This study compares reproductive biology estimates of female red snapper among three habitat types (natural shelf-edge banks, standing petroleum platforms and toppled petroleum platforms) and among six regions in the Gulf of Mexico (central Florida, northwest Florida, Alabama, Louisiana, north Texas and south Texas). In both studies, batch fecundity and spawning frequency increased with length, weight and age, and batch fecundity was best correlated with maternal length. Gulf-wide, 75% maturity was achieved by age-3 and 100% maturity was reached by age-8. Sizes- and ages-at-maturity differed among habitat types and regions. Females from natural habitat reached 50% maturity the slowest (age-5, 450 mm TL), while fish from toppled platforms reached this benchmark the fastest (age-3, 400 mm TL). Among regions, 100% maturity was reached fastest in north Texas (age-6, 625 mm TL), followed by conspecifics in Alabama (age-6, 675 mm TL), while this benchmark was reached slowest in northwest Florida (age-9, 775 mm TL). Mean batch fecundity was significantly greater in Alabama (283,051 ± 35,761) compared to the other regions. Spawning frequency was significantly greater in north Texas (1.9-3.4 days) compared to the other regions. The differences in reproductive biology estimates among habitat types and regions presented here exemplify the diversity of intraspecific life history traits, which may be influenced by varying ecologies, environments and/or fishing pressures. These findings may supplement managers with important knowledge regarding red snapper vital rates, which may be useful for future management decisions.

There is evidence that significant reductions (about 50%) in surface water nitrate concentrations within coastal deltaic wetlands receiving diverted Mississippi River water can be contributed to denitrification. Yet there is also contrasting evidence that other processes could be responsible for this nitrate reduction. As Louisiana plans the implementation of major Mississippi River sediment diversions, a thorough understanding of nitrogen dynamics is necessary to reduce risks of coastal eutrophication and offshore hypoxia. A mechanistic numerical computer model has been developed to simulate nitrogen biogeochemistry within the wetlands of the prograding Wax Lake Delta. This model is calibrated to observed fluxes within laboratory experiments and validated against observed gradients in field observations, as well as against literature reports of other estuarine systems. Calibration of biogeochemical rate constants to the extremes of their bounds set by literature values, as well as the differences in effective rates exhibited between core incubation simulations and ecosystem simulations, suggests that laboratory experiments alone cannot account for full ecosystem biogeochemistry. Sensitivity analysis showed that, within soil core incubation simulations, nitrification had the greatest influence on nutrient fluxes. Dissimilatory nitrate reduction to ammonium (DNRA) had a similar influence on nitrate flux as denitrification and neither of these processes affected ammonium flux. In ecosystem simulations, denitrification exhibited the largest biogeochemical rate at 50 µmol/m²h, with vegetation uptake, DNRA, and nitrification at 27, 17, and 0.6 µmol/m²h, respectively. Retention efficiency of the study site fluctuated between 4% of loaded nitrogen in December and 16% in May. Temperature was found to have little effect on this efficiency, however loading rates and residence times were found to influence the nitrogen retention efficiency according to the same relationships of other wetland systems. Understanding the observed differences of nitrogen biogeochemistry operating at the laboratory and landscape scales, will aid in the interpretation of measured results. Further, consideration of DNRA as a significant influence on surface water nitrate, and understanding the influences of residence time and nitrogen loading rate, will help in determining the fate of nitrogen in similar systems.

