Browsing by Subject "Seagrass"
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Item Acoustic characterization of Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa(2019-08-12) Johnson, Jay Richard, Ph. D.; Wilson, Preston S.; Hamilton, Mark F; Haberman, Michael R; Rahman, Abdullah FSeagrasses are a vital part of the coastal ecosystem; they serve as a habitat for fish, stabilize the seabed, are significant primary producers, and act as efficient carbon sinks. Increased anthropogenic pressures have put stress on seagrass ecosystems, and thus, there is a growing need to map and monitor seagrass meadow growth and decline. Acoustic mapping and monitoring techniques show a potential to be both cost-effective and robust, but there is still a lack of a physics-based model of acoustic propagation in seagrass meadows. This dissertation expands the knowledge necessary for such a predictive forward model of sound propagation in seagrass meadows by conducting various acoustic experiments on the two endemic Mediterranean seagrass species Posidonia oceanica and Cymodocea nodosa. The mean tissue density was measured for both species using direct mass and indirect volume measurements, and the bulk modulus of the leaf tissue of both species was measured using a one-dimensional acoustic resonator technique and finely divided tissue pieces via an effective medium model. The mean tissue density and adiabatic bulk modulus of P. oceanica were measured as 1115 kg m⁻¹ and 1.9 GPa, respectively, and measured for C. nodosa as 930 kg m⁻¹ and 1.5 GPa, respectively. Confidence intervals on the bulk moduli measurements were calculated using a Monte Carlo uncertainty simulation. The effect of leaf morphology and epiphytic coverage on the low-frequency (1 to 5 kHz) acoustic response of seagrass leaf-blades was also explored using a similar one-dimensional acoustic resonator. Variability in response was measured across three different length scales: between species, within species, and within individual leaves. These acoustic measurements are compared with microscopic images of transverse leaf cross-sections, measurements of leaf size, and estimates of aerenchyma void-fraction. A high-frequency (0.5 to 5 MHz) acoustic pulse transmission apparatus was developed to measure the variability in acoustic response along individual seagrass leaves. The system was modeled with a three-layer medium model to get estimates of transmission loss (TL) and phase speed c [subscript ph] in the leaves. In P. oceanica, TL abruptly decreased by up to 25 dB/mm and c [subscript ph] increased from as low as 0.4 to 0.95 times the sound speed in the surrounding water at 14 cm from the leaf base. In C. nodosa, while TL decreased and c [subscript ph] increased towards the apex of the blade, the changes were at a more consistent rate, unlike in P. oceanica.Item The acoustic ecology of submerged macrophytes(2011-12) Wilson, Christopher James, 1985-; Wilson, Preston S.; Dunton, Kenneth H.; Shank, G. Christopher; McClelland, James W.; Fuiman, Lee A.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.Item Assessing seagrass ecosystem status and condition : multi-scale applications of a long-term monitoring program(2021-04-29) Congdon, Victoria Marie; Dunton, Kenneth H.; Hardison, Amber K; Hall, Margaret O; McClelland, James WFluctuations in seagrass abundance and distribution often signify changes in abiotic conditions, including irradiance, temperature, salinity, and nutrient concentrations that can have long-term effects on coastal ecosystems. Stress responses by seagrasses trigger physiological effects that modify morphology and if conditions persist, alters structure at the meadow-scale. Since seagrasses are important indicators of ecosystem condition, long-term monitoring can help us identify factors that influence seagrass habitats through time and space. Such relationships between seagrass structure (i.e., plant physiology, architecture) and environmental conditions can better inform resource managers on the status and trends of seagrass ecosystems. This work investigates the applicability of long-term seagrass monitoring programs in Texas and Florida to: (1) estimate organic carbon stores along the Texas coast; (2) assess seagrass responses to a major disturbance in Texas; (3) characterize seagrass edge effects; and (4) evaluate the effectiveness of ecological indicators in identifying changes in seagrass condition within Florida and Texas. Areas with greater carbon stocks (up to 400 g C m⁻² in living biomass) corresponded to increases in cover and biomass; moreover, greater carbon storage capacity was associated with Thalassia testudinum, a more physically robust species (i.e., wide leaves, thick belowground tissues), than Halodule wrightii or Syringodium filiforme. Interestingly, despite the robust architecture of T. testudinum, this late successional species was more sensitive to a Category 4 hurricane