Potential methane production in oligohaline wetlands undergoing erosion and accretion in the Mississippi River Delta Plain, Louisiana, USA.

Methane (CH4) is steadily increasing in the atmosphere from different sources including wetlands. However, there is limited landscape level CH4 flux data in deltaic coastal systems where the availability of freshwater is impacted by the combined effect of climate change and anthropogenic impacts. Here we determine potential CH4 fluxes in oligohaline wetlands and benthic sediments in the Mississippi River Delta Plain (MRDP), which is undergoing the highest rate of wetland loss and most extensive hydrological wetland restoration in North America. We evaluate potential CH4 fluxes in two contrasting deltaic systems, one undergoing sediment accretion as result of a freshwater and sediment diversions (Wax Lake Delta, WLD), and one experiencing net land loss (Barataria-Lake Cataouatche, BLC). Short- (<4 days) and long-term (36 days) incubations using soil and sediment intact cores and slurries were performed at different temperatures representing seasonal differences (10, 20, 30 °C). Our study revealed that all habitats were net sources of atmospheric CH4 in all seasons, and CH4 fluxes were generally the highest for the 20 °C incubation. The CH4 flux was higher in the marsh habitat of the recently formed delta system (WLD) with total carbon content of 5-24 mg C cm-3 compared to the marsh habitat in BLC, which had high soil carbon content of 67-213 mg C cm-3. This suggests that the quantity of soil organic matter might not be a determining factor in CH4 flux. Overall, benthic habitats were found to have the lowest CH4 fluxes indicating that projected future conversions of marshes to open water in this region will impact the total wetland CH4 emission, although the overall contribution of such conversions to the regional and global carbon budgets is still unknown. Further research is needed to expand the CH4 flux studies by simultaneously using several methods across different wetland habitats.

Scavenging of select radionuclides and trace elements by pelagic Sargassum in the Caribbean Sea.

In recent years, the North Atlantic and the Caribbean Sea have experienced unusual and unprecedented pelagic Sargassum blooms, which may adversely affect coastal ecosystems and productive ocean. Sargassum has the potential to scavenge trace elements and radionuclides from seawater, and when bioaccumulated and thus concentrated, can pose a potential threat to higher trophic organisms, including humans that consume impacted seafood. In this study, trace elements and naturally-occurring U/Th-series radionuclides were measured in Sargassum that were collected in the coastal waters of the Caribbean Sea (Antigua/Barbuda, Belize, and Barbados) to better define baseline concentrations and activities, and to assess the scavenging potential for these trace elements and radionuclides. The mean concentration of trace elements observed in Sargassum collected across these three Caribbean Sea are ranked accordingly to the following descending order: Sr > As>Fe > Mn > Zn > Ni > V > C > Cd > Se > Co > Cr > Pb > Ag > Hg. 210-Po and 210Pb activities in Sargassum were observed to be more elevated than previously reported values.

Dissimilatory nitrate reduction to ammonium (DNRA) is marginal relative to denitrification in emerging-eroding wetlands in a subtropical oligohaline and eutrophic coastal delta.

Nitrate (NO3-) and ammonium (NH4+) are reactive nitrogen (Nr) forms that can exacerbate eutrophication in coastal regions. NO3- can be lost to the atmosphere as N2 gas driven by direct denitrification, coupled nitrification-denitrification and annamox or retained within the ecosystems through conversion of NO3- to NH4+ via dissimilatory nitrate reduction to ammonium (DNRA). Denitrification and DNRA are competitive pathways and hence it is critical to evaluate their functional biogeochemical role. However, there is limited information about the environmental factors driving DNRA in oligohaline habitats, especially within deltaic regions where steep salinity gradients define wetland spatiotemporal distribution. Here we use the Isotope Pairing Technique to evaluate the effect of temperature (10, 20, 30 °C) and in situ soil/sediment organic matter (OM%) on total denitrification (Dtotal = direct + coupled nitrification) and DNRA rates in oligohaline forested/marsh wetlands soils and benthic sediment habitats at two sites representing prograding (Wax Lake Delta, WLD) and eroding (Barataria- Lake Cataouatche, BLC) deltaic stages in the Mississippi River Delta Plain (MRDP). Both sites receive MR water with high NO3- (>40 µM) concentrations during the year via river diversions. Denitrification rates were significantly higher (range: 18.0 ± 0.4-113.0 ± 10.6 µmol m-2 h-1) than DNRA rates (range: 0.7 ± 0.2-9.2 ± 0.3 µmol m-2 h-1). Therefore, DNRA represented on average < 10% of the total NO3- reduction (DNRA + Dtotal). Unlike denitrification, DNRA showed no consistent response to temperature. These results indicate that DNRA in wetland soils and benthic sediment is not a major nitrogen transformation in oligohaline regions across the MRDP regardless of wide range of OM% content in these eroding and prograding delta lobes.

