Many coastal ecosystems around the world are dominated by submerged aquatic vegetation (SAV) habitats. These SAV habitats are known to provide many highly valuable ecosystem services such as habitat for commercial important species and increased water clarity. Water flow is an environmental variable which can have measurable effects on the ecosystem services provided by SAV, but is often not considered in studies assessing these services. This dissertation sought to investigate the links between SAV, primarily seagrasses, and hydrodynamics, paying special attention to the effects on sediments and fauna. Three main areas are discussed: (1) the effects of SAV on flow, (2) the effects of SAV and flow on deposition in SAV beds, and (3) the effects of SAV and flow on faunal communities in SAV beds. Seagrasses and other SAV reduce currents, attenuate waves, and dampen turbulence within their vegetative canopies, which in turn can enhance deposition and reduce the resuspension of sediment, organic matter, and passively settling larvae. The ability of SAV to retard flow may be further enhanced by increases in vegetated structure, such as shoot density, biomass, or canopy height, which can promote increased abundance and diversity of in- and epifauna within SAV beds. Ultimately, it is clear that hydrodynamics is an important factor that shapes SAV communities both physically (e.g. deposition, sediment structure, etc.) and biologically (e.g. faunal community composition, predation pressure, food availability, etc.).

Seagrass meadows, like coral reefs, are in decline globally but are often neglected in marine policy and conservation despite their equally critical ecosystem services. Both habitats can be heavily impacted by wave surges, rainfall-induced earth movement and flooding, changes to water temperature, salinity, and acidity, and increased levels of turbidity—all occurring at increased rates due to a changing global climate. We demonstrate that multispectral satellite imagery, geospatial tools, and classification techniques can be used to inform management by identifying and quantifying changes in seagrass distribution and the presence of sediment-related threats. Results from Dominica indicate near-shore seagrass habitat area increased by 195.7 hectares between 2016 and 2019, suggesting a continued expansion of Halophila stipulacea. Further analysis showed 22.4 hectares of accreted coastal sediment and 1362.2 hectares of suspended sediment captured, placing 424.4 hectares of sensitive reef area at risk of experiencing tissue abrasion or reduced photosynthetic activity. Our methods can be used by marine resource managers and policy makers to inform decisions relating to fisheries production, emissions trading, disaster risk mitigation, and invasive species monitoring, facilitating sustainable growth in the blue economy.

Seagrass beds serve critical functions in coastal Florida ecosystems. The beds serve as nursery habitat for many juvenile reef fish species and provide protection for many types of benthic organisms found in Biscayne Bay. They help stabilize sediment that would otherwise increase turbidity around coral reefs, filter the water of contaminants, and help support an entire food web. Three species of seagrass were found at the study sites in northern Biscayne Bay: Thalassia testudinum, Halodule wrightii, and Syringodium filiforme. This study focused on understanding the organism habitat interaction by determining the species diversity, seasonal densities, and the correlation between population size and individual size for Penaeid shrimp, juvenile fish, and small adult fish at each site over a one year period. Habitat selectivity of various species was determined based on the habitat complexity derived from the various different seagrasses found in each of the beds. Animals predominantly favored H. wrightii habitat (Kruskal-Wallis H test: p< 0.0001) and this was likely the result of a decrease in predation risk due to the increased habitat complexity of the seagrass beds. Species diversity did not vary significantly over the course of a year (p= 0.7790), likely due to the lack of large abiotic disturbances (e.g. boating, hurricanes, and extreme salinity changes) to the seagrass beds. Densities of inhabitants changed significantly on a monthly basis, with the overall epifauna densities greatest at the end of the wet season (p< 0.01). The lack of correlation between individual size and overall population size likely indicated the majority of the species caught did not exhibit ontogenetic migration or live in the seagrass beds for the entirety of their life cycle.

Seagrass habitats are critical habitat for many fish species and are currently threatened by anthropogenic and natural factors, such as coastal development, pollution, global climate change, and sea level rise. There are few studies that have tracked long- term changes in seagrass habitat and their associated fish communities. This project addressed this need using data collected by the United States Geological Survey (USGS) from two South Florida sites, North Biscayne Bay, FL (NBB) and Port of Miami, FL (POM). The USGS sampling was part of ongoing monitoring projects designed to assist future management decisions that would enhance the protection of these valuable habitats. Data were collected biannually at the conclusion of the dry (April) and wet (September) seasons from 30 cells at each site. In each cell, the data collected included: six replicates for seagrass species and cover, five sweep net collections for fish species and abundance, as well as abiotic variables (water temperature, salinity, turbidity, water depth, and sediment depth). A distinct loss in fish and seagrass species were observed, particularly between the years of 2011-2014. These years coincided with several events including: the Port Miami Deep Dredge (PMDD) project during the years 2013-2015; periods of drought; and major storm events. Changes in fish community structure over this time period were largely driven by loss of species and increased homogenization of fish communities at both locations. More specifically, the NBB community shifted to resemble that of POM by 2014. These changes mirrored the loss of seagrass cover at both locations. Further studies are required to assess the extent to which ongoing dredging activities and other factors might be affecting seagrass cover, which ultimately affect fish communities.

