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.

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.

Blue runner (Caranx crysos), common around offshore petroleum platforms in the Gulf of Mexico, were found to forage extensively on meso- and macro-zooplankton during the summer months. Larval decapods and stomatopods, hyperiid amphipods, pteropods, and larval and juvenile fishes were common components of their diet. Feeding periodicity results suggest that blue runner around platforms were capable of feeding throughout the night at the same intensity as the day. Size selectivity indicated that larger prey, such as fish were consumed during the night and smaller decapod crustaceans were consumed during the day. Blue runner collected from open waters away from platforms also consumed decapods and larval fish as well as other abundant zooplankton such as chaetognaths. Although certain zooplankton taxa exhibited increased densities near the platform during some cruises, plankton net sampling did not demonstrate consistently elevated densities of zooplankton in proximity to petroleum platforms. Zooplankton density was generally greater during the night samples, however a platform enhancement effect was not observed. Results from ADCP surveys suggested that zooplankton and ichthyoplankton contributed to the measured volume backscattering strength. Diel and between-cruise changes in zooplankton density were correlated with changes in acoustic scattering levels during acoustic surveys conducted around the platforms. These surveys demonstrated the utility of an ADCP to examine small-scale patterns in sound scattering layers in surface waters around platforms, however preliminary analysis did not establish that elevated (or reduced) levels of scattering existed near platforms. Examination of more surveys along with including data on current velocity will provide further insight into the localized effects of these structures. The net and acoustic data collected in this study suggested that more intensive sampling closer to platforms is needed to understand how these structures support large aggregations of fishes such as blue runner.

Vegetative response to saltwater intrusion into coastal freshwater wetlands is governed by the combined effects of increased salinity and porewater sulfide concentrations. I conducted a series of experiments to address the primary hypothesis that growth of Panicum hemitomon is adversely affected by an interaction between salinity and sulfide stresses associated with saltwater intrusion, and the mechanisms for decreased growth are alterations in the metabolic and morphological adaptations needed for a plant to survive in a flooded environment. I exposed marsh sods to a factorial treatment arrangement of three salinities (0, 2, and 4 ppt) and three porewater sulfide concentrations (0, 0.5 and 1 mM) for 19 and 39 weeks. While salinity and sulfide both decreased relative growth rates in P. hemitomon, the salinity-induced growth inhibitions were more severe, particularly with regards to the belowground tissue. Additionally, there was a sulfide-induced stimulus in the production of adventitious tissue that was completely inhibited by elevated porewater salinities. After 19 weeks, salinity at 4 ppt and elevated sulfide concentrations were deleterious to overall plant growth. A sulfide-induced growth stimulation in adventitious root production was inhibited at elevated salinities. After 39 weeks, elevated salinity at all concentrations was so stressful that the long-term effects of sulfide became inconsequential. Root respiration under anaerobic conditions was higher under elevated sulfide, but this stimulation was also eliminated at higher salinity. A 12-week hydroponic exposure to elevated salinity and sulfide showed opposite effects of stressor treatment, with salinity stimulating and sulfide inhibiting root ethanol production. A 3-month field experiment intended to validate the growth chamber experiments supported the sensitivity of P. hemitomon belowground tissue to saltwater flooding, and potential reductions in the capacity to form aerenchymatous tissue for root tip aeration. I concluded from these data that the loss of Panicum hemitomon from the fresh marshes of coastal Louisiana is caused by both reduced growth and a reduced ability to adapt metabolically and morphologically to the highly-reduced edaphic conditions of a saltwater-flooded marsh.

I examined patterns and timing of ontogeny and relative growth in five species of blenny (Teleostei: Blenniidae) from the northern Gulf of Mexico by assigning a suite of discrete character state scores to ontogenetic events (10 external traits; 218 total specimens). This is the first study to evaluate developmental patterns in reef-associated fishes relative to the timing of metamorphosis and settlement by applying scaling techniques and statistical methods to quantify, differentiate, and select criteria for defining intervals of development across taxa. Blennies settle at a common state of ontogeny and share a common pattern of body and fin/cirrus growth. Three 'natural' intervals of development (labeled 'larvae', 'metamorphs', and 'settlers') were consistently identified based on scoring and summing character states, and cluster analysis. Shape differences separate larvae from metamorphs, but not metamorphs from recent settlers. The common growth pattern consists of a general deepening of the head and abdomen, a narrowing of the interorbital region, and elongation of the pectoral and pelvic fins. These changes during metamorphosis produce the common shape and basic adult body form at settlement. Differences in shape show little relationship to phylogenetic distance. Estuarine blennies settle at a smaller size but similar state of ontogeny as coastal/shelf species, which suggests the timing, rate, and state of ontogeny at important periods of ecological transition, may influence survival. The smaller size at settlement in estuarine blennies is consistent with natural selection emphasizing rapid ontogeny in species or areas where competition for available habitat or resources is great. Differences in fin and body pigmentation patterns and in the number of teeth between estuarine and coastal/shelf blennies suggest that development reflects adaptive convergence to similar ecological niches and habitats, rather than revealing any evolutionary relationship. In blennies, ontogeny progresses gradually and continuously rather than in a stepwise fashion, as postulated by saltatory theory. Differential growth rates of individual body parts provide a similar conclusion. Variability in the timing and magnitude of ontogeny make recognizing proposed thresholds between 'steps' difficult, if not impossible. Blennies are not juveniles at settlement as commonly accepted for many other demersal and reef-associated species.