than the prolific pioneer species (H. wrightii) as measured by greater reductions in cover and blade length in mixed (−16 vs. +1 %) and monospecific (−20 vs. +2 %) beds. We identified 11 metrics contributing to the dissimilarity between edge and interior habitats, with greater leaf widths, leaves shoot⁻¹, δ¹⁵N values, and epiphytes specific to edges. Importantly, many of the same metrics overlap as indicators for assessing ecological condition. Cover, shoot allometry and species composition were sensitive indicators of large-scale climatic disturbances (i.e., droughts, hurricanes). Our findings illustrate the breadth of long-term monitoring data in assessing differential responses to carbon stores and disturbances arising from distinct physiology and structure. Moreover, the use of common ecological indicators acquired from long-term monitoring programs highlight the prospect of broad-scale applications which can be used to develop seagrass management and conservation initiativesItem The effects of habitat fragmentation on the diversity of nekton inhabiting subtropical seagrass meadows(2011-05) Hensgen, Geoffrey Michael; Holt, G. Joan; Munguia, Pablo; Ojanguren, Alfredo F.; Stunz, Gregory W.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.Item Effects of post-settlement habitat use and biotic interactions on survival of the seagrass-associated fish red drum (Sciaenops ocellatus)(2009-12) Fencil, Megan Christine; Holt, J. (Joan); Dunton, Kenneth H.; Montagna, Paul A.; Rocha, Luiz A.; Rooker, Jay R.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.Item Impacts of human disturbances on seagrass communities in the Padre Island National Seashore(2004-12-18) Fellows, Kelly Ann; Dunton, Kenneth H.Human trampling effects, such as those from wade fishermen, on seagrasses and surrounding sediments were examined using experimental trampling lanes in the Padre Island National Seashore (PAIS) in South Texas. Six sites were established throughout three regions in PAIS (Bird Island, Yarborough Pass, and Nine-Mile Hole). At each site control, low intensity, and high intensity treatments were randomly assigned to one of three 2.5-m by 5-m trampling lanes. The plots were trampled monthly from April to June 2003 and then repeated at six different sites from September to December 2003. Water column, sediment, and seagrass characteristics were measured prior to initial trampling, and two, four, and 10 months after trampling initiation. Sites trampled in spring were also sampled 16 months after initial sampling event to assess long-term seagrass recovery. After four months of spring and fall trampling, reductions in Halodule wrightii percent cover were evident in all low and high intensity lanes except for one low intensity lane during the fall trampling. By the end of the spring and fall experiments, two low intensity and six high intensity lanes remained lower than the control lanes. Resistance (condition immediately after trampling), tolerance (condition 6-12 months after trampling) and resilience (recovery over a defined period after trampling) indices were calculated to compare trampling responses in the three regions. Seagrasses in Bird Island, where higher shoot densities were consistently measured, were the most resistant to trampling in the spring (59.0%) and fall (77.5%) experiments. Yarborough Pass seagrass beds were more resilient to trampling by the end of the spring (76.3%, month 16) and fall (98.8%, month 10). The lower resilience index in the spring combined with extremely high root:shoot ratios (10.0 to 79.2) at the start of the spring experiment suggest a seasonal response to trampling. However, no long-term effects were detected as the low and high intensity lanes in this region all returned to 100% H. wrightii cover by the end of the experiment. Seagrass response in Nine-Mile Hole was extremely variable and a complete seagrass die-off in all lanes (control, low intensity, high intensity) at one site in that region occurred during the spring experiment. This die-off could possibly be a result of ammonium toxicity. Excessive porewater ammonium concentrations were measured at this site throughout the entire experiment (352 to 1000 μM) and appear to contribute to the instability of this region. Despite the general trends detected in each region, variability in the data makes it difficult to conclude with certainty whether H. wrightii was or was not affected by trampling in PAIS. Natural variability inherent to the system can be more important in determining seagrass distribution than localized disturbancesItem Modeling the low-frequency response of seagrass in a resonator tube(2022-05-04) Torres, Nicholas Antonio; Wilson, Preston S.; Lee, Kevin M; Ballard, Megan SSeagrasses are a vital part of coastal ecosystems. They serve a number of important environmental functions, but are currently declining at rapid rates. A necessary requirement to preserving