No description available.

No description available.

No description available.

Intertidal grazing gastropods play an important role in structuring rocky shore communities. For many years the main food resource to these animals was considered to be epilithic biofilms. This study shows that macroalgae are a more important component of gastropod diets than was previously appreciated and that biofilms contribute surprisingly little to their trophic ecology. Stable carbon and nitrogen isotope analyses were used to examine grazer diets, comparing sources of assimilated carbon and nitrogen in gastropod tissues between moderately wave exposed and sheltered sites in northern and southern Britain. Carbon isotope (δ13C) values of biofilms were considerably lower than those of grazing gastropods at all of the sites examined. The δ13C values of grazer tissues were much more closely related to the carbon isotope values of macroalgae, demonstrating the importance of these foods to intertidal grazing gastropods. Nitrogen isotope values were consistent with this conclusion and reflected differences in the source pools of nitrogen between the two sampling regions. Gut contents analysis was used to directly quantify the diet of the intertidal limpet Patella vulgata in detail. Biofilms and corticated/leathery macroalgae contributed most substantially to patterns of ingested materials and limpet diet was strongly influenced by food availability. Animals from sheltered and northern sites consumed more macroalgae whilst animals from southern sites consumed more biofilm. Stable isotope data from mensurative experiments confirmed the important role of food availability in controlling limpet diets. Data from manipulative caging experiments provided further evidence for this effect and also showed that species identity and interspecific competition influence trophic ecology in intertidal gastropods. Patella depressa isotope values remained different from those of Patella vulgata irrespective of food availability and competition in experimental cages. Stable isotope ratios of Patella vulgata and Littorina littorea, distinct in single species cages, converged towards a common value when held together.

The prediction of wave impact loads on prototype caisson breakwaters from the results of physical model tests is considered, with particular attention given to the effects of air, breaker shape, structural response and scale. A review of related literature is presented from which it is concluded that the different aspects of the problem may be related through the force impulse. Large scale soliton impacts are used to show the importance of entrapped air in determining the form of the load time history. Small scale waves with artificially high levels of entrained air and highly controlled drop impacts are used to show and quantify an inverse relationship between entrained air and impact load maxima. Specially developed aeration probes and analysis techniques are used to show the influence of entrapped air on pressure maxima and quantify entrained air levels in small scale fresh water breaking waves. A definition of the force impulse is proposed and used to investigate its variation with breaker shape. The impulse magnitude is shown to be relatively invariant for regular wave impacts compared to a large scatter in impulse form. A numerical model of caisson dynamics is used to predict structural motion and to calculate a series of dynamic amplification factors. The prediction of structural response to obtain effective static loads through the use of these factors is investigated and achieved through the adoption of an 'equivalent impulse' concept. The scatter in impulse form is found to cause large variations in effective static loads between nominally identical impact events. The equivalent impulse concept is used to solve this problem. A comparison is made between the form and magnitude of the force impulses of the small and large scale waves. The results indicate that the impulse magnitude may be relatively free of scale effects. An example is given in which the results of a small scale test are interpreted, scaled and processed to account for the effects of entrained air and structural response in order to predict large scale effective static loads. These are shown to compare well with predictions made using measured large scale force time histories and the numerical caisson model.

Many studies have focused on hydrological and geochemical fluxes from land to the ocean via groundwater discharge, however few have assessed groundwater flow in deltaic settings. Hydrological budgets indicate that 1,000 to 5,000 m3s-1 of water flow from the Mississippi River (MR) to its delta (MRD) via subterranean pathways, however the spatial and geological controls on these processes are less known. This study suggests that deltaic lithofacies of paleoenvironments related to the delta cycle allow for groundwater to discharge out of the MR main channel and into the MRD through organic sediment-rich and silt-sand overbank and mouth bar facies. This study employs geophysical data, including chirp sonar subbottom profiling and continuous resistivity profiling (CRP), to detect the location of these sediments in Barataria Bay, a coastal bay located in the MRD. Chirp data indicate paleochannel features in the MRD, whereas CRP data indicate freshwater seepage into MRD embayments during high river stage events. Analyses of bulk properties of sediment cores are used to characterize delta facies sediments and determine variability in hydraulic conductivity values, which range from 10-7 to 10-2 ms-1. These geophysical and sediment core data show the potential for groundwater flux through deltaic sediments, and will contextualize geochemical tracer data collected by project collaborators. Results indicate that groundwater discharge in the MRD is directly controlled by the geological constraints of the delta region. These results illustrate the potential for substantial groundwater fluxes in other large river deltas, and present implications for urban and coastal infrastructure planning, as many large global deltas sustain significant populations. / 1 / Alexander M Breaux

Thesis (M. Phil.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

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.