In this thesis I review how Sargassum muticum (Yendo) Fensholt, an invasive alga from Asia, has spread globally due to human activities and describe how this species can affect seagrass ecosystems. Abiotic factors such as nutrient and substratum availability may facilitate the spread of S. muticum into Zostera marina L. meadows, but analyses of seawater nutrients, and sediment particle size and % organic content revealed no significant differences between experimental quadrats in seagrass meadows either with, or without the presence of S. muticum. Phenolic compounds were examined because they form the basis of defensive mechanisms in plants and algae, therefore any change in phenolic content may affect the ability of Z. marina to protect itself from disease, herbivory and invasive species through allelopathic interactions. Results from a four year field study and multiple annual laboratory experiments showed significant reductions (p = 0.034 and p = 0.002, respectively) in the caffeic and tannic acids equivalents content of Z. marina when in the presence of S. muticum. As the abundance of S. muticum increases, other changes in the physiology of Z. marina may occur including variations in growth rates, nutrient partitioning and chlorophyll fluorescence, but data from multiple laboratory experiments illustrated no significant differences in growth. Chlorophyll fluorescence analyses revealed significant differences between treatments with and without S. muticum (p = 0.008), but pairwise comparisons indicated these differences only occurred in 2008 (p < 0.001). Significant differences were also found in nutrient partitioning amongst functional regions of the shoots (p = 0.024), but pairwise comparisons detected these differences between a biomass control treatment (ZZ: Zostera + Zostera) and the ZS (with S. muticum) and ZM (Z. marina on its own at a lower biomass per replicate) treatments (p = 0.013 and p = 0.019, respectively), but not between ZS and ZM. Previous in situ research has found negative effects of S. muticum presence on densities of kelp and other algae. Results from the long-term field study indicated significantly lower mean in situ Z. marina densities within the ZS treatment (p < 0.001). Epibiota found living on the blades of Z. marina provide food for organisms within seagrass ecosystems and also create microhabitats for other species to occupy. Alterations in the abundances of epibiota and microhabitats formed could further modify seagrass ecosystems through shifts in timing of food availability, food preferences and microhabitats created. The long-term field study data revealed significantly lower epibiota abundances within the ZS treatment (p = 0.019), but differences in biomass between treatments were not detected. Changes in the biochemistry, physiology, vegetative physiognomy and epibiota assemblages of Z. marina revealed during experimental manipulations are presented and considered within the context of long-term seagrass survival in light of increasing S. muticum invasion.

Due to high mortality encountered by marine fish larvae during their first weeks of life, small changes in the number of individuals surviving through this period can cause large fluctuations in year-class strength. Larval Red Drum (Sciaenops ocellatus) are dependent upon structured estuarine habitat to avoid predation. A study of post-settlement larval Red Drum distribution in a subtropical seagrass meadow in Mission-Aransas Estuary, Texas, USA indicates that larvae settle over approximately two months. Abundance of larger settled larvae was significantly different among sites. The areas of highest larval abundance varied temporally, indicating that the entire extent of the seagrass bed is utilized. Regression analysis of abiotic environmental factors did not explain why larvae were more abundant at particular sites. To characterize the structure and variability of the fish species assemblage that Red Drum encounter upon settlement, larvae and juveniles were captured in the seagrass meadow during weekly collections. Of the 32 fish species collected, seven represented 92% of the assemblage. Multivariate species analysis indicated that collections widely separated in time and space shared the lowest Bray-Curtis similarity. Because Red Drum settle over a relatively long period and co-occur at body sizes known to cause cannibalism under laboratory conditions, I tested combinations of small and large Red Drum larvae at various field-realistic densities and at different levels of seagrass habitat structure to determine potential for cannibalism. Artificial seagrass did not protect small (5 – 6 mm SL) larvae from cannibalism, but natural dense seagrass had a protective effect relative to edge habitat. The final component of this research examined the emergent impacts of a common predator pinfish (Lagodon rhomboides) on mortality and cannibalistic interactions between small and large Red Drum larvae. Both pinfish and large Red Drum larvae alone readily consumed small Red Drum in all seagrass habitat structures tested. However, the combined treatment of pinfish and large Red Drum together led to reduced mortality of small Red Drum. Predation can significantly affect Red Drum survival during the post-settlement period, and multiple predators may have a protective effect on the smallest settlers if predation pressure is re-directed towards a larger size class. / text

