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.

Split-beam hydroacoustic sampling with two, 420 KHz, 2x6 degree elliptical transducers was evaluated as a tool to quantify nekton movements in a Louisiana estuary. I measured the effects of atmospheric cold front passages on nekton in a tidal creek in Port Fourchon, LA during the fall of 2000. Six fronts were sampled between October 6 and December 18, 2000. Nekton density ranged from 0 to 24.4 fish/m³ with an overall mean of 2.44 fish/m³. Net samples were taken to supplement the acoustic data with species composition data. Species composition changed from October to December. A higher number of smaller targets were found with the acoustic gear than with the nets, evidence of gear bias. Hydroacoustic based density was generally greater during falling tide (3.17 fish/m³) than rising tide (1.81 fish/m³). Fish Energy, a proxy for biomass, ranged from 1.76*10^-7 units to 2.06*10^-3 units, with an overall mean of 5.96*10^-5 units. Fish Energy was higher on rising tides (6.10*10^-5) than falling tides (5.79*10^-5). Biomass and density were highly variable and similar to previous studies. Barometric pressure combined with tide affected nekton movement. The range of barometric pressures during a sampled front had a significant effect on biomass. When interacted with tidal stage, the range of barometric pressure significantly affected biomass and nekton density (α=0.05). Hydroacoustics were valuable, although sampling during high densities confounded data acquisition. Higher biomass on incoming tides and higher density on outgoing tides suggest an emigration of small nekton during fronts. Thus, judging by total biomass measured over the study, a front may not be as significant a cue in moving estuarine biomass to emigrate as had been hypothesized.

Little is known regarding the feeding behavior of many shark species. Even less is known about shark feeding habits in Louisiana coastal waters. The stomach contents of gillnet-captured blacktip sharks (n=356), Carcharhinus limbatus, and Atlantic sharpnose sharks (n=55), Rhizoprionodon terraenovae, were examined in this study. Roughly half of the blacktip stomachs (52%) and sharpnose stomachs (45%) contained prey items. The primary prey item in terms of percent number, occurrence, and weight for blacktips and sharpnose was the gulf menhaden, Brevoortia patronus. Both blacktips and sharpnose appear to mainly be piscivores; however, no members of Sciaenidae, the most common family of teleosts in the sampling area in terms of number of species, were found in sharpnose stomachs. Based on temporal gillnet sampling, neonate blacktips undergo an increase in feeding activity in the late afternoon /early evening hours, whereas blacktips without an umbilical scar do not appear to follow this same pattern. Using a combination of the stomach content analysis and the derived Scale of Degradation for menhaden found in the stomachs of the blacktips, a digestion rate of approximately 24 hours was estimated for blacktips. Zero and one year old blacktips from this study grew at a rate of approximately 7.6 g/day and 0.47 mm/day. A comparison of the growth rate to the top 10% of stomach content weights yielded growth efficiencies between 13-25% depending on the frequency in which blacktips filled their stomachs with prey. I concluded that menhaden are an important food source for both blacktips and sharpnose in the area, providing the sharks with an abundant and nutritional food source, and directly contributing to the high growth rates for blacktips in the area.

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.