these resources is an ability to monitor them. Seagrass can strongly affect acoustic propagation in seawater, and this effect can be exploited to monitor these resources. Seagrass impacts the acoustic propagation through the release of bubbles from photosynthesis as well as gas contained within the seagrass tissue itself. Previous mathematical models that treat the seagrass tissue as a fluid have not sufficiently explained acoustic propagation through seagrass. In the present work, a Finite Element Method (FEM) model was used alongside data from the literature to determine if an elastic model for seagrass captures the effects it has on acoustic propagation. The results of this model were then compared against a recently-published measurement of the shear modulus of Thalassia testudinum. Additional resonator experiments were then used to inform refinements of the FEM model. The improved FEM model resolved a number of discrepancies in the initial model. Finally, the improved model was used with resonator measurements to infer the shear modulus of the two different types of tissue present in seagrass, the epidermis and aerenchyma tissue in T. testudinum.Item Nitrogen budget of the seagrass Thalassia testudinum in the western Gulf of Mexico(1998) Lee, Kun-seop, 1961-; Dunton, Kenneth H.The nitrogen (N) budget of the seagrass Thalassia testudinum was examined with respect to inorganic-N acquisition and the effects of sediment NH₄⁺ enrichment on two distinct populations in south Texas. The two populations exhibit different biomass allocation patterns at Corpus Christi Bay (CCB) and lower Laguna Madre (LLM): plants at CCB have a higher above-ground biomass while plants at LLM have a higher below-ground biomass. Ambient sediment pore water NH₄⁺ concentrations at CCB (ca. 100 μM) were significantly higher than at LLM (ca. 30 μM). Therefore, it was hypothesized that 1) differences in biomass allocation are a result of the differential sediment N availability, 2) sediment NH₄⁺ enrichment will affect growth, leaf morphology and tissue nutritional content of T. testudinum to a greater degree at low sediment N conditions, and 3) the relative contributions by leaf and root tissues to total N acquisition will differ between the two study sites. To examine the effects of sediment NH₄⁺ enrichment, the seagrass bed sediments were fertilized with commercial N fertilizer, and changes in production, biomass, leaf morphology, tissue nutritional content and carbon (C) reserves were monitored. Additionally, N uptake by leaves and roots of T. testudinum from the two sites were measured seasonally. After fertilization, leaf production rates and shoot height at LLM increased to reach levels equivalent to CCB. However, sediment NH₄⁺ enrichment had little effect on production and leaf size of T. testudinum at CCB. These results suggest that sediment N availability at LLM limits seagrass production. Rhizome non-structural carbohydrates (NSC) decreased in response to sediment NH₄⁺ enrichment during the early periods of the experiment which suggests that C was reallocated from rhizome to leaf tissues to support the stimulated leaf growth. Thus, the NH₄⁺ enrichment affected concentration and allocation of C as well as N. Root NH₄⁺ uptake accounted for about 52 % of total N acquisition, while leaf NH₄⁺ uptake contributed about 38 % and leaf NO₃⁻ uptake accounted for the remaining 10 % at both sites. The high biomass, chlorophyll, and C content in leaf tissues at CCB and the high biomass, C and NSC content in rhizome tissues at LLM demonstrated that plants responded to high sediment N conditions by enhancing leaf function, and to low N conditions by enhancing function of below-ground tissuesItem Patterns in iscoscapes and N:P stoichioscapes of the dominant seagrasses (Halodule wrightii and Thalassia testudinum) in the western Gulf of Mexico(2018-06-26) Cuddy, Meaghan Ruth; Dunton, Kenneth H.Seagrasses assimilate carbon (C), nitrogen (N), and phosphorus (P) from the water column and sediment, making their tissue nutrient content an excellent bioindicator of long-term, system-wide environmental conditions. I examined the role of seagrasses as ecological indicators of water quality and nutrient loading in three Texas estuarine systems through examination of their tissue isotopic (δ¹³C and δ¹⁵N) and stoichiometric (N:P) ratios over a period of five years using maps of spatial patterns in isotopic and stoichiometric regimes (“isoscapes” and stoichioscapes”). Leaf tissue samples were collected from the dominant Texas seagrasses, Halodule wrightii and Thalassia testudinum, at 567 stations during annual sampling between 2011 and 2015. Tissues were analyzed for C, N, and P content and C and N isotopic composition. Data were used to develop interpolated maps of variations in seagrass δ¹³C and δ¹⁵N signatures and N:P ratios in the Mission-Aransas NERR, Corpus Christi Bay, and upper and lower