An important Athecate genus, Eudendrium, and a group of species of the Thecata, the latter ecologically related by life on a common substrate, are reviewed. Eudendrium, hitherto poorly known in Australia, comprises 17 species, including 10 undescribed species with 71% Australian, and high provincial endemicity. Eudendrium may be a shelf genus avoiding turbulent oceanic waters. Species of Eudendrium are predominantly epizoic and some gregariously settling colonies may live for five years. Identification of sterile material is refined by using the cnidome in a key to classification. The species and population dynamics of hydroid epiphytes of the endemic southern Australian marine angiosperm Amphibolis were investigated with revision of historically vexatious taxa. In contrast with the northern hemisphere, no Athecata are associated with southern Australian seagrasses. Seventeen species from eight thecate families are associated with the two species of Amphibolis, including one undescribed species, H&lecium amphibolum, and one new record for Australia, Aglaophenia postdentata. The Lineolariidae is revised and a new genus, Millardaria, erected for a species from seagrass in Madagascar. The high endemicity (58%) and host-specificity of hydroids to Amphibolis is an evolutionary consequence of isolation of the seagrass dating from break-up of the Tethyan Sea. Hydroids occur throughout the year in the Amphibolis leaf canopy with a mean annual epiphytism of 44% on A. antarctica in the eastern continent and 86% in the western continent; epiphytism is 52% on A. griffithii in the western continent. Half of the eight important species are dominant epiphytes across the southern continent but the species and order of abundance varies regionally. Most are pioneer colonists with short, repetetive life-cycles lasting from weeks to a few months. Three species epiphytise the seagrass stems but only one is a leaf-canopy dominant. The canopy community comprises small, fast-growing species or dwarfed variants of species larger in other habitats: these ecomorphically constant forms are associated only with seagrass. Strategies for survival in the harsh Amphibolis environment include adnate colonies and gonothecae adnate or recumbent to the substrate, marked strengthening of the hydrorhiza, various hydrodynamic adaptations of the hydrotheca, early maturation and production of numerous small ova.

Seagrass bed is a highly productivity ecosystem, it also provides habitats for animals and plays an important role in stabilizing the substrate. The sea anemone Phymanthus strandesi is very abundant in the seagrass bed of Thalassia hemprichii on Hsiao-Liuchiu Island. Structural Equation Modeling (SEM) was used to investigate the relationship between P. strandesi and some environmental factors, which affect the distribution of this species at Tuozaiping tidal flat (N 22¢X20"55' E 120¢X21"49'), Hsiao-Liuchiu Island. Light and temperature were also manipulated in the laboratory to test their effect on the hiding response of P. strandesi. The results of SEM show that the abundance of T. hemprichii showed very weak positive relation with P.strandesi. On the other hand, soil depth on the seagrass bed might be the main factor that affects the distribution of P. strandesi. In high a temperature situation (i.e. over 38¢XC), all the sea anemones in the experimental container hided into the sand. However, only some sea anemones hid when exposed to strong light (i.e. 5030 lum/ft²) after one and half hours.

