Kanchan Maiti

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 Hawai`i. 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.

Insights into particle formation and remineralization using the short‐lived radionuclide, Thoruim‐234

Simple mass balance models are applied to a high resolution 234 Th profile from the northwest Pacific to examine the magnitude, rate, and depth distribution of particle remineralization processes below the euphotic zone (E z ). Here, excess 234 Th ( 234 Th > 238 U) below the E z is attributed to fragmentation processes that result in the conversion of sinking to non-sinking particles. By considering particulate organic carbon (POC) to 234 Th ratios on particles, we show that POC flux attenuation is larger than for 234 Th, which we attribute to bacterial and zooplankton consumption of sinking POC. Three case studies are used to demonstrate how different combinations of particle fragmentation and POC respiration impact flux attenuation below the E z . When sampled with high vertical resolution and precision, 234 Th and POC/ 234 Th ratios provide insights into both export from the E z and the extent to which sinking particle fluxes and associated minerals are attenuated with depth.

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.

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.

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.

Release and Sorption Pattern of Monosilicic Acid from Silicon Fertilizers in Different Soils of Louisiana: A Laboratory Incubation Study

ABSTRACT The amount of monosilicic acid (H4SiO4), which is the plant-available form of silicon (Si), released from fertilizers can be influenced by Si source and soil properties. A series of laboratory experiments were conducted using six soil series from Louisiana to document the differences in the release characteristics of H4SiO4 from wollastonite and slag. Monosilicic acid in solution released from slag declined with time while wollastonite consistently increased its concentration across all soil suspensions well above 40 µg mL−1. Among these soil series, soils high in organic matter and clay were seen to have maximum percent sorption (up to 79%) with minimum polymerization of H4SiO4. The presence of ions like aluminum (Al) and magnesium (Mg) enhanced the process of H4SiO4 polymerization, which led to a decrease in H4SiO4 concentration in solution. Evident relationships were observed between H4SiO4 concentration in solution with added Si sources and sorbed quantity from soil solutions.

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.