for the sediment denitrification rate decrease. A significant inverse relationship of denitrification rate and salinity was obtained [denitrification rate (mg N kg−1 day−1) = −0.20 × salinity(‰) + 10.41, R2 = 0.91]. Under highest sea water condition (salinity = 36‰), denitrification rate of the sediment would be 30.8% of its original activity (salinity = 0‰).