An experimental approach to assess the post-depositional mobility of 134Cs.

The partitioning coefficient, Kd, which is defined by the reversible sorption processes between a solid and an aqueous phase at equilibrium, is one of the most important parameters to assess environmental transport and risk. In this study, a series of simple laboratory experiments were conducted to investigate sorption properties of 134Cs on a model sediment under two treatments (shaken vs non-shaken) and with three (small: <75 µm, large: > 75 µm and bulk i.e., composite) particle size fractions. Vertical transport of 134Cs across the water-sediment interface and into sediment was also evaluated. As expected, grain size had the strongest influence on 134Cs Kd values, with the small particle size fraction yielding significantly higher Kd values than the large and bulk fractions. The mean Kd values obtained from the various experiments ranged from 89 ± 13-130 ± 5 L kg-1 (small), 44 ± 10-91 ± 13 L kg-1 (large), 73 ± 3-112 ± 11 L kg-1 (bulk, shaken) and 73 ± 5-110 ± 4 L kg-1 (bulk, non-shaken). Most of the 134Cs partitioning processes occurred rapidly (<2 h) into the experiment. Physical mixing (shaken) did not appear to significantly affect the 134Cs Kd values. In complement, a separate experiment on the vertical penetration of 134Cs into a bulk sediment column showed that 134Cs was able to penetrate up to 5 cm into the sediment column after 88 days (∼0.6 mm d-1) and this flux rate is comparable to natural settings. Adsorption and contact time were found to be key for the 134Cs penetration process. Results from these experiments add to the literature on post-event radionuclide transport studies in marine settings and provide an experimental perspective that can be built upon to complement field observations.

Printed Electrode for Measuring Phosphate in Environmental Water.

Phosphate is a major nonpoint source pollutant in both the Louisiana local streams as well as in the Gulf of Mexico coastal waters. Phosphates from agricultural run-off have contributed to the eutrophication of global surface waters. Phosphate environmental dissemination and eutrophication problems are not yet well understood. Thus, this study aimed to monitor phosphate in the local watershed to help identify potential hot spots in the local community (Mississippi River, Louisiana) that may contribute to nutrient loading downstream (in the Gulf of Mexico). An electrochemical method using a physical vapor deposited cobalt microelectrode was utilized for phosphate detection using cyclic voltammetry and amperometry. The testing results were utilized to evaluate the phosphate distribution in river water and characterize the performance of the microsensor. Various characterizations, including the limit of detection, sensitivity, and reliability, were conducted by measuring the effect of interferences, including dissolved oxygen, pH, and common ions. The electrochemical sensor performance was validated by comparing the results with the standard colorimetry phosphate detection method. X-ray photoelectron spectroscopy (XPS) measurements were performed to understand the phosphate sensing mechanism on the cobalt electrode. This proof-of-concept sensor chip could be utilized for on-field monitoring using a portable, hand-held potentiostat.

The impact of recently excavated dredge pits on coastal hypoxia in the northern Gulf of Mexico shelf.

Large volumes of sand are needed in order to combat coastal land loss due to global sea-level rise for restoration of barrier island systems and beaches undergoing rapid erosion and submergence. The sediment required for such projects often originates from dredging of sand deposits on the adjacent shelf. Two dredge pits, with contrasting geology and located at varying distances from the Mississippi River Delta in the northern Gulf of Mexico shelf were sampled during spring and summer. Samples were also collected concurrently from surrounding continental shelf stations that are subject to seasonal hypoxia every summer. The bottom water dissolved O2 inside the dredge pits were found to be consistently hypoxic or near hypoxic throughout both seasons, with high sediment O2 consumption (SOC) rates of 23.7 to 51.8 mmol m-2 d-1 in spring and 34.3 to 51.3 mmol m-2 d-1 in summer. In contrast, control stations immediately outside the dredge pits showed lower SOC rates ranging between 6.3 and 35.9 mmol m-2 d-1. The SOC rates of the surrounding continental shelf subjected to annual seasonal hypoxia ranged between 25.7 and 59.6 mmol m-2 d-1 indicating that the dredge pits experienced similar high rates of SOC. Our results suggest that sluggish water circulation inside these topographic depressions coupled with higher SOC rates does result in persistent low bottom O2 conditions inside these dredge pits well beyond the duration of the seasonal hypoxia period in this region. This is the first study to provide insight on the impacts of dredge pits to surrounding hypoxia in this region which is critical as future dredging operations are expected to increase worldwide with projected sea-level rise.

A continuous flow membrane bio-reactor releases the feedback inhibition of self-generated free organic carbon on cbb gene transcription of a typical chemoautotrophic bacterium to improve its CO2 fixation efficiency.

