Seagrass Thalassia Testudinum Research Articles

Reduced seagrass resilience due to environmental and anthropogenic effects may lead to future die-off events in Florida Bay

Coastal systems often depend on foundation species such as seagrasses that are supported by self-facilitation. Seagrass meadows, however, are threatened worldwide due to climate change and local human impact, disrupting self-facilitation leading to system instability. Florida Bay is a large seagrass dominated coastal ecosystem that suffered from multiple seagrass mortality events over the last half century driven by hypoxia, high water temperatures, hypersalinity, and high biological oxygen demand. These conditions reduce the amount of photosynthetically-derived oxygen in the plant causing sulphide intrusion into meristematic tissues. Using a bay-wide sampling design and long-term monitoring trends of seagrass condition, we investigated the current state of the meadows, sediment characteristics (e.g., organic matter, sulphide, nutrients) and discuss how climate stressors interact with plant and sediment oxygen dynamics. Our survey revealed that at sites where seagrass had been previously denuded by die-off, the dominant seagrass Thalassia testudinum had not recovered, while the pioneering seagrass Halodule wrightii recolonized the impacted areas. Organic matter and sulphide levels were higher at the impacted sites, apparently a persistent characteristic of the formerly dense T. testudinum meadows in central and western Florida Bay. These sediment conditions promote sulphide intrusion of T. testudinum belowground tissue under anoxic conditions. Plant oxidation initially buffers sulphide intrusion, but disruption of this oxidation mechanism due to changing environmental conditions results in widespread mortality and seagrass community collapse. Climate change cannot be fully mitigated by local management, however, attempts can be made to control critical salinity and oxygen levels by increasing freshwater input, reducing hypersalinity and aiming to keep the internal seagrass oxidation feedback intact. Our study shows that the Florida Bay seagrass ecosystem is still recovering four years post die-off and continues to be susceptible to future climate change and system degradation.

Coralline Algal Population Explosion in an Overgrazed Seagrass Meadow: Conditional Outcomes of Intraspecific and Interspecific Interactions

Interactions such as mutualism and facilitation are common in ecosystems established by foundation species; however, their outcomes vary and show conditionality. In a Mexican Caribbean Bay, a seagrass-coralline algae (rhodoliths) mutualism protects the seagrass Thalassia testudinum from green turtle overgrazing. We postulate that the state of the seagrass meadow in this bay depends on the strengths of the interactions among seagrasses, green turtles, and coralline algae. Spatio-temporal changes through satellite imagery showed rhodolith bed developed rapidly from 2009 (undetected) to 2016 (bed of 6934 m2). Typically, such rapid expansion of the rhodoliths does not occur in seagrass meadows. An in situ growth experiment of coralline algae showed that a combination of reduction in light and wave movement (usual in dense seagrass meadows) significantly reduced their growth rates. In the rhodolith beds, the growth rates of the coralline algae Neogoniolithon sp. and Amphiroa sp. were high at 9.5 mm and 15.5 mm per growth tip y−1, respectively. In a second experiment, we found lower mortality in coralline algae within a rhodolith bed compared to algae placed outside the bed, likely explained by the reduced resuspension that we found in a third experiment, and this positive feedback may explain the high population increase in the rhodoliths, once established when the turtles grazed down the seagrass canopy. Therefore, the grazing-protection mutualism between seagrasses and coralline algae is thus conditional and came into existence under a co-occurrence of intensive grazing pressure and rapid population growth of coralline algae facilitated by positive feedback from increased growth and reduced sediment resuspension by the dense rhodolith bed.

Warming and grazing independently and interactively impact plant defenses and palatability

The ecological impacts of multiple stressors are hard to predict but important to understand. When multiple stressors influence foundation species, the effects can cascade throughout the ecosystem. Gulf of Mexico seagrass ecosystems are currently experiencing a suite of novel stressors, including warmer water temperatures and increased herbivory due to tropicalization and conservation efforts. We investigated the impact of warming temperatures and grazing history on plant performance, morphology, and palatability by integrating a mesocosm study using the seagrass Thalassia testudinum with feeding trials using the sea urchin Lytechinus variegatus. Warming temperatures negatively impacted T. testudinum tolerance traits, reducing belowground biomass by 34%, productivity by 74%, shoot density by 10%, and number of leaves per plant by 24%, and negatively impacted resistance traits through 13% lower toughness of young leaves and a trend for reduced leaf carbon:nitrogen. Lytechinus variegatus individuals preferred to consume plants grown under heated conditions, which supports findings of enhanced palatability. Simulated turtle grazing impacted more plant traits than grazing by other herbivores, potentially diminishing plant resilience to future disturbances through reduced rhizome non‐structural carbohydrate concentrations and increasing palatability through reduced fiber content and 23% lower leaf carbon:phosphorus. Simulated turtle, simulated parrotfish, and urchin grazing reduced leaf carbon:nitrogen by 11%, also potentially increasing nutritive value. Interactions between warming temperatures and grazers on plant traits were additive for 16 out of 19 response variables. However, the stressors non‐additively impacted the number of leaves per plant, fiber content, and epiphyte load. We suggest that the impacts of grazers on leaf turnover rate and leaf age may vary based on water temperature, potentially driving these interactions. Overall, increased temperatures and grazing pressure will likely reduce seagrass resilience, structure, and biomass, potentially impacting feedback systems and producing negative consequences for seagrass cover, associated species, and ecosystem services.