Seagrasses are ecosystem engineers, forming extensive meadows that provide critical habitat and modulate local morphodynamics. Their canopies induce drag on flow to attenuate mean flow and reduce near-bed flow velocities, which can shield the bed from erosion and sediment suspension. Alternatively, seagrass loss can enhance erosion and sediment suspension, which can be initiated through short-lived extreme events, or chronic long-term disturbances. Physical process and disturbances can govern the evolution of seagrass meadow ecosystems. In two separate chapters, this research examined 1) the influence of climate variability and storms on seagrass loss and erosion at a high spatial resolution, and 2) how flow attenuation by seagrass varies across tidal cycles and at different locations in the Choked Passage meadow, on the Central Coast of British Columbia. We used high resolution multibeam echosounder (MBES) bathymetry and backscatter data from 2018 to 2021, drone mapped seagrass delineations from 2014 to 2021, and wind and wave data from 2014 to 2021. Flow data (i.e. velocity magnitude, velocity direction, and acoustic backscatter) above the seagrass canopy was collected with an Acoustic Doppler Current Profiler (ADCP) along transects and moored to the seafloor over a tidal cycle. Sediment samples were collected from the bed to estimate critical shear stress and verify sediment classes from an acoustic backscatter analysis. From 2018 to 2021, the meadow experienced significant erosion (net surface lowering of -18,768 m3) and loss of seagrass (10% reduction), which we attribute to the preceding winter storm activity driven by moderate La Niña conditions. The spatial patterns of erosion and seagrass loss was non-uniform across the meadow. Coupled erosion and seagrass loss resulted in the generation and/or expansion of blowouts. We observed a trend of a reduction in seagrass coverage following winters with a high number of storm events and/or high recorded storm intensity from 2014 to 2021. We believe the Choked Passage seagrass meadow undergoes cyclic behaviour with reduction in seagrass coverage during energetic ENSO years, followed by a recovery period during weak years. The ADCP was used to detect the seagrass canopy height, measure flow, and estimate shear stress. Overall, flow is fastest in the northern section of the main meadow, particularly in the north-west corner where the meadow is patchy. Moreover, flow appears to accelerate through the meadow interior, which suggests that topographic steering and the strength of incoming currents exceeds the ability of seagrass to dampen flow velocity. During the transition from peak flood to ebb, flow velocity remained heightened for longer above the southern meadow and lagged the other sections. Shear stress results indicate that sediment can be transported as bedload and in suspension under peak flow velocities at some of the sites examined within the meadow. Shear stress is largest in the meadow center and lower towards the southern margin of the main meadow. Based on our results, when sediment transport is initiated under peak tidal and/or extreme conditions, sediment is likely primarily transported as bedload, creating the observed sand wave and blowout bedforms. This research demonstrated linkages between extreme storms (during ENSO years), seabed morphology, and seagrass coverage, and examined the variability in the interaction between flow, seagrass, and sediment transport. Geomorphic processes and disturbances have an important influence on ecosystem structure and function over time, therefore, it is important to understand how these processes operate and are modified by external drivers. The results of this study have significant implications on seagrass conservation, restoration, and the evolution of coastal landscapes. / Graduate

In tropical seascapes, coral reefs often exist in proximity to marine vegetated habitats such as seagrass, mangroves, and macroalgae. This habitat mosaic offers the possibility for connection and exchange of both organisms and nutrition between habitats, mediated by biological and physical processes. This dissertation examines flows of organisms and nutrition between coral reefs and tropical vegetated habitats in the central Red Sea through 3 different mechanisms: 1) Use of multiple habitat types by tropical marine fishes, 2) Transport of algal material to coral reefs via the foraging behavior and movements of herbivorous fishes, and 3) Physical flow of water between coastal habitats. The results of this thesis suggest that coastal tropical habitats maintain a variety of ecological links at different spatial and temporal scales. A large fraction (36%) of fish species found on coral reefs are also found in at least one marine vegetated habitat in the central Red Sea, with many species mainly living in vegetated habitats as juveniles. This demonstrates the value of mangrove, seagrass, and macroalgae habitats to coral reef fishes, and suggests that many species make ontogenetic migrations between reef and non-reef habitats through their lives. Two species of herbivorous reef fishes (Naso elegans and N. unicornis) were found on coral reefs with algae in their guts which likely originated from nearby Sargassum-dominated macroalgae canopies, representing a fish-mediated, cross-habitat flux of nutrition from macroalgae habitats to coral reefs. Finally, we used a combination of remote sensing, a dye tracer study, and in-water measurements to observe water movement from shallow seagrass and mangrove habitats to nearby lagoon and coral reef habitats. Water exiting seagrass and mangrove habitats had altered concentrations of various nutrients (such as increased particulate organic carbon or decreased dissolved nutrients), suggesting that Red Sea mangroves and seagrasses change nutrient concentrations in water and the movement of water from these habitats to coral reefs could supply reefs with an allochthonous source of nutrition. These various linkages, controlled by a range of physical and biological processes, highlight the interconnected nature of tropical coastal ecosystems, and thereby the need to conserve whole habitat mosaics in the pursuit to protect coral reefs and maintain healthy and functioning coastal ecosystems.

