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.

Ecosystems with submersed vegetation are relatively shallow, physically stable and of moderate hydrodynamic energy. Submersed vegetation affect hydrodynamic friction for currents and waves resulting in increased overall frictional loss. Seagrasses shelter the sediment bed and reduce wind-wave resuspension. Bed sheltering factors were estimated from previous flume data on Laguna Madre seagrass species. Data from Laguna Madre indicate that total suspended material levels for bare areas are about an order of magnitude higher than some areas with submersed vegetation. Waves in Laguna Madre at depths less than 2 m were found to be smaller than those expected for the same non-dimensional depths based on studies in slightly deeper waters. Waves were depth-limited and in the transition wave lengths between deep-water and shallow-water waves. A scaling of wave energy and wave period by atmospheric shear stress, rather than the conventional wind speed, was found to improve prediction of wave characteristics. Atmospheric roughness height was related to wave height and age (the ratio of wave celerity to atmospheric friction velocity). Modeling requires process descriptions to be organized and prioritized resulting in model structures which might be different for different aquatic systems. Model formulations were defined with a single grain-class and simultaneous erosion and deposition (type I), and single (type II) and multiple (type III) grain classes with mutually exclusive erosion and deposition. Model formulations were compared. A type I sediment resuspension model was developed for Florida Bay, validated, and coupled to a water quality/ecological model of the system. For Laguna Madre, a two-dimensional depth-averaged type III sediment model was developed to make annual simulations for fixed seagrass characteristics, with and without dredged material disposal. Dredged material disposal involves more sediment than the total natural sediment input to this system, and near-field deposit areas expose an appreciable sediment source to possible resuspension. Measurements near a dredge-pipeline discharge indicated that a highly-stratified fluid-mud underflow slowly moved material hundreds of meters downslope as sediment deposited. Underflow layer-averaged concentration did not change much with distance from the discharge.

The Bab el Mandab Strait is where the transition occurs between two noticeably different tidal regimes: the Gulf of Aden, where tidal fluctuations are mixed and have a range in excess of 2 m, and the Red Sea, where the tides are principally semidiurnal and their range is less than 1 m. Within the Strait, observations collected between May of 1995 and July of 1997 indicate that tidal currents are a mixed type and dominant constituents are the K₁ and M₂. The vertical structure of the tidal currents is complicated, differs between semidiurnal and diurnal constituents, and depends on the location and stratification. In addition, the stratification impacts more the vertical distribution of the diurnal tidal currents. The major part of this signal is barotropic but energetic baroclinic currents are observed near Perim Narrows and the Hanish Sill during the winter stratification. Results of the EOF analysis and internal velocity modes estimated from observed density profiles indicate that the baroclinic currents of diurnal frequency have vertical structure, which is a combination of the second and third baroclinic modes near Perim Narrows and of the first and second modes near the Hanish Sill. To examine in more details the barotropic tides in the Strait, a two-dimensional finite element model (ADCIRC-2DDI) was implemented. Results from this model indicate that the largest elevations and most energetic currents are observed in the region located between Perim Narrows and Assab-Mocha line. They also imply that residual circulation induced by the barotropic tides is generally weak in the Strait and consists of mean and fluctuating components. Average barotropic energy fluxes over a tidal period are small and their direction depends on the constituent. The K₁ component has one source of energy, which is the flux from the Gulf of Aden, while there are two sources of energy for the M₂: one from the Gulf of Aden and another from the Red Sea. In addition, these results show that the major part of the tidal energy for both constituents is dissipated within the Strait itself.