Epiphytic diatom production on the seagrass Thalassia testudinum in northern Gulf of Mexico

Epiphytic diatom production on the seagrass Thalassia testudinum in northern Gulf of Mexico

Food selection and habitat use patterns of immature green turtles (Chelonia mydas) on Caribbean seagrass beds dominated by the alien species Halophila stipulacea

Marine herbivores face rapid changes in the coastal ecosystems where they forage. In the Caribbean, the recent and fast expansion of the invasive phanerogam species Halophila stipulacea is threatening native seagrass ecosystems. So far, H. stipulacea is escaping most Caribbean herbivores, certainly because of its recent introduction or lower nutritional value. We investigated the impact of H. stipulacea invasion on fine-scale foraging habitat selection and food resource selection of immature green turtles at critical foraging sites in Les Anses d′Arlet, Martinique. The analysis of seagrass distribution and nutritional content, together with turtle behaviour and resource selection, showed that H. stipulacea may be of contrasting interest to green turtles. Compositional analysis confirmed the lower nutritional value of H. stipulacea compared to the native species, but the invasive species showed higher digestibility than native ones, which calls into question the energetic advantage of consuming the native plants over the exotic plant. Thus, although green turtles mostly selected the native seagrass Thalassia testudinum in multispecies seagrass beds, some individuals fed on H. stipulacea . Accordingly, in bays entirely invaded by H. stipulacea , one possibility for resident green turtles is to increase foraging on this species, but, if so, the consequences on their growth and survival still remain to be determined. As the expansion of H. stipulacea may have been facilitated by factors such as shipping, anchor scarring and fishing activities, protection of native seagrass beds and immature green turtles from human disturbances is urgently required to ensure the long-term adaptation of green turtles to this new foraging environment. • High variability of diet composition in immature green turtles in Martinique. • Turtles preferentially selected T. testudinum in multi-species seagrass beds. • H. stipulacea may represent 0–100% of the diet whatever native plants availability. • H. stipulacea has lower nutritional value but higher digestibility than native plants. • Human activities promote H. stipulacea development to the detriment of native species.

Hypersalinity effects on O2 flux across the diffusive boundary layer of leaves in the tropical seagrass Thalassia testudinum

Hypersalinity is a major stressor to seagrasses, particularly in highly evaporative coastal estuaries and lagoons as well as those subjected to brine effluent from desalination systems, a condition likely to be heightened under a warming climate. While hypersalinity has been well established to cause physiological dysfunction, the effects on internal seagrass pO2 and O2 flux to the water column has not been explicitly studied. We examined the pO2 across the leaf diffusive boundary layer and estimated O2 flux rates from T. testudinum leaves at 35, 45, 55, and 65 salinity in the light and dark using O2 microsensors. Leaf segments were used to examine photophysiological responses and leaves of shoots in intact cores with multiple shoots to evaluate internal leaf pO2 flux that incorporates whole-plant sinks. Steady-state pO2 levels internally and at the leaf surface were also quantified at light saturation and in the dark. We found hypersalinity (45–65) primarily affected the O2 dynamics of T. testudinum leaves in the light based on leaf segments and whole plants. Net and gross photosynthetic rates of leaf segments were significantly lower at 55 and 65 salinities compared to controls (35), and net photosynthesis at 65 salinity was significantly lower than at 55 salinity. In contrast, the light required to compensate for leaf respiration (Icomp) was not affected by hypersalinity, and photosynthetic efficiency was only significant between 35 and 55 salinities above compensation irradiances (Φ(I comp – 200)), suggesting lesser effects of hypersalinity on photosynthesis at low irradiance below compensation (0–50 μmol photon m−2 s−1). We propose that hypersalinity, particularly ≥55 causes photosynthetic dysfunction limiting leaf O2 production, specifically Pmax at high irradiance. Further, our data indicate that with increasing hypersalinity more of the O2 produced by leaves in intact shoots is consumed by meristems and/or belowground tissues and the sediment. These O2 sinks are likely more significant than O2 lost to leaf respiration (RD) that was not shown to increase with salinity in experiments using leaves in isolation. Thus, hypersalinity not only affects the physiology, growth and reproduction of seagrasses, but also their O2 balance. For seagrass species growing at their upper salinity thresholds, a management strategy that lowers hypersalinity exposure would limit loss of seagrasses to O2 imbalance from hypersalinity effects on photosynthesis and ecosystem O2 demand.