Since the free organic carbon (FOC) generated by chemoautotrophic bacterium self has a feedback inhibition effect on its growth and carbon fixation, a continuous flow membrane bio-reactor was designed to remove extracellular FOC (EFOC) and release its inhibition effect. The promotion effect of membrane reactor on growth and carbon fixation of typical chemoautotrophic bacterium and its mechanism were studied. The accumulated apparent carbon fixation yield in membrane reactor was 3.24 times that in the control reactor. The EFOC per unit bacteria and cbb gene transcription level in membrane reactor were about 0.41 times and 11.18 times that in control reactor in late stage, respectively. Membrane reactor separated out EFOC, especially the small molecules, which facilitated the release of intracellular FOC, thereby releasing the inhibition of FOC on cbb gene transcription, thus promoting growth and carbon fixation of the typical chemoautotrophic bacterium. This study lays a foundation for enhancing carbon fixation by chemoautotrophic bacteria and expands the application field of membrane reactor.

Microbial mediated sedimentary phosphorus mobilization in emerging and eroding wetlands of coastal Louisiana.

The interactions between the microbial reduction of Fe (III) oxides and sediment geochemistry are poorly understood and mostly unknown for the Louisiana deltaic plain. This study evaluates the potential of P mobilization for this region during bacterially mediated redox reactions. Samples were collected from two wetland habitats (forested wetland ridge, and marsh) characterized by variations in vegetation structure and elevation in the currently prograding Wax Lake Delta (WLD) and two habitats (wetland marsh, and benthic channel) in degrading Barataria Bay in Lake Cataouatche (BLC). Our results show that PO43- mobilization from WLD and BLC habitats were negligible under aerobic condition. Under anaerobic condition, there is a potential for significant release of PO43- from sediment and wetland soils. PO43- release in sediments spiked with Fe reducing bacteria Shewanella putrefaciens (Sp-CN32) were significantly higher in all cases with respect to a control treatment. In Wax Lake delta, PO43- release from sediment spiked with Sp-CN32 increased significantly from 0.064±0.001 to 1.460±0.005µmolg-1 in the ridge and from 0.079±0.007 to 2.407±0.001µmolg-1 in the marsh substrates. In Barataria bay, PO43- release increased from 0.103±0.006µmolg-1 to 0.601±0.008µmolg-1 in the channel and 0.050±0.000 to 0.618±0.026µmolg-1 in marsh substrates. The PO43- release from sediment slurries spiked with Sp-CN32 was higher in the WLD habitats (marsh 30-fold, ridge 22-fold) compared to the BLC habitats (marsh 12-fold, channel 6-fold). The increase in PO43- release was significantly correlated with the Fe bound PO43- in sediments from different habitats but not with their organic matter content. This study contributes to our understanding of the release mechanism of PO43- during bacterial mediated redox reaction in wetland soils undergoing pulsing sediment deposition and loss.

The Effect of Atrazine on Louisiana Gulf Coast Estuarine Phytoplankton.

Pesticides may enter water bodies in areas with a high proportion of agricultural land use through surface runoff, groundwater discharge, and erosion and thus negatively impact nontarget aquatic organisms. The herbicide atrazine is used extensively throughout the Midwest and enters the Mississippi River through surface runoff and groundwater discharge. The purpose of this study was to determine the extent of atrazine contamination in Louisiana's estuaries from Mississippi River water under different flow and nutrient regimes (spring and summer) and its effect on the biomass and oxygen production of the local phytoplankton community. The results showed that atrazine was consistently present in these systems at low levels. Microcosm experiments exposed to an atrazine-dilution series under low and high nutrient conditions to determine the phytoplankton stress response showed that high atrazine levels greatly decreased phytoplankton biomass and oxygen production. Phytoplankton exposed to low and moderate atrazine levels under high nutrient conditions were able to recover after an extended acclimation period. Communities grown under high nutrient conditions grew more rapidly and produced greater levels of oxygen than the low nutrient treatment groups, thus indicating that atrazine exposure may induce a greater stress response in phytoplankton communities under low-nutrient conditions. The native community also experienced a shift from more sensitive species, such as chlorophytes, to potentially more resilient species such as diatoms. The phytoplankton response to atrazine exposure at various concentrations can be especially important to greater trophic levels because their growth and abundance can determine the potential productivity of the entire ecosystem.

Changing Dynamics of Dissolved Organic Matter Fluorescence in the Northern Gulf of Mexico Following the Deepwater Horizon Oil Spill.

