Polonium-210 (t1/2=138 d) is the most common among the 33 known radioisotopes of Polonium found in the natural environment. It is produced by the radioactive decay of its long-lived grandparent Lead-210(t1/2=22.3 d) via Bismuth-210 (t1/2=5.012 d) and forms as part of the natural Uranium-238 decay series. The primary hazard associated with Polonium-210 is its radioactivity, as an alpha particle emitter. Marine organisms receive their maximum radioactive dose in the natural environment from Polonium-210. Polonium has been known to bioaccumulate in the marine food web and can be potentially harmful to humans via the intake of certain marine organisms. Thus it is important to understand the source and sink of Polonium-210 in the marine environment. A number of studies in the past have observed Polonium-210 remobilization from sediments in anoxic lake environments but the release mechanism has never been studied in low oxygen marine systems. On the other hand, the biological affinity of Polonium-210 allows it to be used as an effective tracer of POC export from the upper ocean, although no such study using Polonium-210 as a tracer of POC flux has been carried out in the northern Gulf of Mexico. The present work is aimed at understanding both remobilization of Polonium-210 from sediments in the hypoxia zone of the northern Gulf of Mexico and utilizing Polonium-210 as a tracer of POC export in the northern Gulf of Mexico. Results from our study indicate Polonium-210 was enriched in bottom waters by as much as 50% compared to surface waters from sample stations in the hypoxia zone in 2011 and 2012. No strong correlation between oxygen concentration and unsupported Polonium-210 activity was apparent. However, there was a good correlation between Polonium-210 enrichment and the release of redox sensitive trace metals like Fe and Mn. Due to the affinity of Polonium-210 to Fe and Mn, the cycling of redox sensitive elements such as Fe and Mn and the degradation of organic matter in the water column are likely the driving mechanisms of Polonium-210 remobilization from sediments to the water column under hypoxic conditions. The second study utilized Polonium-210 as a tracer for POC export and was carried out along a north south transect in the Gulf of Mexico beginning near the mouth of the Atchafalaya River and ending on the slope of the continental shelf. POC fluxes were estimated using the Lead-210 Polonium-210 disequilibria in the water column and varied between 10.4 mg C m-2 d-1 and 85.6 mg C m-2 d-1 and showed a general decreasing trend further offshore similar to the decrease in net primary productivity further from shore. The efficiency of the biological pump was found to decrease from 21% at stations close to shore to 4% at stations further offshore, suggesting a transition in POC export efficiency from nutrient rich eutrophic water to nutrient poor oligotrophic water.

This dissertation attempts to better understand astronomical, meteorological, and oceanographic forcing of offshore-spawned, estuarine-dependent species from continental shelf to estuarine waters through tidal passes. The vertical distribution of zoo-/ichthyoplankton within the inner continental shelf from the Louisiana Offshore Oil Platform monitoring project (1978-1995) provided an important offshore end member for the estuarine recruitment study, and is potentially useful in predicting vulnerabilities to past and future oil spills. The role that atmospheric cold front passages may have on densities and movement of zoo-/ichthyoplankton recruiting through the Bayou Tartellan tidal pass, Louisiana, were analyzed using a Generalized Additive Model. The pre-frontal phase, with dominant southern quadrant winds, leading to coastal setup, can enhance flood tides and increase larval recruitment. Post-frontal phase strong northerly winds can enhance ebb tides, which could have negative estuarine retention implications. Lateral differences across Bayou Tartellan at a bulkheaded northern edge, center channel, and a natural-sloping southern shore edge were analyzed using a Zero Inflated Negative Binomial model to determine if behaviorally-mediated, lateral movements by larger larvae could enhance estuarine recruitment/retention. During inflows, estuarine-dependent larvae generally utilized the surface of the center channel and had much lower densities towards the edges. During outflows, larger larvae were more numerous along the southern edge, where velocities were slower. Finally, otolith age and growth data for Micropogonias undulatus and Brevoortia patronus were analyzed for growth rates and microstructure differences associated with oceanographic variability along their recruitment corridors from offshore spawning grounds through the coastal boundary layer, and into the tidal pass. Growth rates from a Laird-Gompertz model for M. undulatus were similar to previous studies. Otolith microstructure suggested ingress through the coastal boundary layer/estuarine waters occurred at approximately 40 days post hatch, and had a marked effect on growth. A two-cycle, Laird-Gompertz growth model for B. patronus suggested a growth stanza at 35 days post hatch, which most likely reflects changing oceanographic conditions during transport and biological consequences of a shift in ontogenetic feeding strategy from selective particulate feeder to an omnivorous filter feeder, with a strong initial growth rate decreasing rapidly after the beginning of the transition in feeding strategy.