Laguna Madre. Regions where seagrasses had significantly enriched δ¹⁵N signatures, depleted δ¹³C signatures, or elevated N:P ratios were often associated with areas of urbanization or development. This was supported by significant relationships between δ¹⁵N and δ¹³C clusters and distance from outlets draining high population watersheds. I also documented a distinct temporal shift in δ¹³C signatures and N:P ratios across the study areas. The change in δ¹³C signatures was particularly notable in H. wrightii in 2015, when δ¹³C signatures became more depleted and N:P ratios were elevated, following an influx of freshwater and nutrients ending a three-year drought in south Texas. The spatial and temporal variation in seagrass tissue C content reported here reflects inputs of freshwater and riverine DIC as well as changes in the benthic light environment, while the N and P dynamics reported suggest that N:P ratios and δ¹⁵N signatures of seagrasses on the Texas coast are accurate bioindicators of nutrient loading in these estuaries. These metrics may thus serve as early indicators of changes in water quality.Item Patterns in seagrass coverage and community composition along the Texas coast : a three-year trend analysis(2015-05) Wilson, Sara Susan; Dunton, Kenneth H.; Buskey, Edward J.; Maidment, David R.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.Item Scientific analysis of Thalassia testudinum leaves(2022-08-12) Wilson, Nathan Gauntlett; Wilson, Preston S.; Ballard, Megan SSeagrass meadows are an important element of coastal ecosystems and they perform numerous environmental functions. Because of their importance, the ability to properly monitor the health of the seagrass meadows is imperative. Seagrass can greatly affect acoustic propagation and therefore acoustics can be used for remotely sensing the health of the meadow. To do so, an accurate model of sound propagation within seagrass is required. Knowledge of the volume of gas contained within seagrass leaves is a requisite part of the process. The first two chapters of this report describe the microscopic imaging, and image analysis, of seagrass leaves that were used in associated laboratory acoustic measurements. The last chapter of this report describes the preparation of an acoustic measurement apparatus, a one-dimensional resonator tube, that was used to measure the effective sound speed of seagrass tissue in water. The results themselves and discussion of the measurements are outside the scope of the present report but citations that direct the reader to those results are provided.Item Sediment characterization using in situ measurements of acoustic properties(2018-08-17) Dubin, Justin Thomas; Wilson, Preston S.; Ballard, Megan S.; Lee, Kevin Michael, 1977-Three related studies associated with the acoustics of marine sediments were performed and described here. The first study was a direct exploration of the microscopic properties of marine sediment collected in the field. High-resolution images, acquired through scanning electron microscopy, are presented alongside sediment analysis and in situ sound speed data in order to better understand the affect microscopic geometries have on acoustic propagation in naturally occurring marine sediment. This microscopy work is followed by a description of the apparatus, methodology, and results from a field experiment conducted in a shallow water seagrass-bearing environment in which the acoustic properties of both the seagrass canopy and underlying sediment were measured in situ. The results are compared to predictions from several effective medium models to help explain the observed propagation behavior. The final study in this thesis describes acoustic directivity characterization for the compressional wave transducer probes used to collect in situ data presented in the previous studies as well as for a re-designed pair of prototype probes. These laboratory measurements are compared to both analytical and finite element models.Item The impact of salinity diffusion, poroelasticity, and organic carbon in sediment acoustics(2019-05) Venegas, Gabriel Ricardo; Wilson, Preston S.; Hamilton, Mark R; Haberman, Michael R; Salamone, Salvatore; Lee, Kevin M; Ballard, Megan STo optimize the use of sound in waters on the continental shelf for naval, commercial, and environmental monitoring applications, the acoustic properties of the ocean bottom must be well understood. The effects of 1) pore water salinity variability on acoustic reflectivity, 2) poroelasticity on geoacoustic inference, and 3) organic carbon on sediment properties were formerly-considered insignificant in sediment acoustics, but due to advancements in other areas of underwater acoustics systems and modeling, have now become significant. Three separate but related studies were conducted to begin to quantify these effects. 