Subtropical Seagrass Meadows Research Articles

The unique biogeochemical role of carbonate-associated organic matter in a subtropical seagrass meadow

The particulate organic matter buried in carbonate-rich seagrass ecosystems is an important blue carbon reservoir. While carbonate sediments are affected by alkalinity produced or consumed in seagrass-mediated biogeochemical processes, little is known about the corresponding impact on organic matter. A portion of particulate organic matter is carbonate-associated organic matter. Here, we explore its biogeochemistry in a carbonate seagrass meadow in central Florida Bay, USA. We couple inorganic stable isotope analyses (δ34S, δ18O) with a molecular characterization of dissolved and carbonate associated organic matter (21 tesla Fourier-transform ion cyclotron resonance mass spectrometry). We find that carbonate-associated molecular formulas are highly sulfurized compared to surface water dissolved organic matter, with multiple sulfurization pathways at play. Furthermore, 97% of the formula abundance of surface water dissolved organic matter is shared with carbonate-associated organic matter, indicating connectivity between these two pools. We estimate that 9.2% of the particulate organic matter is carbonate-associated, and readily exchangeable with the broader aquatic system as the sediment dissolves and reprecipitates.

Dynamics of methane emissions from northwestern Gulf of Mexico subtropical seagrass meadows

While seagrass meadows are perceived to be pertinent blue carbon reservoirs, they also potentially release methane (CH4) into the atmosphere. Seasonal and diurnal variations in CH4 emissions from a subtropical hypersaline lagoon dominated by Halodule wrightii in southern Texas, USA, on the northwest coast of the Gulf of Mexico were investigated. Dissolved CH4 concentrations decreased in the daytime and increased overnight during the diel observation period, which could be explained by photosynthesis and respiration of seagrasses. Photosynthetic oxygen was found to significantly reduce CH4 emissions from seagrass sediment. Diffusive transport contributed slightly to the release of CH4 from the sediment to the water column, while plant mediation might be the primary mechanism. The diffusive CH4 flux at the sea-air interface was 12.3–816.2 µmol/m2 d, over the range of the sea-air fluxes previously reported from other seagrass meadows. This was related to relatively higher dissolved CH4 concentrations (11.6–258.2 nmol/L) in a mostly closed lagoon with restricted water exchange. This study emphasizes seagrass meadows in the subtropical hypersaline lagoon as a source of atmospheric CH4, providing insights into the interactions between seagrass ecosystems and methane dynamics, with potential implications for seagrass meadow management and conservation efforts.

Herbivory Has a Major Influence on Structure and Condition of a Great Barrier Reef Subtropical Seagrass Meadow

Grazing by all members of an herbivore community can act to structure the ecosystems they feed on. The outcome of this grazing pressure on the plant community also depends on the interaction between different herbivore groups that are present. We carried out a three-month multi-level field exclusion experiment to understand how different groups of herbivores act both individually and interactively to structure a subtropical seagrass meadow in the Great Barrier Reef. Megaherbivore grazing had the largest impact on this seagrass meadow, significantly reducing aboveground biomass and shoot height, whereas there was no measurable impact of meso- or macroherbivores on seagrass metrics or epiphyte biomass. Megaherbivores here grazed broadly across the meadow instead of targeting grazing in one area. The principal seagrass-herbivore dynamic in this meadow is that megaherbivores are the main group modifying meadow structure, and other grazer groups that are present in lower numbers do not individually or interactively structure the meadow. We demonstrate that herbivory by large grazers can significantly modify seagrass meadow characteristics. This has important implications when designing and interpreting the results of monitoring programs that seek to conserve seagrass meadows, the ecosystem services that they provide and the herbivores that rely on them. Collectively, our results and those of similar previous studies emphasize there is unlikely to be one seagrass and herbivory paradigm. Instead, for individual meadows, their unique species interactions and differences in biotic and abiotic drivers of seagrass change are likely to have a strong influence on the dominant seagrass-herbivore dynamic.

Water temperature control on CO2 flux and evaporation over a subtropical seagrass meadow revealed by atmospheric eddy covariance

AbstractSubtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2 exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 ± 0.17 μmol m−2 s−1 (x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2 flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2 exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer.