Wetland groundwater hydrology was investigated at different spatial scales to assess the usefulness of the information to coastal managers. Specific objectives were to: (1) review studies related to coastal groundwater discharge, evaluating techniques and identifying controls; (2) understand regional trends in groundwater flow along the U.S. East and Gulf coasts; (3) evaluate the applicability of naturally occurring radioisotopes as indicators of groundwater in a shallow deltaic system; and (4) evaluate groundwater-surface water exchange within Barataria Basin, Louisiana. Results of the review confirmed discharge estimates vary over several orders of magnitude, due to differences in precipitation and tidal prisms. In addition, in very few areas such as the Sippewissett Marshes, Massachusetts, groundwater information was extensive enough for management planning. A regional water budget study performed for 29 watersheds along the East and Gulf of Mexico indicate a likely insignificant net annual (30-year average) export of groundwater in the northeast (17 cm). However, a net import was found in the southeast (10 cm), and eastern Gulf coast (14 cm). The mid-Atlantic and western Gulf showed no net groundwater flow. In Barataria Basin estuary, ²²²Rn increased exponentially from the mouth to 120 km upstream. Significant excess ²²²Rn activities suggest that an additional source is required to balance the geochemical budget, such as groundwater. Radium-226 activities demonstrated non-conservative mixing, and appear to indicate an additional source at intermediate salinities, likely desorption of ²²⁶Ra from suspended and bottom sediments. Further evaluation of groundwater in Barataria was performed using more in depth experiments with radioisotopes. Tracer mass balance estimates of SGD at three sites produced a range in SGD flux of 1.6 to 9.6 cm/d, with the highest groundwater flux at Kenta Canal. A comparison of results from the water budget and mass balance for Barataria Basin confirm that the water budget was the lower estimate of SGD. Coastal managers can utilize SGD information in planning as better estimates and consistent techniques become available. In Barataria Basin, a further study of stratigraphy, groundwater flows, and SGD-derived nutrients is necessary if Louisianas coastal planners are to fully understand the hydrology and resulting impacts on its wetlands.

Denitrification potential was determined in surface sediment from Lake Cataouatche, the receiving basin for a future Mississippi River diversion located in the northern portion of the Barataria Basin estuary. Nitrate removal and denitrification was measured in the laboratory using static sediment microcosms flooded with lake water. Dissolved potassium nitrate (KNO₃) was added to the microcosms to achieve: 1) an initial nitrate concentration similar to the mean Mississippi River concentration (~1.4 mg NO₃-N l^-1), and, 2) a high initial nitrate concentration to elicit a denitrification potential (~50 mg NO₃-N l^-1). Denitrification was determined by the acetylene inhibition technique. The denitrification potential during the most active period (day 3-10 ) of nitrate removal in September, December, and March studies ranged from 37 to 55, 29 to 60, and 34 to 111 mg N m^-2 d^-1, respectively. A mean denitrification potential of 49±17 mg N m^-2 d^-1 was estimated from the three sampling periods. The denitrification potential was 16 times greater than estimates of denitrification under nitrate concentrations similar to the Mississippi River (3.0±1.2 mg N m^-2 d^-1), which ranged from 1.7 to 4.1, 0.89 to 3.8, and 2.3 to 4.5 mg N m^-2 d^-1 in September, December, and March studies, respectively. Mean nitrate removal from the 50 mg NO₃-N l^-1 addition to the microcosm floodwater was estimated at 177±25 mg N m^-2 d^-1 (with 6.3-18.5% error using an assigned estimate of floodwater volume). However the quality of data was poor resulting from not measuring initial floodwater volumes and evaporation. The nitrate removal rate estimate is qualitative and may vary as much as 100% at the high nitrate addition rate (see Table 4). At the low nitrate addition (1.4 mg NO₃-N l^-1) it was difficult to assign a removal rate since many measurements were below the analytical method detection limit used. Results demonstrated that Lake Cataouatche sediment has a large capacity to remove nitrate from the water column, and also suggest that denitrification could remove a significant portion of the nitrate inputs from the Davis Pond diversion.

In order to understand the physical processes in the Barataria estuary, a previously developed two-dimensional, depth-integrated hydrodynamic model is applied to simulate estuarine processes. The equations for conservation of mass and momentum predict specific physical processes when forced by tidal and salinity variations at the open boundaries, wind forcing patterns, precipitation and evaporation over the model domain, and freshwater runoff as point sources. This study is focused on examining the impact of freshwater dispersion from the freshwater sources. Thus, a hydrologic model was developed to estimate runoff. Furthermore grid size was reduced, a new advective code added, and baroclinic effects included. The model was run with and without freshwater diversions from the Mississippi River. When compared to observations the correlation coefficients ( ) of model water levels are larger than 0.9 at all but one station. For the hydrologic calibration, a big flood event was tested. Agreement between observed and model results with runoff is surprisingly good. The observed agreement provides a justification for adopting 100 % coupling between the hydrologic model and hydrodynamic model, at least for this flood event. In terms of water level within the basin, the freshwater diversions seem to affect most of the Barataria Basin system water level within 3 days. Using a Hovmüller diagram, tidal phase speed was estimated within the basin as 16 hours to travel from the mouth of Barataria Bay to the top of the basin, Lac des Allemands. In terms of salinity within the basin, the impact of freshwater diversion reaches Barataria Bay within 5 and 10 days from Naomi and West Pointe à la Hache, respectively, and the Gulf of Mexico 15 to 20 days after freshwater release.