Nutrient dynamics and seasonal marsh growth were examined in a newly formed Panicum virgatum floating marsh at Thibodaux LA. The floating marsh formed in a cleared area of forested wetland receiving secondarily treated effluent. Net Areal Primary Productivity (NAPP), total belowground biomass, NO₃, and δ¹⁵N ratios varied significantly (P <0.05) along a 75m marsh transect while mean δ¹³C varied between plant species. The upland end of the transect had the highest NAPP (3876 g m^-2y^-1), total belowground biomass (4079.0 ± 298.5 g m^-2), and mean NO₃ (5.4 ± 2.9 mg l^-1). The mean floating-marsh δ¹⁵N of H. umbellata was less enriched at 0-75 m (9.69 ± 1.9) compared to 100-200 m (20.99 ± 3.8). The δ¹³C of the belowground peat mat of the floating marsh was similar to P. virgatum but not H. umbellata, indicating that P. virgatum was forming the mat. There was a significant decrease in NAPP, total belowground biomass, NO₃ and δ¹⁵N enrichment across the 75 m transect. Nutrient availability affected NAPP and δ¹⁵N. Floating marsh NAPP in the 0-45 m was greater than most reported values for floating marsh. These results suggest that nutreint rich freshwater can promote restoration of floating marshes.

Land-building sediment diversions, such as the Mississippi River Diversion at West Bay, can be used as important building blocks in the conservation and restoration of Mississippi Delta wetlands. Sediment deposition and retention patterns were studied in the West Bay diversion outfall area through analyses of sediment cores, hydrodynamics, and bathymetry. Cores and x-ray samples were collected at twenty stations in March 2004, August 2004, November 2005, and April 2006. Cores were analyzed for grain-size distributions and ⁷Be activities, in order to examine patterns of recent sediment deposition. In comparing ratios of total ⁷Be inventory to atmospheric ⁷Be deposition rates, it was estimated that 10%, 60%, and 30% of the cores experienced net accretion, net erosion, and no deposition, respectively. Turbidity and velocity data collected during instrument deployments along with historical Mississippi River flows and sediment loads were used to estimate an average sediment influx of 2.9 x 10⁶ tons/year (2.6 x 10⁹ kg/year). A sediment trapping efficiency of 25 - 50% was estimated by comparing sediment influx with ⁷Be penetration depths observed. This estimation could not be verified by bathymetric surveys conducted in April 2006 and compared to pre-construction surveys from 2003. Volumetric change calculations estimated a loss of over 4.16 x 10⁶ yd³ (3.18 x 10⁶ m³) of sediment from the bay bottom. It is speculated that these losses were caused by Hurricane Katrina which made landfall on August 29, 2005 approximately 15 nautical miles (28 km) from the study area. Potential deltaic growth rates of between 51 ac/year (21 ha/year) and 143 ac/year (59 ha/year) were estimated for the diversion by comparing delta growth parameters estimated in this study with the Wax Lake Delta. These estimates suggest that delta-building processes at West Bay may continue following a typical subdelta growth curve of 100 - 150 years, meaning peak development of deltaic wetlands could be decades away. Therefore, future planning for diversions should consider coastal process which could potentially counter environmental benefits and engineering strategies should place as much focus on receiving area configuration and trapping efficiency as sediment delivery in order to maximize sediment retention.