On April 20, 2010, the BP Deepwater Horizon (DWH) offshore oil platform exploded releasing ~ 795 million L of southern Louisiana (LA) light sweet crude oil into the Gulf of Mexico. Approximately 7.9 million liters of dispersant, Corexit EC9500A, were applied for remediation. The effect of BP DWH crude oil and Corexit EC9500A on two marsh soil microbial processes, (mineralizable nitrogen and denitrification), were examined in the laboratory. Surface soil samples were collected from an unimpacted salt marsh site proximal to areas that suffered light to heavily oiling in Barataria Bay, LA. Additions of oil were at a ratio of 1:10 (crude oil:wet soil), mimicking a heavy oiling scenario. Carbon and nitrate based compounds were added to investigate the toxicology of oil and dispersant on denitrifier activity. Potential denitrification rates at the 1:10 weathered crude oil:wet soil ratio were 46 ± 18.4% of the control immediately after exposure and 62 ± 8.0% of the control following a two-week pre-incubation period. Potential denitrification rates of soil oiled with fresh crude oil were 51.5 ± 5.3% of the control after immediate exposure and significantly lower at 10.9 ± 1.1% after two-week exposure. Potential denitrification rates (acetylene blockage) after immediate exposure to Corexit:wet soil at ratios of 0:10 (control), 1:10, 1:100, 1:1,000, and 1:10,000 were below detection for the 1:10 treatment while the 1:100 was 7.6 ± 2.7% of the control and the 1:1,000 was 33 ± 4.3% of the control. The 1:10,000 treatment was not significantly different from the control. Denitrification rates measured after two-week pre-incubation were below the detection limit for the 1:10 treatment and the 1:100 treatment was 12 ± 2.6% of the control. Both fresh and weathered crude oil and Corexit can significantly impact activity of denitrification in the short-term. Corexit also negatively affected other microbial measures. Microbial biomass nitrogen (N) values were below detection for the 1:10, 1:100 and 1:1,000 Corexit:wet soil treatments. Potentially mineralizable N rates were significantly lower for 1:10 and 1:100 Corexit:wet soil treatments. Future research should include additional oiling levels and extended exposure periods to determine the recovery of key wetland soil microbial processes.

The first part of this thesis includes evaluating and developing empirical band ratio algorithms for the estimation of colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) for SeaWiFS, MODIS and MERIS ocean color sensors for the northern Gulf of Mexico. For CDOM, matchup comparison between SeaWiFS-derived CDOM absorption coefficients and in situ absorption measurements at 412 nm (aCDOM(412)) were examined using the DSa et al. (2006) and the Mannino et al. (2008) algorithms. These reflectance band ratio algorithms were also assessed to retrieve aCDOM(412) from MODIS and MERIS data using the Rrs(488)/Rrs(555) and Rrs(510)/Rrs(560) band ratios, respectively. Since DOC cannot be measured directly by remote sensors, CDOM as the colored component of DOC is utilized as a proxy to estimate DOC remotely. A seasonal relationship between CDOM and DOC was established for the summer and spring-winter with high correlation for both periods. Seasonal band ratio empirical algorithms to estimate DOC were thus developed. In the second part of this study, a numerical model to study CDOM dynamics in the northern Gulf of Mexico was examined. To derive surface CDOM concentration maps from simulated salinity output from the Navy Coastal Ocean Model (NCOM), a highly correlated linear inverse relationship between CDOM and salinity is required which was examined for both inner-shelf and outer-shelf areas for the spring-winter and the summer periods. Applying these relationships on NCOM simulated salinity resulted in hourly maps of CDOM exhibiting high consistency with CDOM patterns derived from SeaWiFS sensor. Overlaying the NCOM-derived CDOM maps on the simulated currents showed the profound effect of currents on CDOM advection. Cold fronts strongly impact CDOM advection in both the inner and outer shelves by flushing CDOM-laden waters out of the coastal bays.