Feeding habits of juvenile Siganus fuscescens in a subtropical seagrass meadow estimated by rearing experiments and dual stable isotope analysis

Feeding habits of juvenile <i>Siganus fuscescens</i> in a subtropical seagrass meadow estimated by rearing experiments and dual stable isotope analysis

Net heterotrophy and carbonate dissolution in two subtropical seagrass meadows

Abstract. The net ecosystem productivity (NEP) of two seagrass meadows within one of the largest seagrass ecosystems in the world, Florida Bay, was assessed using direct measurements over consecutive diel cycles during a short study in the fall of 2018. We report significant differences between NEP determined by dissolved inorganic carbon (NEPDIC) and by dissolved oxygen (NEPDO), likely driven by differences in air–water gas exchange and contrasting responses to variations in light intensity. We also acknowledge the impact of advective exchange on metabolic calculations of NEP and net ecosystem calcification (NEC) using the “open-water” approach and attempt to quantify this effect. In this first direct determination of NEPDIC in seagrass, we found that both seagrass ecosystems were net heterotrophic, on average, despite large differences in seagrass net above-ground primary productivity. NEC was also negative, indicating that both sites were net dissolving carbonate minerals. We suggest that a combination of carbonate dissolution and respiration in sediments exceeded seagrass primary production and calcification, supporting our negative NEP and NEC measurements. However, given the limited spatial (two sites) and temporal (8 d) extent of this study, our results may not be representative of Florida Bay as a whole and may be season-specific. The results of this study highlight the need for better temporal resolution, accurate carbonate chemistry accounting, and an improved understanding of physical mixing processes in future seagrass metabolism studies.

The seagrass skeletal assemblage from modern to fossil and from tropical to temperate: Insight from Maldivian and Mediterranean examples

AbstractSeagrasses are marine angiosperms that form extensive submarine meadows in the photic zone where carbonate producing biota dwell as epiphytes on the leaves or as infaunal forms, and act as prolific carbonate sediment factories. Because seagrasses have a low preservation potential and records of exceptionally well‐preserved and plant material from marine settings are rare, these palaeoenvironments are difficult to identify in the rock record. Consequently, sedimentological and palaeontological proxies are the main indicators of the presence of seagrass‐dominated ecosystems. This work investigates the skeletal assemblage of Modern (Maldivian and western Mediterranean) and fossil (Eocene; Apula and Oman carbonate platforms and Oligocene; Malta platform) seagrass examples to characterize the skeletal assemblage of modern and fossil seagrasses. Two main types of grains, calcareous algae and foraminifera, constitute around 50% of the bioclastic sediment in both tropical Maldivian and temperate Mediterranean scenarios. However, in the tropical setting they are represented by green algae (Halimeda), while in the Mediterranean they are represented by corallinacean red algae. In contrast, in the Eocene examples, the foraminifera are the most conspicuous group and the green algae are also abundant. The opposite occurs in the Maltese Chattian, which is dominated by coralline algae (mean 42%), although the foraminifera are still abundant. It is suggested to use the term foralgal to identify the seagrass skeletal assemblage. To discriminate between red algae and green algae dominance, the introduction of the prefixes ‘GA’ (green algae) and ‘RA’ (red algae) is proposed. The investigated examples provide evidence that the green algae–foralgal assemblage is typical of tropical, not excessively dense seagrass meadows, characterized by a well‐illuminated substrate to support the development and calcification of the Halimeda thallus. Contrarily, the red algae‐foralgal assemblage is typical of high density tropical to subtropical seagrass meadows which create very dense oligophotic conditions on the sea floor or in temperate settings where Halimeda cannot calcify.

Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15NN2 tracer method

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 605:87-101 (2018) - DOI: https://doi.org/10.3354/meps12755 Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15N-N2 tracer method Natasha L. Carlson-Perret*, Dirk V. Erler, Bradley D. Eyre Centre for Coastal Biogeochemistry, School of Environmental Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia *Corresponding author: natasha.carlson-perret@scu.edu.au ABSTRACT: This study used a direct 15N stable isotope labelling technique to measure rates of dinitrogen (N2) fixation within above- and belowground loci of a subtropical seagrass meadow (Zostera muelleri). The total rate of N2 fixation (i.e. sum of the above- and belowground rates) was ~38 µmol N2 m-2 h-1, similar to other rates measured in subtropical systems. Rates of N2 fixation were higher when the 15N-N2 label was added to the surface water compared to when it was added to the sediments. Furthermore, the lowest rates of N2 fixation were observed in the root/rhizome material regardless of whether the 15N-N2 label was added directly to the rhizosphere (0.12 µmol N2 m-2 h-1) or the overlying water column (7.3 µmol N2 m-2 h-1). These results suggest that there was active transport of fixed N from the leaves to the roots of the seagrass plants, in contrast to other studies in which N2 fixation was more active in the rhizosphere. Our study demonstrates the utility of the direct 15N-N2 tracer approach for quantifying the spatial heterogeneity of N2 fixation in complex seagrass environments. KEY WORDS: Nitrogen fixation · Stable isotope tracer · Zostera muelleri Full text in pdf format Supplementary material PreviousNextCite this article as: Carlson-Perret NL, Erler DV, Eyre BD (2018) Dinitrogen (N2) fixation rates in a subtropical seagrass meadow measured with a direct 15N-N2 tracer method. Mar Ecol Prog Ser 605:87-101. https://doi.org/10.3354/meps12755 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 605. Online publication date: October 26, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Seasonal variations in leaf growth of Cymodocea serrulata in subtropical seagrass meadows

We estimated the leaf growth of the tropical–subtropical seagrass species Cymodocea serrulata using the conventional leaf-marking method in a subtropical seagrass bed on Ishigaki Island, southern Japan, which is near the northern distributional limit of this species. This survey was conducted over an annual cycle at two sites. The deep site was located in a dense meadow in the subtidal zone. The shallow site was located around the lowest part of the intertidal zone, forming a tide pool during ebb tides of the spring tide, which was considered to approximate the upper depth limit of C. serrulata. The relationships between temperature and shoot weight, and the number of leaves per shoot were significantly influenced by site, resulting in a lower biomass in the shallow site. Leaf growth and shoot length increased with temperature without any spatial variation. Leaf growth and the number of leaves per shoot particularly declined during periods of low temperature in both sites. These results suggest that leaf growth and phenology of C. serrulata in this subtropical area are largely regulated by temperature.

Vertical accretion and carbon burial rates in subtropical seagrass meadows increased following anthropogenic pressure from European colonisation

Abstract: Seagrass meadows serve as marine carbon sinks, though rates of carbon burial and sediment accretion may vary within and among seagrass meadows due to spatial and temporal variability in sedimentary and biophysical conditions. However, few data are available to evaluate variation in carbon burial over decades to centuries, particularly in subtropical seagrass meadows. Here we assess historical trends in rates of vertical accretion and carbon burial in subtropical seagrass sediments in Moreton Bay, Australia. A total of 19 sediment cores varying in length from 22 to 205 cm were collected from six locations, including two locations where seagrass meadows occurred historically but were lost in recent decades. Cores were dated using 210Pb and 14C. Geochronologies were modelled based on the successful 210Pb dates for two of the locations combined with 14C at five locations. Organic and inorganic carbon content, dry bulk density and stable carbon and nitrogen isotope composition (δ13Corg, δ15N) were measured throughout the cores. Sediments underlying seagrass meadows in Moreton Bay have organic carbon stored up to a depth of at least 2 m which dates to over 7000 yr BP. Across the entire time series, organic carbon burial rates averaged 19 g m−2 yr−1 and inorganic carbon burial rates averaged 27 g m−2 yr−1. Overall mean rates of vertical accretion were estimated to be 0.2 cm yr−1. Rates of vertical accretion accelerated 10-fold following European colonisation which commenced in 1824, from 0.06 ± 0.06 cm yr−1 to 0.66 ± 1.19 cm yr−1. Similarly, rates of organic carbon burial averaged 7.2 ± 5.5 g m−2 yr−1 but then increased 7-fold post-colonisation to 50.5 ± 82.1 g m−2 yr−1. Thus, in Moreton Bay, European settlement and associated land-use change has enhanced sediment accretion and carbon burial in seagrass sediments.