Although the Earth may be in the early stages of the projected trends of global warming, several ecological responses to recent warming are already evident. The timing of seasonal activities of plants and animals has begun to shift, and the advancement of species poleward is occurring in several regions of the world. One unique distributional shift is the movement of Avicennia germinans (black mangrove) northward into temperate salt marshes dominated by the marsh grass, Spartina alterniflora. In Louisiana, black mangroves were historically restricted to the southernmost barrier islands and beaches; however, in recent years a noticeable expansion of Avicennia northward into Spartina marshes has been observed. To date, no research documents the effects mangroves may have on ecosystem processes within salt marshes. The goal of this research was lessen this gap in scientific knowledge by examining the effects of expanding populations of Avicennia on the ecosystem processes of accretion, organic matter production and decomposition, and carbon assimilation. Study results indicate that, to date, Avicennia expansion has had no major effects on essential ecosystem processes within the salt marsh. No differences were detected in accretion rates, using sediment trap, feldspar, or 137Cs techniques, decomposition rates, belowground production, or carbon assimilation rates between Avicennia and Spartina areas. However, edaphic parameters such as elevation, redox potentials, bulk density, and soil ammonium were slightly higher, while soil moisture and porewater salinity were somewhat lower where Avicennia expanded into the surrounding Spartina salt marsh. Also, differences were detected in the relative rates at which mangrove and salt marsh tissues decomposed. Decomposition of Avicennia leaves (0.63-1.0% loss d-1) and roots (0.055-0.30% loss d-1) was quicker than that of Spartina leaves (0.52-0.83 % loss d-1) and roots (0.074-0.25% loss d-1). Moreover, root biomass belowground degraded quite slowly, with 50-60% of roots remaining after 384 days. In contrast to quick leaf degradation and the export of leaf litter, slow root degradation and retention within the soil suggest that roots have more potential to contribute organic matter to this Louisiana salt marsh, particularly Spartina roots which degraded even more slowly than Avicennia roots.

Increased flooding, nutrient and sediment deprivation, and saltwater intrusion have been implicated as probable causes of coastal swamp deterioration in the Mississippi Delta. An understanding of the interactive effects of these factors is required to enable successful planning of wetland restoration activities. I used field data collected from 2000 till 2005 at forty study sites to characterize the baseline conditions of the Maurepas swamp. I used a cluster analysis to identify four swamp habitat clusters, and characterized the clusters on the basis of soil properties, salinity, basal area, stem density, and other tree-related variables. ANOVA and related statistical techniques showed that three of the four habitat clusters exhibited tree biomass and densities indicative of flooding stress, and one cluster showed high tree mortality in response to salt-water intrusion. I then developed a two-species individual-based forest succession model (IBM) of a coastal swamp. The IBM followed the weekly growth, mortality, and reproduction of individuals of Taxodium distichum and Nyssa aquatica trees in a 1-km2 spatial grid, using historical time-series of stage and salinity data as inputs. IBM simulations predicted that increased flooding leads to swamps with reduced basal areas and stem densities, while increased salinity (~1-3 psu) resulted in lower basal areas. The IBM showed a tendency to overestimate wood production and the dominance of T. distichum in comparison to field data. Lastly, I compared the predictions of the IBM and a widely-used landscape model. I used salinity and flooding conditions simulated by the landscape model in eight of its 1-km2 cells as input to the IBM, and compared both models’ predictions of habitat change over 100 years. The models showed good agreement in their predictions of marsh persistence and swamp to marsh conversion. The IBM, however, showed higher sensitivity to changes in both salinity and flooding than the landscape model, and never predicted swamp persistence. The next generation of models for forecasting coastal habitat change in the Mississippi Delta will likely be a combination of the individual-based and landscape models used in this dissertation.