Seagrasses are extremely productive coastal plant communities that serve as habitat for various types of marine and estuarine fauna and provide numerous ecosystem services. Seagrass meadows around the world have become threatened by environmental and anthropogenic pressures such as altered hydrologic regimes, physical disturbances, and eutrophication. Monitoring programs that provide high-resolution information and document changes in cover, morphometric characteristics, species composition, and tissue nutrient content across large spatial scales are critical in global conservation and management efforts. In an attempt to address the uncertainties regarding the current distribution and condition of seagrasses in the southwest Gulf of Mexico, I conducted annual sampling from 2011-2013 to examine seagrass cover and condition at 558 permanent stations. Sampling occurred in three regions of the Texas coast: the Coastal Bend (CB), Upper Laguna Madre (ULM), and Lower Laguna Madre (LLM), which together comprise over 94% of the seagrasses in Texas. Significant trends in seagrass coverage and tissue elemental composition were highly location- and species-specific. In the CB, I did not observe significant changes in seagrass cover and no spatial patterns in tissue nitrogen (N) or phosphorus (P) were apparent. However, I observed a species shift in the northern ULM, where significant decreases in Syringodium filiforme cover were coupled with significant increases in Halodule wrightii cover. Long-term salinity records at four stations throughout the study area suggest that S. filiforme mortality in the ULM in 2013 was a product of an extended period of high salinity (> 55) that began in late 2012. In LLM, there were significant increases in H. wrightii cover in the north and significant decreases in T. testudinum cover in the south, which cannot be explained based on underwater light levels, salinity, or nutrient availability. Both H. wrightii and T. testudinum displayed lower C:N, C:P, and N:P ratios, along with enriched δ¹⁵N signatures nearest urban areas, particularly in the LLM. This study illustrates the value of integrating rapid-assessment field sampling and rigorous statistical and spatial analysis into a large-scale seagrass monitoring program to uncover patterns in seagrass community structure. I detected significant trends in seagrass coverage and condition across multiple spatial and temporal scales, including a massive species replacement that coincided with a prolonged period of hypersaline conditions.

Habitat fragmentation is often regarded as a biodiversity threat associated with habitat degradation; however, research has also revealed beneficial effects on biodiversity as well, depending on the ecosystem and species community. This study examined the biodiversity of small nekton residing in seagrass meadows characterized by three levels of habitat fragmentation, and as a habitat gradient comprised of measures such as habitat amount, connectivity, patch shape, and proximity. Landscapes were mapped using recent advances in GPS and GIS technology, and analyzed using established methods from research in terrestrial ecosystems. Species richness was not significantly different as a function of fragmentation regardless of season, suggesting that the amount of habitat and configuration of several patches in fragmented habitats is sufficient to support comparable numbers of species in several patches compared to communities in large, continuous seagrass meadows. Species evenness declined significantly in fragmented habitats versus continuous ones in both seasons. Within fragmented landscapes, evenness progressively declined as habitat amount and connectivity decreased and patch isolation and density increased, suggesting that changes in landscape qualities can differentially impact processes supporting metapopulations such as dispersal and reproduction in certain species, thereby influencing community structure. Analyses that included measures of habitat connectivity, proximity, and patch density in addition to habitat amount accounted for more variability in species evenness than those just containing percent cover, and showed that fragmentation’s impacts can differ geographically. These data suggest that community resilience to fragmentation can differ between similar animal communities residing in separate locations, and that landscape configuration plays an important role in determining how communities respond to fragmentation after a threshold of change in habitat amount has been exceeded. / text

Underwater acoustics has recently emerged as a viable tool for assessing ecosystem health and exploring the estuarine soundscape. Recent acoustic surveys have mapped distributions of both seagrass meadows and kelp forests, and scientists are currently developing remote sensing capabilities to improve ecological assessments of these communities. Furthermore, researchers are beginning to focus on the propagation and ecological significance of bioacoustic signals within estuarine landscapes. The research presented here includes a thorough examination of the interaction of acoustic energy and macrophyte tissue as it pertains to habitat assessment and ecosystem function. Modeling experiments investigated the interaction of acoustic energy and submerged macrophyte tissue. Both seagrasses and kelp exhibited a similar acoustic response by increasing the acoustic compressibility of a seawater medium. The increase in acoustic compressibility was driven by free-gas volumes contained within the macrophyte tissue. Interestingly, the tissue served to limit the acoustic compressibility of the gas volume below the magnitude predicted by effective medium models. Separate inquiries of high-frequency sound propagation and the seagrass canopy revealed a significant temporal component to acoustic transmission. Specifically, sound transmission throughout a seagrass canopy was altered by the formation of free gas bubbles and the pressurization of aerenchyma channels, which was mediated by photosynthesis. The photosynthetic controls on sound propagation were species-specific, and patterns of acoustic transmission provided a reasonable proxy for gross primary production in Syringodium filiforme plants. Finally, the interaction of sound energy and submerged macrophytes appears to have important ecological implications. This research suggests that seagrass meadows scatter high-frequency sound energy and provide an acoustic refuge to fish from marine mammal predators. This refuge is highly seasonal, specific to different seagrass species and dependent on the abundance of above-ground biomass. Seagrasses also may influence the transmission of low-frequency sounds used by soniferous fish. Propagation characteristics of low-frequency sounds are highly dependent on frequency and result in differential transmission distances among individual fish species. It is clear from this body of work that submerged macrophytes are an important feature of the underwater soundscape. Future research should continue to exploit this feature for remote sensing purposes and examine its ecological significance. / text