Since the industrialization of the Haber-Bosch process in the 1940s, anthropogenic activity has nearly doubled the Earths nitrogen fixation. Furthermore, nitrate has become the number one groundwater contaminant in the United States and has harmful effects such as eutrophication, algal blooms, and pollution of drinking water. Soils from two sites influenced by high nitrate loading were examined to determine their biogeochemical integrity. First, the Loosahatchie Bar, located northwest of Memphis, Tennessee, is influenced by excess surface water nitrate loading by the Mississippi River. The Loosahatchie Bar is a newly restored wetland that now has similar hydrologic influence to an upstream control site. The upstream control site and the restored bar sites are both bottomland hardwood forest but exhibit very dissimilar soil properties and microbial functions. Significant differences (P < 0.05) between the control and restored sites were observed for moisture content, bulk density, total carbon, nitrogen, and phosphorus, microbial biomass nitrogen, potentially mineralizable nitrogen, and potential denitrification. Second, the Tallahassee Wastewater Treatment Plant, located just southeast of Tallahassee, Florida, receives high nitrate loads to spray field pivot soils from Tallahassee, Floridas municipal wastewater. Although the intended function of the spray field pivots is to remove excess nitrate from the wastewater, there has been observed eutrophication in Wakulla Springs 17.5 km south of the treatment facility. Soil analysis was conduced to compare the pivot soils with an up gradient control site. Significant differences (P < 0.05) were observed for moisture content, percent organic matter, total carbon, nitrogen, and phosphorus, and denitrifying enzyme activity. Carbon amendment experiments were conducted on the pivot soils with residual biosolids and corn plants. There were no significant differences (P > 0.05) observed for potential denitrification with either carbon amendment. More research should be done to understand water retention at these sites and the microbial communities involved in denitrification.

Understanding the role that habitat plays in the life history of reef-associated fishes is particularly significant given the dramatic increase in the number of artificial reefs deployed in coastal ecosystems over the past 50 years. In the Gulf of Mexico, the oil and gas industry has added a significant amount of structure to the Louisiana continental shelf, creating the largest de facto artificial reef deployment area in the world. Noting their usefulness as fish habitat, the Louisiana Artificial Reef Program was established to convert decommissioned platforms into artificial reefs. However, very little quantitative information exists on how these habitats affect the associated fish assemblage. The two objectives of this study were to examine high-resolution spatial and temporal distribution around two standing and two toppled platforms, and to examine the trophic ecology of common reef-associated fishes, such as red snapper (Lutjanus campechanus). Spatial distribution of fish biomass was examined using a multifrequency hydroacoustic approach to examine the extent of the area of influence around the two habitats and to examine diel changes in distribution. Standing platforms supported roughly two times higher biomass than toppled platforms, particularly in the upper water column at close ranges to the structures. Diel periodicity was evident, with higher biomass in the upper water column during the night and higher biomass in the lower water column during daylight hours. Diel periodicity was dependent on habitat and distance from the reef, breaking down at close range to standing platforms, likely a result of the light field emitted by working platforms. Trophic ecology was assessed with a combination of gut content and stable isotope analyses to examine both prey and sources of basal resources to the reef habitats. Results indicate that red snapper are opportunistic feeders, and that artificial reef structures do not provide a unique set of prey items, indicating that prey and basal resources are consistently sourced from the surrounding water column and soft bottom sediments. Additionally, no evidence of an area of prey depletion (feeding halo) was found around the two habitats, further indicating that prey is derived opportunistically from areas surrounding the reef structures.