A comprehensive and integrated approach involving sedimentology, shallow surface geophysics and radio-chemistry was used to understand lakebed sediment dynamics in Little Lake. This methodology attempted to (1) define the morphology and origin of the lake, (2) understand the variability in lake-bottom sediments, (3) assess short-term and long-term sediment accumulation rates, and (4) image lakebed features. Subbottom chirp, single beam echo sounder, and side scan sonar data were collected to define hydrographic depths, lateral variability in seabed sediment type, lakebed features, and shallow subsurface structure. Sediment samples were taken at representative locations throughout the lake and particle size distributions were determined. Radionuclide dating was performed on selected samples to gain an understanding of lake sediment accumulation rates. The results indicate the lake formed as a consequence of subsidence and the amalgamation of four sub-basins to form the current lake extent. The distribution of surface sediment is controlled by basin morphology and in situ relic channel/levee deposits. The surface sediment in Little Lake is organic rich clay near tidal channels (from erosion of tidal channel banks), silt near the perimeter of the lake (winnowing by shallow wave action), and sand in the center of the lake (erosion of relic channels). The shallow surface sediments showed three discernable accumulation layers defined as (1) short-term, < 200 days, Be-7 in the upper few centimeters with a potential riverine source, (2) event deposition, days to weeks, constant excess Pb-210 activity up to 10 cm thick interpreted as a storm deposit (Isidore and Lili, 2002) and (3) long-term, 150 years, excess Pb 210 with classic decay, 1-5 mm/year accumulation rates. Identified lakebed features include bottom scars, marine pipelines, channels and shell beds. Bottom scars cover approximately 25% of the lake bottom and are preserved in silts and clays whereas sands are mostly devoid of recognizable scars. This thesis is the first comprehensive study of a coastal lake/bay and represents a baseline dataset for future studies trying to understand the affects of the Davis Pond River Diversion restoration project on lakebed sediment dynamics. Data indicate that the Davis Pond River Diversion has not significantly affected Little Lake.

Coastal hypoxia (< 2 mg O2L-1) represents a global problem that continues to worsen as nutrient fluxes to these areas increase. The second largest zone of human-induced hypoxia is located on the Louisiana continental shelf where hypoxic bottom waters commonly occur during summertime. This region is strongly impacted by the large flux of freshwater and nutrients from the Mississippi River, which influences both biological and physical processes that control oxygen dynamics. Yet, based on oxygen concentration measurements alone, it is difficult to separate the effects of biological factors from physical factors. To address this problem, I used a dual budget approach to assess the importance of oxygen sources and sinks on the Louisiana continental shelf. The dual budget was based on using stable oxygen isotopes (ä18O) in combination with conventional oxygen concentration measurements. To analyze temporal trends, surface and bottom water samples were collected monthly between July 2001 and July 2003 along an onshore-offshore transect. For better spatial resolution, shelfwide sampling was conducted extending from the Mississippi River Delta to the Louisiana-Texas border in the month of July of 2001, 2002, and 2003. Oxygen saturations values ranged between 180% at the surface and almost 0% close to the bottom with a corresponding range of ä18O values from 15 to 50. Biological parameters were important during all seasons for surface oxygen dynamics. The effects of physical factors were less apparent, except during severe physical disturbances. Bottom water oxygen dynamics showed clear seasonal signals of high oxygen depletion and larger contributions of benthic respiration during the summer, which corresponded to the strong stratification of the water column. In bottom waters, summer oxygen depletion was predominantly due to benthic respiration, accounting for about 73%, 80% and 60% of the total oxygen loss for 2001, 2002 and 2003 respectively. Model estimates of production/respiration (P/R) ratio during the July shelfwide cruises indicated that surface waters were productive with an average calculated P/R above 1. Depth stratified sampling (5 m intervals), which started in July 2002, showed that productivity in the mixed layer (5 to 10 m) was not homogeneous. Calculated P/R exceeded 1 only in the surface layer, while at 5 m P/R was approximately 1 and at a depth of 10 m, P/R was less than 1. Additionally, hypoxic conditions were only detected within 5 m of the bottom sediments. The dual budget approach yielded new estimates of productivity dynamics in surface waters and of sediment oxygen demand in bottom waters. For the first time, this study provided routine insight into productivity and respiration dynamics over large temporal and spatial scales. This could not have been accomplished using traditional methods because they commonly rely on time-consuming incubations. The study has shown that respiration dynamics in bottom waters vary seasonally with higher contribution of benthic respiration during stratified summer conditions and prevalent water column respiration during fall and winter. In contrast, seasonality in surface waters was less pronounced as productivity was more dependent on (salinity-inferred) nutrient supply than climatic forcing.