The ¹⁴C method has been used extensively by both limnologists and oceanographers to measure photosynthetic rates in aquatic systems, and the large database of ¹⁴C measurements that now exists is the ground truth with which satellite algorithms for estimating marine photosynthetic rates on a basin and global scale have been calibrated. However, disconcerting uncertainties still remain with respect to whether and under what circumstances the ¹⁴C method provides an estimate of net or gross photosynthesis, or something in between. My study combined batch and continuous culture studies to clarify this ambiguous issue. The batch culture work with seven species of marine phytoplankton indicated that the ¹⁴C method should estimate net photosynthesis for <em>Isochrysis galbana</em> and <em>Dunaliella tertiolecta</em>, gross photosynthesis for <em>Chlorella kessleri</em>, and a rate in between for the remaining four species. Follow-up chemostat studies with <em>I. galbana</em> and <em>C. kessleri</em> grown under both light- and nitrate-limited conditions produced results consistent with the implications of the batch culture work. For <em>I. galbana</em> the photosynthetic rates estimated by ¹⁴C uptake were in good agreement with the actual net fixation, but for <em>C. kessleri</em> the ¹⁴C method overestimated TOC fixation by roughly 50100%, the degree of overestimation depending on incubation length and growth condition. Time-course studies with <em>C. kessleri</em> indicated that at a high nitrate-limited growth rate recently fixed carbon began to enter the respiratory substrate pool after a time interval of about four hours. Results of 12:12 L:D cycle incubations were not as straightforward to interpret as the continuous culture results, but the calculated photosynthetic rates relative to net carbon fixation were clearly a function of species and growth rate. The fact that the specific activity (SA) of organic carbon respired in the dark period was less than the SA of the inorganic carbon in the growth medium implies that carbon respired in the dark was a combination of recently fixed carbon and old carbon. These results imply that in field studies the uptake of ¹⁴C during the photoperiod overestimates net photosynthesis, the degree of overestimation depending on the growth conditions and composition of the phytoplankton community.

Cold seep mussels, Bathymodiolus childressi, are common cold seep constituents that form large populations at upper continental slope (500-1000 m) cold seep sites in the Northern Gulf of Mexico. These mussels utilize methane present through symbiotic relationships with methanotrophic bacteria. This study uses a coupled isotope technique to determine the relative incorporation of respiratory carbon in the shell as a measure of the availability of methane between different seep sites. This method indicates a higher abundance of methane at the Brine Pool site than at the Bush Hill site which appears significantly more resource limited and that changes in methane availability can arise on both decadal and yearly time scales. The method has implications for determining long term methane abundances in both archived samples and disarticulated shells with a relative minimum of additional cost. Additionally, analysis of the means and standard deviations of & #948;15N, & #948;13C, and & #948;34S of mussel soft tissue can provide indications of the presence and variability of those resources across time and space. These analyses indicate the utilization of unique resources, specifically ammonium and thermogenic or biogenic methane, between the two sites. The difference in resources at each site can support further development of unique mixing models for each site that utilize the resources present and not a single blanket analysis using similar resource values for all cold seep sites.

Near Inertial Oscillations (NIOs) are widely observed in ocean current data after severe weather. In this work, we analyzed the NIOs in the Central Gulf of Mexico (GoM) associated with 6 hurricanes/tropical storms in 2005, including Katrina and Rita, based on the deep ocean mooring from Coastal Studies Institute (CSI), LSU. The basic characteristics of the NIOs, such as phase speed, group speed, frequency, energy were discussed. The phase speed ranged from 0.56 cm/s to 2 cm/s above the thermocline for the NIOs during Hurricane Katrina and Rita, while the group velocity varied from 0.047 cm/s above the thermocline to 0.25cm/s below it for the NIOs during Hurricane Karina. The theory of NIO frequency: f<SUB>eff</SUB>=f+ζ/2 were examined with Sea Surface Height (SSH) data, and a correlation between the real and theoretical frequencies was established (R²=0.32). A comparison between the strength of Loop Current (LC) and NIOs induced by severe hurricanes was also made. The result shows that the LC could have comparable instant high energy as severe hurricane induced NIOs, while the impact of a strong LC could last longer.