Evaluation of Relationships Between Cover Estimates and Biomass in Subtropical Seagrass Meadows and Application to Landscape Estimates of Carbon Storage

Seagrass habitats provide invaluable ecosystem services, including the ability to sequester carbon through primary production. In the absence of significant grazing pressure, changes in seagrass biomass reflect net photosynthetic activity in response to the light environment. We developed linear models to convert a commonly measured parameter, percent cover, into above- and below-ground biomass for three tropical seagrass species, Thalassia testudinum, Halodule wrightii and Syringodium filiforme . Linear models were estimated from the analysis of 126 biomass cores collected at five levels of percent cover from three coastal bays in Texas. The models showed an increase in aboveground biomass with increased percent cover (R2 = 0.52–0.77), but belowground biomass displayed greater variability with respect to percent cover (R2 = 0.15–0.63). Such linear biomass models can provide valuable estimates of carbon storage potential in living seagrass tissues using monitoring data collected across large spatial scales within the Gulf of Mexico. Resumen: Los habitats de hierba marina proporcionan servicios inestimables de los ecosistemas, incluyendo la capacidad de secuestrar el carbono a traves de la produccion primaria. En ausencia de una presion de pastoreo significativa, los cambios en la biomasa de los pastos marinos reflejan la actividad fotosintetica neta en respuesta a la luz ambiental. Se desarrollaron modelos lineales para convertir un parametro comunmente medido, por ciento de cobertura, en biomasa subterranea y subterranea para tres especies de pasto marino tropical, Thalassia testudinum, Halodule wrightii y Syringodium filiforme. Los modelos lineales se estimaron a partir del analisis de 126 nucleos de biomasa recolectados en cinco niveles de cobertura porcentual de tres bahias costeras en Texas. Los modelos mostraron un aumento de la biomasa aerea con mayor cobertura porcentual (R2 = 0,52–0,77), pero la biomasa subterranea mostro mayor variabilidad con respecto al porcentaje de cobertura (R2 = 0,15–0,63). Tales modelos de biomasa lineal pueden proporcionar valiosas estimaciones del potencial de alma-cenamiento de carbono en los tejidos vivos de los pastos marinos usando datos de monitoreo recolectados a traves de grandes escalas espaciales dentro del Golfo de Mexico.

Impacts of physical disturbance on ecosystem structure in subtropical seagrass meadows

Impacts of physical disturbance on ecosystem structure in subtropical seagrass meadows

Microbial community structure and dynamics in restored subtropical seagrass sediments

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 74:43-57 (2015) - DOI: https://doi.org/10.3354/ame01725 Microbial community structure and dynamics in restored subtropical seagrass sediments Amanda S. Bourque1,2,*, Rebecca Vega-Thurber3, James W. Fourqurean2 1Habitat Restoration Program, Biscayne National Park, National Park Service, 9700 SW 328 Street, Homestead, Florida 33033, USA 2Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, Miami, Florida 33199, USA 3Department of Microbiology, Oregon State University, Corvallis, Oregon 97331, USA *Corresponding author: amanda_bourque@nps.gov ABSTRACT: Microorganisms in seagrass sediments facilitate many key ecosystem processes, yet current knowledge of microbial facilitation of seagrass community recovery following disturbance or restoration is limited. We studied microbial community responses to restoration of a subtropical seagrass meadow disturbed by vessel groundings in south Florida, USA, and relationships between microbial communities and sediment properties at the study sites using terminal restriction fragment length polymorphism. Two restoration methods were evaluated: the installation of bird roosting stakes as a means to provide a nutrient source, and placement of sand fill into excavations to prevent erosion. Both disturbed and restoration sites had less complex microbial community structure than undisturbed reference seagrass sediments. Microbial community structure varied little between disturbed and fertilized sites, but was distinct in filled sites. Sediment bulk density, sediment organic matter and total phosphorus content, porewater ammonium, soluble reactive phosphorus, and dissolved sulfide concentrations were important environmental predictors of microbial community structure across the restoration treatments. We show that community structure and diversity varied with sediment depth, among restoration treatments, and through time. Our results indicate that microbial communities in seagrass meadows are changed by physical disturbance of the rhizosphere, and that common restoration techniques lead to the formation of distinct microbial communities during the first year of recovery. KEY WORDS: Microbial diversity · Restoration · Disturbance · Soil structure · Biscayne National Park Full text in pdf format PreviousNextCite this article as: Bourque AS, Vega-Thurber R, Fourqurean JW (2015) Microbial community structure and dynamics in restored subtropical seagrass sediments. Aquat Microb Ecol 74:43-57. https://doi.org/10.3354/ame01725 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 74, No. 1. Online publication date: January 12, 2015 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2015 Inter-Research.