Recently we have begun to understand the importance of inshore hard-bottom substrate, including oyster reefs, to estuarine fish communities in the Gulf of Mexico, especially in the context of identifying Essential Fish Habitat (EFH). However, problems such as habitat loss, disease, overharvest, and failure to replace shell have severely decreased the amount of high-relief oyster reef habitat available to finfish. The purpose of this project was to establish an artificial high-relief mimic-oyster reef in Barataria Bay, Louisiana, and monitor its use by economically and ecologically important finfish, including spotted seatrout (Cynoscion nebulosus) and Atlantic croaker (Micropogonias undulatus). The finfish and invertebrate communities over the artificial reef site were compared to a mud bottom reference site, using two different gear types to sample the entire water column. I also examined site-specific trophic linkages by enumerating gut contents and performing stable isotope analyses of spotted seatrout, Atlantic croaker and bay anchovy (Anchoa mitchilli). There was no overall difference in the community structure between sites, though there was a seasonal difference in the numbers of individuals found at both sites. Species richness also varied by season, with highest number of species present in summer months. Results of the gut-content analysis showed that diets of spotted seatrout do not differ significantly between sites. Spotted seatrout consumed mostly fish and anchovies by number, and penaeid shrimp by weight. Stable isotope analysis indicated that while there were no overall differences in mean stable isotope values, the dietary breadth of spotted seatrout was greater over the artificial reef. In contrast, results indicated that there were significant differences in the diets Atlantic croaker between sites. Atlantic croaker diets consisted of mud crabs (Xanthidae) and other, unidentifiable crabs over the reef, and bivalves and fish over the reference site. Stable isotope analysis of Atlantic croaker indicates that overall dietary breadth was similar between sites, though 15N values were significantly higher over the artificial reef. Results of the stable isotope analysis for bay anchovy indicate that there was a greater dietary breath over the artificial reef.

The Davis Pond freshwater diversion discharges nutrient-rich Mississippi River water to a 3,760 ha receiving marsh in upper Barataria Basin, LA. Excess nitrate in the Mississippi River has been linked to algal blooms and hypoxia in the Gulf of Mexico with potential to negatively impact Barataria Basin. We hypothesized that 1) soil denitrification enzyme activity (DEA) will increase with higher surface water nitrate concentrations, and 2) the spatial distribution of DEA in Davis Pond marsh will provide information about the extent nitrate loading at a specific discharge rate. Intact soil cores collected from the marsh received a continuous flow of nitrate solution (0.0, 0.5, 1.0, or 2.0 mg NO3-N l-1) for a period of 7, 20, or 45 days. Overall, DEA for the 1.0 mg NO3-N l-1 was significantly higher than the control treatment (P < 0.05). A strong positive correlation between DEA and surface water nitrate in the 0-5 cm (P < 0.05) and 5-10 cm (P < 0.001) soil horizons was observed on day 20. However, the correlation between DEA and nitrate was not significant on days 7 and 45. Measureable DEA was observed in the 0.0 mg NO3-N l-1 on all days, indicating the contribution of internal biochemical N cycling to DEA in organic wetland soils. Approximately 92% of all DEA was observed in the top 5 cm of soil, 7% occurred at 5-10 cm, and <1% below 10 cm. DEA was also quantified for 88 randomly distributed soil cores in Davis Pond marsh collected May - July, 2007. At a mean discharge rate of 39 m3 s-1, high rates of DEA (0.41 to 2.10 mg N2O-N kg-1 h-1) occurred in a 715 ha area proximal to the diversion inflow, while background rates (0 to 0.30 mg N2O-N kg-1 h-1) were observed outside this area. The 715 ha area contained > 80% of all the DEA observed in Davis Pond marsh, yet encompassed only 19% of the total marsh area. The area of elevated DEA included the highest observed surface water nitrate concentrations, suggesting DEA is a potential indicator of nitrate loading.

