[Truncated abstract] Biogeochemical processes in seagrass sediments influence growth and nutrition of seagrasses. This thesis investigates the below-ground interactions between biotic and abiotic factors that influence seagrass nutrition and growth, with focus on a small species of seagrass, Halophila ovalis (R. Br.) Hook ƒ., from the Swan River Estuary, Western Australia. Seagrass showed significantly lower growth and an increase in leaf nitrogen and phosphorus concentrations with increased organic matter loading. With maximal light reduction, lower growth rates and average leaf weights were observed, and leaf nitrogen and phosphorus concentrations were higher. Light reduction was also shown to increase bioavailability of inorganic nutrients within porewater of seagrass sediment . . . Sulphide was hypothesised to have an inhibitory effect on nutrient uptake of Halophila ovalis. Below-ground sulphide inhibits the photosynthetic efficiency of photosystem II at sulphide concentrations greater than 1 mM. Sulphide exposure enhanced phosphate uptake, with no significant effect on ammonium uptake of H. ovalis. This thesis demonstrates that biogeochemical processes both constrain the potential growth of seagrasses and influence the nutrient status of seagrass tissue. Consideration of the influence of sulphide stress on seagrasses is likely to be particularly important for anthropogenically influenced aquatic systems, where inputs of organic matter are enriched relative to pristine ecosystems. A better understanding of biogeochemical processes will allow researchers to predict how future changes in sediment chemistry will influence seagrass meadows.

Green sea turtles in Bermuda are overgrazing the seagrasses on which later life stages are thought to specialize. I hypothesized that larger green turtles in Bermuda would display individual diet specializations during seagrass scarcity. Stable isotope methods were used to determine the diet composition of green sea turtles from the Bermuda Platform as a function of size class and in turtles captured in successive years. Individual turtles had a wide range of diets, however, the variation in diets was driven by differences among size class rather than within the size classes of larger turtles, indicating that green turtles undergo a dietary ontogenetic shift during their residency on the Bermuda Platform and no clear specialization of diets among late-stage individuals. The apparent lack of dietary specialization of larger turtles indicates that older turtles are not diversifying their diets in response to the drastic reductions in seagrass in Bermuda.

Anthropogenic disturbances are ubiquitous in coastal marine ecosystems. As such, more intensive monitoring efforts are necessary to conserve these valuable habitats. Bioindicators, organisms that predictably respond to changes in environmental variables, may be utilized in monitoring efforts to assess ecosystem functioning. To incorporate organisms into monitoring programs as bioindicators managers need to first understand the difference between the natural phenology of the focal organisms and their responses to different forms of anthropogenic disturbance. To determine if gammaridean amphipods could be used as indicators of changes in environmental quality in sub-tropical seagrass ecosystems, I conducted spatial and temporal surveys of amphipod communities in south Florida. Amphipod community structure varied significantly across sites and seasons. Variation in community structure was largely driven by macrophyte biomass, food availability, seasonally variable factors (epiphyte abundance, dissolved oxygen, salinity, and temperature), water-column nitrogen concentration, and factors related to freshwater input, including low Thalassia testudinum and high Halodule wrightii densities, and salinity. Amphipods are also susceptible to mechanical damage in seagrass habitats and could be used as indicators of ecological functioning of a region. A major source of mechanical damage in seagrass ecosystems is caused by boat propellers. I simulated propeller scars in continuous seagrass beds to investigate the effects of scarring on seagrass ecosystem functioning. Seagrasses located adjacent to propeller scars experienced a shift in the limiting resource from light to phosphorus. Amphipod community structure, however, was not impacted by scarring, but amphipod density was reduced in fragmented patches. To determine if plant-herbivore interactions were impacted by propeller scarring, we removed amphipods from half of the experimental plots and measured epiphyte biomass and community composition. Top-down control on epiphyte biomass or community composition by amphipods was not affected by fragmentation, despite reduced amphipod densities. My dissertation research demonstrates that amphipods could be incorporated into existing management programs in sub-tropical seagrass ecosystems as environmental indicators. Reduced amphipod densities in fragmented seagrass beds suggests that amphipods could also be used as ecological indicators, but more research is needed to determine the extent of the impacts of fragmentation on higher trophic levels.