1) A high-frequency acoustic reflection experiment was performed on a water-clay interface, while varying the salinity of the water. Results demonstrated significant changes in reflectivity at high incident angles, as well as a transient effect explained by a new coupled salt diffusion/reflection model. Using the model, the effective diffusion coefficient of salt in clay was inferred from the experiment, and reflectivity was then simulated at lower frequencies and longer time-scales. From this modeling effort, at a given time-scale of fluctuation, a characteristic frequency was identified, below which the reflectivity should not be assumed temporally invariant. 2) A model geoacoustic inference procedure was performed on a layered waveguide consisting of water and water-saturated glass beads contained within a glass tube. The resonance frequencies of the system were measured and compared with simulations of the experiment. Within each simulation, various sediment acoustics models were used. The only model that allowed for self-consistency between the inference and an independent set of high-frequency sound speed measurements, was a model that accounted for poroelastic effects. 3) A sediment constituent that has great value to the planet and is ubiquitous in natural marine sediment, organic carbon, has been ignored in sediment acoustics models. To begin to explore this relationship, sediment cores were extracted from a T. testudinum seagrass meadow in the Lower Laguna Madre, Texas, USA. A strong correlation between organic carbon and the primary-wave modulus was identified using a custom-built automated broad-band core and resonance logger and an elemental analyzer. The sediment properties attained from the cores were compared, and a theory explaining the correlations was developed. The acoustic sensitivity to organic carbon in a seagrass meadow has demonstrated promise toward developing an acoustic tool to more rapidly quantify marine organic carbon stores, which is needed in climate science. However, a larger-scale study is required to determine its applicability across a broader range of seagrass meadows and sediment typesItem Two studies on the acoustics of multiphase materials : seagrass tissue and encapsulated bubbles(2014-05) Enenstein, Gregory Robert; Wilson, Preston S.There are two focal points of this thesis: the acoustics of seagrass and the acoustical properties of encapsulated bubbles for underwater noise abatement. The acoustical properties of seagrass have applications in mine hunting, shallow water sonar, and environmental acoustic remote sensing. In order to optimize these applications, a predictive model of acoustic propagation in seagrass beds is sought. Previous laboratory research has indicated that the tissue acoustic properties of seagrass as well as the tissue physical structure and entrained air masses inside the leaves contribute to the overall acoustic behavior. The present research utilized a glass laboratory resonance tube to find the low frequency (1 kHz-4 kHz) acoustic compressibility of two species of seagrass, Thalassia testidinum and Halodule wrightii. By using a mixture of finely divided seagrass tissue suspended in seawater, the bulk moduli of the seagrass species were extracted. In the second section, encapsulated bubbles were analyzed as a method of abating underwater anthropogenic noise sources, since these sources, including marine piledriving and oil and gas exploration and production, pose potential harmful effects to marine life. Previous research, which used an array of rubber-shelled encapsulated bubbles, found the attenuation from these bubbles in be in close accordance with an existing encapsulated bubble model. Experiments were performed in a small laboratory resonance tank, a large outdoor acoustic tank, and at Lake Travis Test Station (LTTS) in order to determine the effects of varying an encapsulated bubble's wall thickness and fill material on bubble resonance frequency and damping. Results found that increasing the wall thickness tended to increase the balloon resonance frequencies measured in the small tank, which was strongly correlated to the frequency of maximum noise reduction in the large outdoor test tank and LTTS tests. The addition of polyester fibers and aluminum wool as fill materials decreased both the resonance frequency and quality factor, whereas helium-filled filled encapsulated bubbles had an increased resonance frequency but decreased quality factor as compared with air-filled bubbles. The resonance quality factor and void fraction further proved to affect the noise reduction near bubble resonance in the outdoor acoustic tank and LTTS tests. The measurements made with a single bubble in a small laboratory tank were correlated to measurements with a full-size system composed of many bubbles operating in open water.Item Understanding factors that control seagrass reproductive success in sub-tropical ecosystems(2014-08) Darnell, Kelly Marie; Dunton, Kenneth