Net uptake of atmospheric CO 2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air-sea CO2 fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air-sea CO2 flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2. This finding is contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO2 may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2.

Effects of common seagrass restoration methods on ecosystem structure in subtropical seagrass meadows

Seagrass meadows near population centers are subject to frequent disturbance from vessel groundings. Common seagrass restoration methods include filling excavations and applying fertilizer to encourage seagrass recruitment. We sampled macrophytes, soil structure, and macroinvertebrate infauna at unrestored and recently restored vessel grounding disturbances to evaluate the effects of these restoration methods on seagrass ecosystem structure. After a year of observations comparing filled sites to both undisturbed reference and unrestored disturbed sites, filled sites had low organic matter content, nutrient pools, and primary producer abundance. Adding a nutrient source increased porewater nutrient pools at disturbed sites and in undisturbed meadows, but not at filled sites. Environmental predictors of infaunal community structure across treatments included soil texture and nutrient pools. At the one year time scale, the restoration methods studied did not result in convergence between restored and unrestored sites. Particularly in filled sites, soil conditions may combine to constrain rapid development of the seagrass community and associated infauna. Our study is important for understanding early recovery trajectories following restoration using these methods.

Amphipod control of epiphyte load and its concomitant effects on shoalgrass Halodule wrightii biomass

We investigated the interaction between epiphyte-grazer abundance and eutrophi- cation to assess the relative importance of top-down and bottom-up effects in subtropical seagrass meadows. In field experiments using a cageless technique to control amphipod abundance, we measured the effects of grazing and nutrient supply on the growth and productivity of shoalgrass Halodule wrightii and its epiphytes at both protected and wave exposed sites. Amphipod removal at the protected site resulted in 70% greater epiphyte loads on shoalgrass leaves and a 36% reduction in leaf biomass after 10 wk. At the wave-exposed site, where amphipod abundance was consistently low, we found no significant effects of grazer presence or nutrients on epiphyte load or leaf biomass. Average leaf length, however, was significantly reduced in nitrogen-enriched plots. Our results indicate that natural densities of amphipods can reduce seagrass loss by control- ling epiphytes.

Leaf Nutrient Resorption of Seagrass &lt;i&gt;Zostera japonica&lt;/i&gt; Aschers. et Graebn. in Beihai, Guangxi, China

Resorption of nutrients (C, N, P, Ca, Mg, Fe and Mn) from senescent leaves of Zostera japonica and the concentration of nutrients (C, N, P, Ca, Mg, Fe and Mn) in the sediment from three subtropical seagrass meadows (Guchenling, Zhulin and Ronggenshan) in Beihai, Guangxi, China were studied. Resorption efficiencies were quantitatively assessed by comparing nutrient content between the leaf with the highest content and the senescent leaves. In addition, the relationship between nutrient resorption efficiencies and nutrient sources (the sediment and the adult leaf) were evaluated. During the course of leaf development in Z. japonica (young leaf – adult leaf – senescent leaf), C content first rose and then declined; N and P contents gradually declined; but Ca, Mg, Fe and Mn gradually rose. During leaf senescence, resorption efficiencies of C, N, P, Ca, Mg, Fe and Mn were 2.1%, 19.5%, 45.1%, −61.1%, −12.0%, −27.1% and −72.3%, respectively. For Z. japonica, C, N and P contributed to new leaf growth, whereas Ca, Mg, Fe and Mn was not. The nutrient contents of the sediment and adult leaf were not significantly correlated to leaf nutrient resorption efficiencies. It was calculated that for Z. japonica approximately 13.8% of total N demand and 31% of total P demand could be met by internal resorption.