Coastal marine ecosystems are among the most impacted globally, attributable to individual and cumulative effects of human disturbance. Anthropogenic nutrient loading is one stressor that commonly affects nearshore ecosystems, including seagrass beds, and has positive and negative effects on the structure and function of coastal systems. An additional, previously unexplored mechanistic pathway through which nutrients may indirectly influence nearshore systems is by driving blooms of benthic jellyfish. My dissertation research, conducted on Abaco Island, Bahamas, focused on elucidating the role that benthic jellyfish have in structuring systems in which they are common (i.e., seagrass beds), and explored mechanistic processes that may drive blooms of this taxa. To establish that human disturbances (e.g., elevated nutrient availability) may drive increased abundance and size of benthic jellyfish, Cassiopea spp., I conducted surveys in human-impacted and unimpacted coastal sites. Jellyfish were more abundant (and larger) from human-impacted areas, positively correlated to elevated nutrient availability. In order to elucidate mechanisms linking Cassiopea spp. with elevated nutrients, I evaluated whether zooxanthellae from Cassiopea were higher from human-disturbed systems, and whether Cassiopea exhibited increased size following nutrient input. I demonstrated that zooxanthellae population densities were elevated in human-impacted sites, and that nutrients led to positive jellyfish growth. As heightened densities of Cassiopea jellyfish may exert top-down and bottom-up controls on flora and fauna in impacted seagrass beds, I sought to examine ecological responses to Cassiopea. I evaluated whether there was a relationship between high Cassiopea densities and lower benthic fauna abundance and diversity in shallow seagrass beds. I found that Cassiopea have subtle effects on benthic fauna. However, through an experiment conducted in a seagrass bed in which nutrients and Cassiopea were added, I demonstrated that Cassiopea can result in seagrass habitat modification, with negative consequences for benthic fauna. My dissertation research demonstrates that increased human-driven benthic jellyfish densities may have indirect and direct effects on flora and fauna of coastal marine systems. This knowledge will advance our understanding of how human disturbances shift species interactions in coastal ecosystems, and will be critical for effective management of jellyfish blooms.

The availability of light is a primary limiting factor for seagrass recovery and sustainability. Understanding not only the quantity but the quality of light reaching the bottom is an important component to successful seagrass management and the key focus of this study. This study explores the spectral properties of the sub-surface light field in four shallow Seagrass Management Areas (SMA) in Tampa Bay. Wavelength-specific photosynthetically active radiation (PAR(λ)) and the spectral light attenuation coefficient (Kd(λ)) are used to estimate the percent blue, green, and red light remaining at the bottom relative to the surface. LIDAR Bathymetry is combined with Kd(λ) to produce high-resolution maps of percent subsurface light along the seagrass deep edge. The absorptance spectra from two seagrass species together with PAR(λ) is used to calculate the photosynthetically useable radiation (PUR(λ)), a term describing the actual wavelengths of light being used by the seagrass. Based on the average annual Kd(λ) , 32% - 39% percent of PAR reached the bottom at the seagrass deep edge, while only 14% - 18% of blue light reached bottom, suggesting that seagrass may be blue-light limited. Analysis of PUR(λ) data further confirmed that seagrass are blue-light limited. Each SMA was characterized in terms of the inherent optical properties (IOP) of absorption and scatter. Tampa Bay is considered a chlorophyll-dominated estuary. However, in this study, colored dissolved organic matter (CDOM) was the major absorber of blue light, accounting for 60% of the total absorption. To infer past light conditions, the IOPs were related to parameters more commonly used in routine monitoring programs. To estimate Kd(λ) an empirically-derived model using only the total absorption and scatter coefficients was used and resulted in a good fit between measured Kd(480) and modeled Kd(480). A deck-mounted flow-through system was used to survey each SMA for CDOM and chlorophyll a fluorescence, among other properties. A series of SMA-specific predictor equations were empirically derived to relate raw fluorescence to the IOPs. The Kitchen SMA was used as a case study. Survey results show a strong connection between CDOM-rich waters and the mangrove-dominated shoreline.