H.Seagrasses are submerged marine plants that provide essential ecosystem functions, but are declining in abundance worldwide. As angiosperms, seagrasses are capable of sexual reproduction, but also propagate asexually through clonal rhizome growth. Clonal growth was traditionally considered the primary means for seagrass propagation. Recent developments in genetic techniques and an increasing number of studies examining seagrass population genetics, however, indicate that sexual reproduction is important for bed establishment and maintenance. Few studies have investigated the reproductive biology and ecology of sub-tropical seagrass species, although this information is necessary for effective management and restoration. This work investigates the influence of pore-water nutrients on flowering, water flow on seed dispersal, consumption on seed survival, and describes the reproductive phenology in Texas for the two dominant seagrass species in the Gulf of Mexico: turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii). These species exhibit distinctive reproductive seasons that span summertime months, but reproductive output varies spatially and temporally. Results of an in situ nutrient enrichment experiment indicate that turtle grass produces fewer flowers (but more somatic tissue) when exposed to high pore-water ammonium than when exposed to low pore-water ammonium, suggesting that nutrient loading has the potential to reduce seagrass reproductive output. Seed consumption may also limit reproduction and recruitment in some areas, as laboratory feeding experiments show that several local crustaceans consume shoal grass and turtle grass seeds and seedlings, which do not survive consumption. Dispersal experiments indicate that seed movement along the substrate depends on local water flow conditions, is greater for turtle grass than shoal grass, and is related to seed morphology. Under normal water flow conditions in Texas, turtle grass secondary seedling dispersal is relatively minimal (< 2.1 m d⁻¹) compared to primary dispersal, which can be on the order of kilometers, and shoal grass secondary seed dispersal can be up to 1.1 m d⁻¹, but seeds are likely retained in the parent meadow. Results from this work can be used when developing seagrass management, conservation and restoration actions and provide necessary information concerning a life history stage whose importance was historically under-recognized.Item What defines an edge? : quantifying edge effects across multiple trophic levels(2020-08-14) Estrada, Jenelle Marie; Erdner, Deana L.Habitat fragmentation is often cited as a primary driver of biodiversity loss across biomes. Spatial habitat fragmentation causes an increase in edge habitat relative to interior which may provide different value as they often vary in functionality. There is a general lack of consensus regarding the distance that delineates edge versus interior limiting the ability to accurately capture edge effects on seagrass ecological function. As fragmentation is accelerating, it is necessary to understand the factors driving edge effects to better predict the effect of changes in habitat configuration. My work measured community metrics in seagrass ecosystems at varying distances from the edge to understand edge effects driving organismal response to habitat fragmentation. First, I sampled seagrass beds in 2018 for a post-disturbance study one year after Hurricane Harvey hit Texas. I sampled along seagrass patch edges and interior to quantify seagrass morphometrics and biomass, benthic macrofauna and nekton communities. Next, I conducted a fine-scale study in 2019 sampling seagrass beds along 10 meter transects from the edge in towards bed interiors in the same region to quantify nekton and macrofaunal communities, seagrass metrics, predation risk, and flow at a precise scale. Across both studies, I found increased seagrass biomass in habitat interiors that was most pronounced around 2 meters in one study. I saw higher temperature and epiphyte biomass with increased distance from seagrass edge in the fine-scale study and across studies there were no demonstrated edge effects on benthic macrofaunal community composition, predation risk or flow. There was a positive effect on nekton diversity in the post-disturbance study driven by matrix spillover of species associated with soft muddy bottoms. Results suggest pronounced edge effect for seagrass itself, and ecologically defined edges may be larger than the edge sizes assumed in many past studies highlighting the need for edge designations to be scaled to the response variables tested in order to accurately capture edge effects. Multi-trophic and continued empirical measurement of habitat edges may help resolve differential responses across studies and predict the effects of anthropogenic habitat fragmentation on the ecological function of critical nearshore habitat like seagrass