The characteristics of fluorescent components of dissolved organic matter (DOM) were examined using excitation emission matrix (EEM) fluorescence spectroscopy combined with parallel-factor analysis (PARAFAC) for seawater samples obtained from the northern Gulf of Mexico (NGoM) before, during, and after the 2010 Deepwater Horizon (DwH) oil spill. An EEMs PARAFAC modeling of samples collected within 16 km of the wellhead during the oil spill in May 2010, which included one typical subsurface sample with a PAH concentration of 1.09 µg/L, identified two humic-like and two previously reported oil-like components. Compared to prespill levels, however, there were order-of-magnitude higher fluorescence intensities associated with these components that are consistent with an oil-spill source. The spectral decomposition of the EEMs data using individual and combined data sets from coastal and offshore waters impacted by the DwH spill further revealed the changing nature of fluorescent DOM composition. Although the PAHs concentrations were at prespill conditions after the spill in 2012 and 2013 near the DwH site, the variable and anomalous levels of fluorescence intensities and DOC concentrations three years after the spill suggest the potential long-term persistence of the oil in the DOC pool in the NGoM.

(234)Th as a tracer of vertical transport of polycyclic aromatic hydrocarbons in the northern Gulf of Mexico.

Particle-mediated vertical flux of polycyclic aromatic hydrocarbons (PAHs) plays an important role in their removal from upper oceans and sets a limit on the amount delivered to the deep-sea sediments. In this study, we applied a one-dimensional steady-state (234)Th scavenging model to estimate vertical flux of PAHs in the northern Gulf of Mexico and compared them with sediment trap based flux estimates. The (234)Th-based ∑PAH43 fluxes were 6.7±1.0µgm(-2)d(-1) and 3.7±0.6µgm(-2)d(-1) while sediment trap-based fluxes were 4.0±0.6µgm(-2)d(-1) and 4.5±0.7µgm(-2)d(-1) at 150m and 250m, respectively. Alkylated homologues contributed to 80% of the total PAH fluxes which is in contrary to other regions where combustion derived parent PAHs dominate the fluxes. The results indicate that the (238)U-(234)Th disequilibria can be an effective tracer of particulate PAH fluxes in upper mesopelagic zones and can provide flux estimates with high spatial coverage needed to quantify their long term fate and transport in the marine systems.

Distributions and accumulation rates of polycyclic aromatic hydrocarbons in the northern Gulf of Mexico sediments.

Sediment samples collected from shelf, slope and interior basin of the northern Gulf of Mexico during 2011-2013, 1-3 years after the Deepwater Horizon (DWH) oil spill, were utilized to characterize PAH pollution history, in this region. Results indicate that the concentrations of surface ΣPAH43 and their accumulation rates vary between 44 and 160 ng g(-1) and 6-55 ng cm(-2) y(-1), respectively. ΣPAH43 concentration profiles, accumulation rates and Δ(14)C values are significantly altered only for the sediments in the immediate vicinity of the DWH wellhead. This shows that the impact of DWH oil input on deep-sea sediments was generally limited to the area close to the spill site. Further, the PAHs source diagnostic analyses suggest a noticeable change in PAHs composition from higher to lower molecular weight dominance which reflects a change in source of PAHs in the past three years, back to the background composition. Results indicate low to moderate levels of PAH pollution in this region at present, which are unlikely to cause adverse effects on benthic communities.

Improved estimates of India's HIV burden in 2006.

BACKGROUND & OBJECTIVE: HIV estimates in India were based on HIV sentinel surveillance (HSS) data and several assumptions. Expansion of sentinel surveillance to all districts and community based HIV prevalence measured by National Family Health Survey-3 (NFHS-3) in 2006 provided opportunity to replace many of the assumptions with evidence based information and improve the HIV estimate closer to reality. This article presents a detailed account of the methodology used for the 2006 HIV burden estimates for India. METHODS: State-wise adult HIV prevalence among different risk groups observed from HSS 2006 was adjusted for site level variations using a random effects model and for the previous four years the same was back calculated using trend equations derived from a mixed effects logistic regression model based on consistent sites prevalence. The adjusted HIV prevalence among the general population was calibrated to the estimates from NFHS-3. Overall point estimates of adult HIV prevalence in each State for 2002-2006 were derived from the UNAIDS Workbook and projected for the period 1985-2010. The results were put into Spectrum to derive estimates of the number of people living with HIV in all ages and other epidemic impacts. RESULTS: National adult HIV prevalence was 0.36 per cent (range 0.29-0.46%) and the estimated number of people living with HIV was 2.47 million (range 2.0-3.1 million) in 2006. The national adult HIV prevalence remains stable around 0.4 per cent between 2002 and 2006. The States with the highest estimated prevalence were Manipur, Nagaland and Andhra Pradesh. The States with the highest burden were Andhra Pradesh, Maharashtra, Karnataka and Tamil Nadu. INTERPRETATION & CONCLUSION: The improvement in the 2006 estimates of the HIV burden in India is attributable to the expanded sentinel surveillance and representative data from the population-based survey in 2006, combined with an improved analysis. Despite the downward revision, India continues to face a formidable challenge to provide prevention, treatment and care to those in need.

Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean.

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.