The Holocene Mississippi River Delta is arguably the worlds most thoroughly researched deltaic system. However, much of this research has occurred predominantly within the present-day subaerial delta, whereas comparatively little research has been attempted downdip within the offshore, more difficult-to-reach parts. This study advances our understanding of the Holocene delta by examining deltaic sediments within the Trinity-Tiger Shoals Complex region, located ~ 30 40 km offshore of central Louisiana. Specifically, this study addresses two questions. Which delta complex prograded into this offshore region? Do the overlying transgressive deposits reflect that predicted by the prevailing transgressive depositional systems model? To answer these questions, this study uses an integrated data set consisting of ~ 1,350 km of high-resolution subbottom sonar profiles and various sedimentological data gleaned from 60 vibracores, including 22 AMS radiocarbon dates. Results from this study depict an intricate stacking arrangement of 12 distinct depositional units. Near the base of the stratigraphic framework above antecedent topography are the remnants of a middle Holocene Mississippi Delta complex. Prodelta and delta-front facies along with the partially-preserved southwest trending distributary system that fed these depositional environments are recognized. Furthermore, analysis of the regressive component reveals an offsetting, overlapping, and stacked arrangement of four delta lobes and one subdelta. Five AMS radiocarbon dates (~ 4,820 yr BP to 5,980 yr BP) measured from these regressive sediments confirm a Maringouin Delta Complex origin. A ~ 4,000 yr hiatus ensued following the abandonment of the Maringouin Delta Complex, as indicated by 17 AMS radiocarbon dates measured from the overlying transgressive component. During this hiatus, erosional shoreface retreat produced a prominent ravinement surface along the entire top boundary of the Maringouin Delta. It was not until ~ 1,000 yr BP that significant amounts of sediments began migrating into the study area from the east, which have remained, at least for the time being, mostly preserved. Among these recent post-Maringouin depositional units are a detached down-drift subaqueous delta related to the Lafourche Delta Complex as well as the most-recent sandy deposits of Trinity Shoal and an unnamed shoal. This new understanding of post-Maringouin deposition is considered incompatible with the prevailing transgressive depositional systems model, particularly stage 3 of that model.

Simulating animal movement in spatially-explicit individual-based models (IBMs) is both challenging and critically important to accurately estimating population dynamics. I compared four distinct movement approaches or sub-models (restricted-area search, kinesis, event-based, and run and tumble) in a series of simulation experiments. I used an IBM loosely based on a small pelagic fish that simulated growth, mortality, and movement of a cohort on a 2-dimensional grid. First, I tested the sub-models calibrated (i.e., trained) with a genetic algorithm in one set of environmental conditions in three other novel environments. The sub-models performed well, except restricted-area search and event-based that needed to be trained in environments with gradients similar to the test environment. Also, run and tumble only trained in steep habitat quality gradients. The sub-models were then trained and tested across a range of spatio-temporal resolutions (cell size and time step). The sub-models generally performed well across resolutions, but the sub-models did not perform equally well at all resolutions. Kinesis and run and tumble performed better at coarser resolutions, and restricted-area and event-based performed better at finer resolutions. I attributed the trends across resolution to differences in how the habitat quality individuals experienced changed at each time step. Finally, I trained and tested the sub-models in an IBM with dynamic prey and predator fields. I trained and tested the sub-models in dynamic and static versions of the environment. Sub-models trained in the dynamic environment performed well in both dynamic and static test environments; however, sub-models trained in static environment did not perform consistently well in dynamic test environment. Overall, restricted-area search, kinesis, and event-based were robust across the range of conditions in which I tested them, but run and tumble only performed well in environments with very steep habitat quality gradients. In selecting a movement sub-model, researchers should consider the assumptions of potential sub-models, the observed movement patterns of the species of interest, the shape and steepness of the underlying habitat quality gradient, and the spatio-temporal resolution of the model. Sub-models that will be applied in dynamic conditions should be calibrated in comparable dynamic conditions.