Water motion has been shown to influence almost every aspect of the ecology of seagrass communities; seagrass communities have likewise been shown to significantly influence water movement around them. This thesis examines the important role of water motion on seagrass ecosystems by integrating field and laboratory studies of several aspects of seagrass ecology influenced by water motion. To facilitate the study of hydrodynamics of seagrass ecosystems, a solid state electronic current meter was designed and developed, using thermistors as flow sensing devices. Important characteristics of the meters include: no moving parts, compact size, high sensitivity (resolution better than ± 0.5 cm s-1), and high sampling rate (greater than 0.2 Hz). Deployment of the meters in field measurements provided reliable and meaningful results of flow conditions through seagrass canopies, and they show great potential for use in many studies of marine ecology. Field studies of water velocity profiles revealed significant differences between the shapes of profiles of different seagrass species, particularly between species of Posidonia and Amphibolis. Of particular note is the observation of a region of high water velocity beneath the leafy canopy of Amphibolis, which was not present in the Posidonia plants. Water velocity profile measurements, sediment grain size analyses and standing stock measurements were conducted across an exposure gradient in a Posidonia sinuosa meadow. These studies revealed that, while the exposed location experienced a higher ambient water velocity than the sheltered site, the baffling influence of the seagrass canopy reduced the water velocity to approximately the same at both sites, within the meadow, although the effects varied seasonally. It was also observed that the seagrass meadow produced apparent skimming flow under the low flow conditions measured at the sheltered location; this phenomenon reflects the capacity for flow redirection over the canopy, and has important implications for the sub-canopy ecosystem and the protective role of seagrasses on the seabed. Field and laboratory studies on the role of seagrass density on the hydrodynamic nature of seagrass ecosystems revealed that water velocity profiles through meadows of reduced densities, and different shoot arrangements, were markedly different to “natural” profiles, implying the existence of a “critical density” (approximately 25 % of natural meadow density) with regard to canopy hydraulics. The role of water motion at an individual leaf scale was investigated with a series of laboratory flume studies of photosynthetic rates of seagrass and algae. The results show that the response of photosynthetic rate to water velocity depends very much on the plant species, with the algae markedly more productive (on a unit chlorophyll basis) than the seagrasses tested. Increases in photosynthetic rate were observed at water velocities above approximately 2.5 cm s-1; negligible photosynthetic activity was observed below this velocity. Calculation of P v. I curves indicated that the Posidonia species had high Ik values at low velocities (1360 :mol quanta m-2 s-1 for P. australis and 250.8 :mol quanta m-2 s-1 for P. sinuosa at 1.58 cm s-1), which decreased with increasing water velocity (to 138.9 and 24.77 :mol quanta m-2 s-1 for P. australis and P. sinuosa respectively), while the algal species had relatively constant values of Ik across all water velocities (85.42 to 312.7 :mol quanta m-2 s-1 for Ulva lactuca and 169.7 to 573.9 :mol quanta m-2 s-1 for Laurencia cruciata). Dye visualization studies showed that the algae remained quite rigid at all the velocities tested, while the seagrass leaves compressed as velocity increased. This resulted in an increased rate of turbulence creation by the algae, which is believed to enhance photosynthetic rates, through improved nutrient exchange rates across the boundary layer adjacent to the thallus. Further dye visualization studies revealed the significance of blade morphology on the creation of microscale turbulence at the surface of seagrass leaves. Epiphytic growth on seagrass leaves was observed to play an important role in breaking up water flow across the leaf surface, thereby enhancing the creation of microscale turbulence. From these studies, it is clear that water motion influences all aspects of the functioning of all components of seagrass communities, playing a role in nutrient supply, reproduction, physical stability, temperature and metabolic functions. The influence of seagrass meadows on coastal hydrodynamics is also apparent, with potential impacts on sediment stability, recruitment of benthic species and coastal erosion. This thesis has clearly demonstrated that water motion is an important parameter in seagrass ecology, and requires serious consideration in seagrass research, conservation and rehabilitation programmes.