Coastal ecosystems occupy an interface between land and ocean, making them vulnerable to a variety of natural and anthropogenic disturbances. Large, episodic disturbances (mega-disturbances) cause immediate and long-lasting changes to coastal wetland plant communities and soils by changing the environmental conditions in which they exist. Here I examined the impacts of storm-induced saltwater intrusion and post-intrusion conditions on the structure and growth of an oligohaline wetland plant community, and on wetland soil biogeochemistry and conditions during and after saltwater intrusion. In the greenhouse, a six-week saltwater intrusion reduced canopy cover and species richness. Once intrusion stress was alleviated, plant community structure and growth were heavily influenced by water level during the 20-month recovery period. Plant resilience after subsequent but non-lethal disturbance (clipping) was dependent on the interaction of flooding and salinity, such that canopy cover recovered to pre-clipping condition more slowly under salty, drained conditions. I also found that sustained high water level favored belowground biomass accumulation, high shear strength, and a relatively low decomposition rate in oligohaline wetland soils in the greenhouse. In the field, plant community structure and growth following saltwater intrusion were heavily influenced by the degree of flooding during the recovery period. High flooding depressed canopy cover and species richness, and influenced species dominance. High flooding also resulted in reduced soil conditions in which sulfide accumulated, and in depressed belowground biomass accumulation. Conversely, sediment inputs enhanced wetland recovery from saltwater intrusion by increasing end-of-season aboveground biomass, providing nutrients, and lowering sulfide concentration when flooding was high. Post-intrusion grazing intensity had few impacts on wetland plants and soils during the recovery phase. Soil response variables measured in intrusion-impacted and reference soils before, during, and after a 6-week saltwater intrusion event indicated that although some significant changes in microbial activity, abundance, and nutrient availability occurred due to saltwater intrusion, these impacts were generally transient, with post-intrusion conditions resembling pre-intrusion conditions. In conclusion, storm-induced saltwater intrusion has some long-lasting impacts on oligohaline wetland plant communities, but mostly transient impacts on oligohaline wetland soils. Possibly more importantly, I found that the oligohaline wetland plant community and soil structure and function was determined by post-intrusion environmental conditions. Because oligohaline wetlands provide vital ecological services in many coastal regions, great effort should be put forth to understand both natural and human impacts to these systems. Information gained through research should be applied in a way that encourages the maintenance of healthy, productive, and diverse wetland communities.

Recent research suggests that oyster reefs provide unique three-dimensional hard bottom habitat for many fish species. Along the northern shore of the Gulf of Mexico, oyster shell bottoms are predominantly flat, subtidal and cultched, lacking the vertical relief and spatial heterogeneity provided by natural reefs. This study compared finfish assemblages, gut contents, and macroinvertebrate assemblages at subtidal oyster shell (cultched oyster lease) and mud bottoms in Barataria Bay, Louisiana. Three mud and three shell sites were sampled on seven dates from October 2001 to October 2002, using gill nets and substrate trays. Data from the gill nets were used to compare fish assemblages, as well as to document feeding habits through gut content analysis. Data from the substrate trays were used to document benthic fish and invertebrate communities associated with the two bottom types. Finfish abundance was greater at shell (N = 234) than mud (N = 179) bottoms. Substrate trays collected significantly greater numbers of benthic fishes (p = 0.001) and decapod crustaceans (p = 0.001) at shell bottoms. Gut contents showed predation on fishes, bivalves, and decapod crustaceans. These results show that cultched shell bottoms support a more abundant finfish assemblage than mud bottoms, and provide a potentially important food source for transient fishes due to abundant benthic fishes and decapod crustaceans.

Adenosine triphosphate (ATP) is a unique biochemical indicator of active microbial biomass and its relationship to environmental conditions. Its assay in sediments is complicated, however, by adsorptive loss to the sediment matrix and subsequent interferences in the luciferin-luciferase assay by compounds released during the extraction process. Corrections must be applied to correct for these losses and we describe a novel approach using radioactive ATP to correct for ATP adsorption. The sediment matrix also plays a significant role in determining both the magnitude of the ATP pool and the extent of the ATP adsorptive loss. Coarser sediments were found to have greater ATP levels and little adsorption, whereas silts and clays had significantly lower ATP levels and up to 95% adsorptive loss. Application of the ATP assay to very fine-grained marine sediments in off shore oil producing areas revealed a sedimentary ATP biomass of approximately 40ngg^-1 prior to oil development but after drilling the ATP level dropped 10 to 15 fold to 2 to 3 ngg^-1. Post-drill sediments contained high levels of barium which is associated with drilling fluids, and had no detectable oxygen at a depth of 3mm, and the mean grain size decreased indicating the bottom was being coated over by the drill spoils.