The move towards an ecosystem approach to fisheries management requires baseline information on the biotic communities and an understanding of the interactions among species. The two objectives of this study were to describe the demersal fish community (DFC) associated with the northwestern Gulf shrimp trawl fishery, and to utilize a multispecies Lotka-Volterra model to examine possible community level effects of fishing. Community level effects include predator-prey interactions and the responses of fish in the same community to fishing pressure. The summer and fall Southeast Area Monitoring and Assessment Program (SEAMAP) bottomfish trawl surveys were used to identify spatial and temporal indicators in the northwestern Gulf. Cluster analysis and nonmetric multidimensional scaling analysis identified four distinct demersal fish communities (DFCs) from the summer survey data and three distinct spatial DFCs from the fall survey data. Indicator species analysis identified two different dominant communities within each DFC during the summer and fall survey periods. No statistically significant temporal trends were identifiable in the SEAMAP data over the selected time period (1986-2007). However, the community metrics from the DFCs were used to create initial parameters for a multispecies Lotka-Volterra simulation model. The Lotka-Volterra model was developed to study the implications of estimating single-species biological reference points from multispecies data and to determine the effect of fishing on other species in the same community. Three series of simulations were created to explore connectance, rebuilding plans, and multispecies fisheries in the Lotka-Volterra model. Across all simulations, the single-species FMSY rates were estimated between 10-75% of the expected values from the Lotka-Volterra model, indicating that fishing mortality rate buffers are needed to account for predator-prey interactions. No trends were detected as to the magnitude of the buffer for the species examined across all levels of community connectance. The magnitude of biomass changes in unfished species decreased as community connectance increased. The projected time for a species to rebuild was longer in the multispecies model versus the single-species model in four of five case studies. Indirect effects were important in all simulations and these results indicate a need for ecological buffers when determining single-species fishing mortality rates.

Extensive wetland loss and nutrient-enhanced eutrophication occur across the Mississippi River delta and include newly emergent landscapes, in the early stages of ecological succession, and older landscape formations, with fully developed ecological communities. Here I tested how the anthropogenic effects of a climate-induced vegetation shift, an oil spill, and nitrate-enrichment regulate the principal environmental factors controlling nutrient biogeochemistry in wetland soils at different stages of development throughout the Mississippi River delta. In the older, transgressing Barataria basin, there was no clear effect of the climate-induced species shift from Spartina alterniflora Loisel to Avicennia germinans L. on soil nutrient chemistry. Observed soil development patterns were attributed to allochthonous sediment deposition from disturbances rather than autochthonous soil development. Throughout the salt marsh-mangrove ecotone, gross denitrification (mean net N2 flux 81.4 µmol N m-2 h-1) was the dominant N2 pathway and low nitrate concentrations (< 10 µM) likely limited direct denitrification. The oiling of Avicennia and Spartina habitats, during the Deepwater Horizon oil spill, doubled soil organic matter stimulating net N2 production and nitrate/nitrite uptake. In the actively regressing Wax Lake delta (WLD), soil nutrient chemistry exhibited patterns characteristic of primary substrate development; total nitrogen and organic matter increased, while total phosphorus remained relatively constant. Under ambient nitrate concentrations (> 60 µM), gross denitrification dominated the mean net N2 flux (163.2 µmol N m-2 h-1). However, under low nitrate concentrations (< 2 µM), soils switched from net denitrification to net nitrogen fixation. As soils in the WLD aged, the subsequent increase in organic matter stimulated fluxes of N2 and nitrate/nitrite in more mature soils. In conclusion, patterns of soil nitrogen biogeochemistry were linked to the distinct stages of delta formation. Low nitrate availability in the older, transgressive regions limited direct denitrification yielding a net N2 flux dominated by coupled nitrification-denitrification fueled by organic matter mineralization. In contrast, young, regressive regions demonstrated a high capacity for direct denitrification of riverine nitrate that was regulated by substrate age and organic matter accumulation. Throughout the delta cycle, nitrate availability and soil organic matter were the principal factors regulating nitrogen biogeochemistry, and thus the anthropogenic impact of nitrate-enrichment had a marked influence on the observed patterns.