Coastline communities have experienced a marked increase in human populations over the last few decades. This increase in population places disproportionate pressure on coastal ecosystems to provide economic services to support local economies. At the same time, overuse of these services can aid in the destruction of the ecosystems responsible for them. Seagrass ecosystems are mainly found near coastlines, and are typically a chief provider of some of these economic goods and services. Many previous studies have documented the ecological functions of this seagrasses. Unfortunately, our increasing knowledge of seagrass structure and function has not been fully incorporated into economic models estimating their value. In this dissertation, I focus on the seagrass ecosystem in southern Biscayne Bay, and simultaneously study the ecological dynamics of the seagrass beds, and estimate its economic value. This value is based on recent ecological models in the literature as well as data I collected from the system. I focused on Biscayne Bay due to, 1) the relevance that this question had to the relationship between Biscayne Bay and the Miami metropolis, and 2) the lack of existing reliable models that explore this relationship in this area. More specifically, I became very interested in this question while working for Biscayne National Park, where such a model would have improved seagrass restoration work taking place there. I found that southern Biscayne Bay is dominated by Thalassia testudinum, with other seagrasses following a spatial pattern primarily determined by salinity and water column nutrient distribution. Syringodium filiforme was mostly found east of the islands, Halodule wrightii was mostly found near the shoreline, and Halophila engelmenii was spotted at only two of the 190 sites visited. T. testudinum distribution was largely unaffected by nutrient enrichment at all sites, but it appeared to induce severe herbivory further from the coastline. For the calendar year 2004, we deduced using a Total Ecosystems Valuation (TEV) model that seagrass ecosystems potentially contributed over $198 million US dollars to the local economy. We argue that a simultaneous understanding and use of both ecological and economic models is important for future conservation efforts of seagrass ecosystems.

The importance of resource supply and herbivory in driving competitive interactions among species has been an important but contentious issue within ecology. These variables exhibit different effects on species competition when manipulated in isolation but interact when manipulated together. I tested the direct and interactive effects of nutrient addition and simulated grazing (clipping) on the competitive performance of primary producers and community structure of a seagrass bed in South Florida. One square meter experimental plots were established in a mixed seagrass meadow from August 2007 to July 2009. The experiment was a 3 x 3 factorial experiment: 3 fertility treatments: control, medium (2.4 mg N d-1 and 80 µg P day-1) and high (4.8 mg N d-1 and 160 µg P day-1) x 3 clipping intensities (0, 25% and 50 % biomass removal (G)) x 5 replicates for each treatment = 45 plots). Nutrient additions and simulated grazing were done every two months. Fertilization and simulated grazing decreased sexual reproduction in S. filiforme. Fertilization increased competitive dominance within the primary producers while simulated grazing counteracted this effect by removal of the dominant species. Fertilization ameliorated the negative impacts of simulated grazing while simulated grazing prevented competitive exclusion in the fertilized plots. Nutrient addition and simulated grazing both exerted strong control on plant performance and community structure. Neither bottom up nor top down influences was eliminated in treatments where both factors where present. The effects of fertilization on plant performance were marked under all clipping intensities indicating that the system is regulated by nutrient availability both in the presence or absence of grazers. Clipping effects were strong under both fertilized and unfertilized conditions indicating that the seagrass bed can be simultaneously under top-down control by grazers.