The endemic eelgrass (Zostera marina) community of Morro Bay Estuary, located on the central coast of California, has experienced an estimated decline of 95% in occupied area (reduction of 344 acres to 20 acres) from 2008 to 2017 for reasons that are not yet definitively clear. One possible driver of degradation that has yet to be investigated is the role of herbicides from agricultural fields in the watershed that feeds into the estuary. Thus, the primary research goal of this project was to better understand temporal and spatial trends of herbicide use within the context of San Luis Obispo (SLO) County and Morro Bay Watershed by analyzing data of application by mass, area, and intensity to identify herbicides with the highest potential for local environmental pollution. California Pesticide Use Annual Summary Reports (PUASR) from the years 2000 to 2017 were used to obtain data for conducting a meta-analysis to estimate total herbicide application by weight within every township, range, and section for each of the eight selected herbicides: oxyfluorfen, glyphosate, diuron, chlorthal-dimethyl, simazine, napropamide, trifluralin, and oryzalin. A second goal was to select an analytical laboratory that would be best suited for herbicide analysis of estuary sediments to determine the presence, or lack thereof, of the eight selected herbicides. Criteria of consideration in laboratory selection included herbicides detection capabilities, detection/reporting limits, testing prices, chain of custody protocols, turnaround times, and laboratory site locations. The meta-analysis yielded results showing high herbicide application rates in SLO County with glyphosate, oxyfluorfen, and chlorthal-dimethyl being identified as three herbicides of elevated risk for local environmental contamination due high rates of use by mass, by area, and/or intensity during the study timeframe. Additionally, Morro Bay Watershed exhibited moderate rates of herbicide application with chlorthal-dimethyl and glyphosate being of highest risk for contamination and accumulation within the estuary because of high application rates by mass, by area, and/or intensity. Finally, Environmental Micro Analysis (EMA) and Primus Group, Inc. (PrimusLabs) were identified as the top candidates for analytical laboratory testing of Morro Bay Estuary sediment samples to be obtained and tested for the selected herbicides. These laboratories provide superior analytical capabilities of the eight herbicides, impressive reporting limits or lower detection limits, competitive testing prices for detecting multiple constituents in multiple samples, robust chain of custody protocols, options for quick turnaround times, and laboratory site locations within California.

Seagrass beds are among the most ecologically important systems in the marine environment. They provide the primary production to nearby coral reef and mangrove communities, and seagrasses comprise a large component of the diets of many marine organisms including fishes, small invertebrate species, and many protected species such as manatees and sea turtles. This consumption provides a pathway for many contaminants to enter the marine food web via the seagrasses. The coastal location of seagrass beds causes them to be especially susceptible to anthropogenic pollution, including accumulation of heavy metals, which has been shown to have many adverse health effects in the seagrasses and marine organisms that feed on them. This study assessed the heavy metal concentrations of seagrasses in three regional locations in South Florida: Port of Miami, Card Sound Aquatic Preserve, and Florida Bay. Three species of seagrasses, Thalassia testudinum, Halodule wrightii, and Syringodium filiforme, which comprise the majority of South Florida seagrass beds, were collected monthly for a period of one year and analyzed for ten heavy metals: (arsenic (As), cadmium (Cd), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), selenium (Se), zinc (Zn)). Concentrations were compared across locations, season, species, and plant part (leaves, shoots, roots, and rhizomes). Concentration ranges, in µg/g (ppm), found in seagrass tissues for all included locations, species, and plant parts were: As (0.02-2.95), Cd (0.09-10.72), Cu (0.38-33.68), Fe (1.52-1877.43), Pb (0.78-156.20), Mn (0.79-300.15), Hg (0.03-16.46), Ni (0.67-87.74), Se (0.01-4.79), Zn (1.48-669.44). Statistical analysis showed significant difference in concentrations among locations, season, species, and plant morphology.

No description available.

No description available.

Marshes and seagrass beds have been widely recognized as important habitat for estuarine species, but less has been done on how these habitats interact and function together, thereby limiting understanding of the variability of juvenile recruitment to coastal systems. Therefore, the objective of this study was to assess the interaction between fringing marsh and adjacent seagrass for the provision of habitat for juvenile nekton. Weekly seine net and benthic seagrass core sampling from June to November 2020 determine the relationship between nekton and marsh-edge and seagrass habitat. This study shows disparate results, in terms of the effects of proximity to marsh edge and seagrass biomass on nekton abundance and size, pointing to different selectivity of marsh edge versus seagrass by different species. In addition, there are no effects of proximity to marsh edge and seagrass biomass on community composition, but an interactive effect on community dispersion.