Short-term, wave induced erosion along bay beaches in the northern Gulf of Mexico has been linked to the postfrontal phase of cold front passages. Not until recently has consideration been given to the importance of wave erosion on marshes fringing large bays during the entire cold front event. Two WAVCIS (Wave-Current-Surge-Information System) stations were established on the north and south flank of a small marsh island in Terrebonne Bay, Louisiana, to measure and elucidate the hydrodynamic response to these events. Data from WAVCIS stations, which includes wind speed and direction, air temperature, significant wave height and water level, were collected between June 1999 and January 2002. These data were coupled to measured shoreline change data obtained from a series of north/south repetitious profiles along the length of the island. Four high-resolution topographic surveys were conducted between April and June 1999 and a fifth in December 1999. These surveys were complimented by annual surveys conducted in spring from 2000 on with an additional survey in October 2003. Data obtained from this effort allows the conclusion that low-energy fringing marshes undergo substantial geomorphological change from locally generated high-frequency waves developed by strong winds (12.99-14.14 m/s maximum wind speed) associated with cold fronts. On marsh islands, 80% to 90% percent of erosion occurs during the winter causing the island to thin in space 2.5 m/yr. Erosional patterns observed on the marsh edge include 1) neck and cleft formation, 2) neck cut off and 3) undercutting and marsh toppling. When compared to tropical storms, erosion associated with a season of cold fronts is equal to erosion from one tropical storm. This work underscores the significance of locally generated waves in marsh loss of coastal Louisiana over short time scales (years).

This dissertation focuses on impact on water quality of two freshwater diversions, the Bonnet Carre Spillway and the Caernarvon freshwater diversion, as well as the Atchafalaya River Estuarine Complex. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82-83%, in nitrite+nitrate of 28-42%, in total nitrogen (TN) of 26-30%, and in total phosphorus (TP) of 50-59%. The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge in Lake Pontchartrain. At the Caernarvon diversion, there were reductions in concentrations of dissolved inorganic Si, N, and P of up to 38%, 57% and 23%, respectively. The DSi:DIN ratio rose from 0.9 to 2.6 at the Gulf end member station, while the DIN:DIP ratio fell from 107 to 26. There were decreases in total suspended sediment, large changes in salinity correlated to diversion discharge, and decreased water temperatures associated with riverine discharge. Chlorophyll concentrations near the diversion were low due to light limitation, but increased after suspended sediments decreased below 80 mg L-1, then decreased Gulfward. In the Atchafalaya River estuarine complex, salinity fluctuated seasonally, with the lowest salinities occurring during high river discharge. There was a 41-47% decrease in NO3- concentrations and total suspended sediments decreased as river water flowed through the estuarine complex. In summary, the following conclusions can be made from these studies of the effect on water quality of riverine input into coastal wetlands: (1) riverine input has a pronounced effect on salinity throughout a receiving basin. Whereas nutrients and sediments are actively deposited and/or transformed as riverine derived water passes through an estuarine system, freshwater passes conservatively with no change, diluting higher salinity waters; (2) There is rapid and effective trapping of suspended sediments in estuarine systems due to decreased water velocity and sediment settling; (3) Nitrate concentrations are substantially reduced in the estuarine environment due to the processes of denitrification, assimilation and reduction; (4) dissolved inorganic molar Si:N ratios increase, and N:P ratios decrease as riverine water flows through an estuarine system.