The influence of hydrology on the mangrove forests of Vietnam has received little scientific attention, even though hydrology is recognized as the primary forcing function in mangrove ecosystems worldwide. The purpose of this dissertation research was to determine the effects of hydrology on specific structural attributes and functional processes within the mangrove forests of Can Gio, a province in southeastern Vietnam. Khe Vinh (KV) and Mui O (MO), two locations within compartment 17 of the Can Gio Mangrove Biosphere Reserve, were chosen as study sites. This research addressed two questions: (1) What are the characteristics of the hydrological regime at the Can Gio mangrove forest? and (2) How does the hydrological regime in the Can Gio mangrove forest affect soil properties, sedimentation, litter decomposition, primary production and species distribution. Tidal effects of the China Sea and the Saigon and Dong Nai Rivers affected the hydrological regime of the Can Gio mangrove forests. Average high tide and low tide were higher in the dry season than in the wet season. The different mangrove vegetation zones had different flooding frequencies at the KV and MO study sites. Zone 1 (nearest to the shoreline) at the KV site had a lower elevation than the other, more inland, mangrove zones at both the KV and MO sites. Overall, flooding frequency and elevation affected various soil properties. Low elevation zones had the highest sedimentation rates and flooding frequency. No sedimentation occurred at the MO site. Litter decomposition at the KV and MO study sites was dependent on the tissue structure of the species and the zones in which they occurred. Species that had thin and soft tissues had a higher decomposition rate than species with thick and hard tissues. The decomposition process was affected by vegetation zone, elevation, and flooding frequency. Flooding frequency and elevation affected primary production and species distribution at the study sites. More species were found in the higher elevation zones, which had dry, compacted soil. However, zones with a single dominant species, such as Rhizophora apiculata or Avicennia alba, had the greatest amount of litter fall.

Dynamic balances between fresh groundwater, saline groundwater, and surface water control the physics and chemistry of subterranean estuaries. Investigations were performed in a subterranean estuary on physical processes contributing to fresh groundwater and saline water mixing, vertical and lateral positioning of this mixing zone, and how this mixing affects spatial and temporal distributions of ^234,238U, ²²⁶Ra, and ²²²Rn. The subterranean estuary is located in an unconfined aquifer beneath Indian River Lagoon, Florida, USA, where I could examine redox responses to altered flow regimes. Continuous groundwater and surface water level measurements suggest a strong hydrologic connection between this lagoon and the groundwater-aquifer system. Periodic forces (e.g. tides, frontal passages) represent a minor contribution to water level variations; however, forcing by tropical cyclones creates large variations in mixing within the seepage outflow. Hurricane Wilma and Tropical Storm Tammy caused hydraulic gradients to reverse, causing lagoon water to recharge the aquifer and shifting the seepage face and subterranean estuary landward about seven meters. Additionally, seasonal distributions of dissolved ²²²Rn, ²²⁶Ra, and ^234,238U in pore and surface waters revealed sensitivity to temporal and spatial mixing and geological heterogeneity. Processes affecting the distribution of these elements include U redox cycling, heterogeneous production of ²²²Rn from sediments, and ²²⁶Ra release during Mn-hydroxide reduction and/or surface exchange. Uranium cycling within the subterranean estuary resulted in a flux of approximately 54 &mu;mol U m^-2 y^-1 to the lagoon. A one-dimensional ²²²Rn transport model was used to quantify fresh and marine sources to submarine groundwater discharge (SGD) through incorporation of heterogeneous production, diffusive, advective, and nonlocal transport mechanisms, and Monte Carlo simulations. Model-based volumetric estimates of fresh and marine SGD components yield ranges of 1.01 to 1.85 and 1.69 to 3.43 m³ d^-1 m^-1 of shoreline, respectively, suggesting fresh SGD contributes approximately one-third of total discharge measured within this subterranean estuary. The 30% fresh component in discharge and the uranium source to coastal lagoon demonstrates SGDs role in global ocean freshwater and elemental inputs. This study highlights the significance of distinguishing fresh and marine groundwater sources and the hydrogeological and chemical complexity of these dynamic subterranean mixing zones.