No description available.

Coastal ecosystems comprise a relatively small area of the ocean, yet they play a disproportionate role in greenhouse gas (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) and nutrient cycling. Vegetated coastal ecosystems (e.g., mangroves, salt marshes, and seagrasses) are key drivers of coastal greenhouse gas and nutrient cycling because of their environmental characteristics (e.g., shallow depths, organic matter rich sediments, etc.). My dissertation addresses the role of vegetated coastal ecosystems in greenhouse gas budgets and biogeochemical cycling. In Chapter 1, I conducted a meta-analysis to quantify the global emissions of CH4 from mangrove, salt marsh, and seagrass ecosystems. Here I show that mangrove ecosystems contribute the most CH4 out of these vegetated areas to the global marine CH4 budget. Further, while a well-known negative relationship between salinity and CH4 fluxes exists for salt marshes globally, this relationship does not hold for mangrove or seagrass meadows, suggesting that other environmental drivers are more important for predicting CH4 fluxes in these ecosystems. In Chapter 2, I present in situ fluxes of CH4 and N2O across the sediment-water interface as well as air-sea fluxes in seagrass meadows and adjacent non-vegetated sediments in two temperate coastal lagoons. Here I demonstrate that seagrass meadows can be sources or sinks of CH4 and that N2O uptake can enhance carbon sequestration in seagrass meadows by ~10%. In Chapter 3, I quantify fluxes of dissolved inorganic carbon, nitrogen, and phosphorous across the sediment-water interface in seagrass meadows and adjacent non-vegetated sediments in the same two coastal lagoons. I found that both seagrass and non-vegetated sediments exhibited dissolved inorganic carbon emission and denitrification, and that dissolved inorganic phosphorous fluxes varied by site and not with vegetation presence. This dissertation highlights the dynamic role coastal ecosystems play in biogeochemical cycling and the importance of vegetated coastal ecosystems in coastal greenhouse gas budgets. / 2024-10-03T00:00:00Z

No description available.

This dissertation serves as a comprehensive, natural-abundance analysis of the present-day spatial and temporal dynamics and trophic linkages of nitrogen from within the Florida Keys National Marine Sanctuary (FKNMS). This work was accomplished by documenting the delta 15 N of particulate organic matter (POM), several genera of aquatic vegetation and herbivorous fish, as well as dissolved water column nitrogen. Seasonal and spatial variations in delta 15 N were assessed, trophic level variations among reef constituents were quantified, and relative contributions of both natural and anthropogenic nitrogen into the reef tract were determined. The measured mean delta 15 N of POM (3.64 per mil), aquatic vegetation (Dictyota = 2.39 per mil; Thalassia = 1.91 per mil; Rhizophora mangle = 1.46 per mil; Halimeda = 1.62 per mil; sponges = 4.13 per mil; turf algae = 2.67 per mil), herbivorous fish (4.92 to 8.47 per mil), as well as the delta 15 N and delta 18 O of nitrate (4.40 and 20.36 per mil, respectively) suggest that the primary nutrient sources directly impacting the reef are from natural sources, principally nitrogen fixation, and not anthropogenic wastes. Clear trophic linkages, without evidence of disturbances from anthropogenic wastes, are apparent in this study; herbivorous fishes show distinct 3 to 4 per mil enrichments over their food source. The presence of anthropogenic wastes was detected in the delta 15 N and delta 18 O of nitrate from Key Largo canal waters (10.09 per mil), however, sewage derived nutrients did not exist in any measurable or detectable amount outside the canals. Additionally, an assessment of the total yearly nitrogen contributions to the FKNMS was conducted, demonstrating that nitrogen fixation was the largest contributor of nitrogen to the ecosystem, delivering approximately 43 percent of all nitrogen. Anthropogenic wastes, however, contributed only about 8 percent to the total nitrogen budget, far less than biotic (non-human) wastes (13 percent), upwelling (10 percent), and gyre waters from the Gulf of Mexico (9 percent). For the first time, a long term, spatially diverse investigation has presented a more complete depiction of delta 15 N composition of various reef components found in the FKNMS, critical and imperative for accurately assessing nutrient pressures on coral reefs. As such, the data presented in this study do not support the theory that continuous anthropogenic nutrient loading from nearshore populations is the sole cause of reef decline in the FKNMS.