Carbon, nitrogen and phosphorus storage in subtropical seagrass meadows: examples from Florida Bay and Shark Bay

Seagrass meadows in Florida Bay and Shark Bay contain substantial stores of both organic carbon and nutrients. Soils from both systems are predominantly calcium carbonate, with an average of 82.1% CaCO3 in Florida Bay compared with 71.3% in Shark Bay. Soils from Shark Bay had, on average, 21% higher organic carbon content and 35% higher phosphorus content than Florida Bay. Further, soils from Shark Bay had lower mean dry bulk density (0.78 ± 0.01 g mL–1) than those from Florida Bay (0.84 ± 0.02 mg mL–1). The most hypersaline regions of both bays had higher organic carbon content in surficial soils. Profiles of organic carbon and phosphorus from Florida Bay indicate that this system has experienced an increase in P delivery and primary productivity over the last century; in contrast, decreasing organic carbon and phosphorus with depth in the soil profiles in Shark Bay point to a decrease in phosphorus delivery and primary productivity over the last 1000 y. The total ecosystem stocks of stored organic C in Florida Bay averages 163.5 MgCorg ha–1, lower than the average of 243.0 MgCorg ha–1 for Shark Bay; but these values place Shark and Florida Bays among the global hotspots for organic C storage in coastal ecosystems.

An evaluation of chemical and physical defenses against fish predation in a suite of seagrass-associated ascidians

The palatability of two solitary and three colonial species of ascidians commonly found in sub-tropical seagrass meadows was evaluated using the abundant, sympatric, omnivorous pinfish Lagodon rhomboides as a model predator. Bite-sized pieces of fresh tissues of both solitary and one of the three colonial ascidian species were unpalatable to fish. Lipophilic and hydrophilic extracts of the three unpalatable species did not cause feeding deterrence indicating that secondary metabolites are not responsible for the lack of palatability. Distaplia bermudensis, the one colonial ascidian that was unpalatable to fish, had a highly acidified outer tunic (pH=1.5). We tested the ability of acidified agar food pellets (pH=1.5) to deter pinfish and found that the fish readily ingested acidified pellets. The toughness of the tunic of all five ascidian species was evaluated by measuring the Force (N) required to penetrate the tunic using a penetrometer. Tunic toughness is likely to explain the lack of palatability of the solitary ascidians Styela plicata and Molgula occidentalis as their tunics required a force of >34N to penetrate. Tunic toughness may be a particularly effective adaptation for ascidian defense in seagrass habitats where fish with strong crushing jaws, such as those that commonly occur in coral reef systems, are rare.

The Effects of Boat Propeller Scarring on Nekton Growth in Subtropical Seagrass Meadows

AbstractAn increasing boating population has led to extensive propeller scarring in many shallow seagrass meadows, and research has focused on relating scarring to nekton abundance; however, little information exists on the impacts on habitat functionality. In this study we moved beyond simple measures of faunal density as an indicator of habitat quality by comparing the growth rates of common estuarine nekton in different levels of propeller scarring in Redfish Bay, Texas. Growth rates of selected fauna were examined by using field enclosures and otolith microstructure analysis. Otolith microstructure analysis on pinfish Lagodon rhomboides indicated no difference in growth rates at various scarring intensities. We conducted field growth enclosure experiments on a common decapod crustacean, the white shrimp Litopenaeus setiferus. White shrimp showed significantly lower growth in highly scarred areas than in reference sites. These results suggest that regions of low‐level propeller scarring (less than 15%) may have little effect on small‐scale habitat quality. However, higher levels of propeller scarring may affect habitat quality; therefore, more information is needed to characterize the large‐scale effects of propellers at higher scarring intensities.