Environmental stressors alter the composition of seagrass phyllosphere microbial communities

Seagrass meadows are among the most valuable habitats in the world as they are used by a wide range of marine organisms and provide important ecosystem services. With increasing human populations and coastal development, seagrasses are under increased stress and coverage is declining worldwide. This is the first experiment to test the effects of two known seagrass stressors, increased temperature and reduced light availability, on the composition of seagrass blade surface microbial communities, which is a relatively understudied community. Analysis of 16S rRNA amplicon (iTag) sequence data revealed that both of these stressors significantly altered microbial community structure, including both taxonomy and abundance, on the blade surfaces of the tropical seagrass Thalassia testudinum. The highest temperature and lowest light treatments showed higher abundances of phyla not commonly reported as indigenous members of seagrass phyllosphere communities, including members of the bacterial phyla Ca. PAUC34f, Ca. Modulibacteria, and Chlamyidae. Despite these compositional difference among treatments, no significant differences in overall microbial diversity or richness were found. These results suggested seagrass phyllosphere microbial communities have the capacity to change significantly and relatively quickly in response to changing environmental conditions due to anthropogenic activity. Further studies are needed to determine if these direct environmental effects on the microbial community or indirect effects that feedback through the seagrass host.

Seagrass Depth Distribution Mirrors Coastal Development in the Mexican Caribbean – An Automated Analysis of 800 Satellite Images

The seagrass Thalassia testudinum is the dominant habitat-builder in coastal reef lagoons of the Caribbean, and provides vital ecosystem services including coastal protection and carbon storage. We used a remote sensing methodology to map T. testudinum canopies over 400 km of coastline of the eastern Yucatán Peninsula, comparing the depth distribution of canopy density, in terms of leaf area index (LAI), to a previously established ecological model of depth and LAI for this species in oligotrophic conditions. The full archive of Sentinel-2 imagery from 2016 to 2020 was applied in an automated model inversion method to simultaneously estimate depth and LAI, covering ∼900 km2 of lagoon with approximately 800 images. Data redundancy allowed for statistical tests of change detection. Achieved accuracy was sufficient for the objectives: LAI estimates compared to field data had mean absolute error of 0.59, systematic error of 0.04 and r2 > 0.67 over a range of 0–5. Bathymetry compared to 46,000 ICESat-2 data points had a mean absolute error of 1 m, systematic error less than 0.5 m, and r2 > 0.88 over a range of 0–15 m. The estimated total area of seagrass canopy was consistent with previously published estimates of ∼580 km2, but dense canopies (LAI > 3), which are the primary contributors to below-ground carbon storage, comprise only ∼40 km2. Within the year-to-year variation there was no change in overall seagrass abundance 2017–2020, but localised statistically significant (p < 0.01) patches of canopy extension and retraction occurred. 2018 and 2019 were affected by beaching of pelagic Sargassum and dispersion as organic matter into the lagoon. The multi-year analysis enabled excluding this influence and provided an estimate of its extent along the coast. Finally, the distribution of LAI with depth was consistent with the ecological model and showed a gradient from north to south which mirrored a well-established gradient in anthropogenic pressure due to touristic development. Denser canopies were more abundant in developed areas, the expected growth response to nutrient enrichment. This increase in canopy density may be a useful early bio-indicator of environmental eutrophication, detectable by remote sensing before habitat deterioration is observed.

Fruit development in the seagrass Thalassia testudinum: Possible relationships between structure, physiology and defense

Even though seagrasses are clonal plants, they also invest substantial energy in sexual reproduction. However, little is known about the development of the reproductive structures of seagrasses and their complex interactions with ambient conditions. Here, we describe the structural and physiological changes during development in the fruit pericarp of the tropical seagrass Thalassia testudinum, and their possible role in defense mechanisms. The fruit exocarp develops protuberances with a monostratified epidermis. The parenchyma below the exocarp epidermis, consists of chlorenchyma with abundant chloroplasts and cells that, in the early stages of development of the fruit, accumulate proanthocyanins and other diverse phenolic compounds, in the course of development, vary in quantity and solubility. During the intermediate and late stages of development, when the fruit emerges above the sediment, idioblasts with oxalate crystals are formed. Changes during the fruit development in morphology, phenolic compounds and oxalate crystal deposits, may be related to various metabolic processes to maintain homeostasis, such as downregulation of photosynthetic ROS. At the same time, these changes may also provide defense mechanisms against herbivores. Parrotfish appear undeterred by phenolic compounds in the exocarp of young manually exposed fruits; but they do not consume the pericarp of mature fruits (naturally emerged above the sediments) with oxalate crystals. These findings suggest that timing and integration of morphological and metabolic changes during fruit development guarantee both successful development and protection of the seeds.

The takeover of Thalassia testudinum by Anadyomene sp. at Biscayne Bay, USA, cannot be simply explained by competition for nitrogen and phosphorous

Apart from direct light effects, we tested whether the takeover of the seagrass Thalassia testudinum by the seaweed Anadyomene sp. in high nutrient areas of Biscayne Bay, Florida, USA, is related to a faster nutrient surge uptake capacity of the seaweed and/or a negative effect on the seagrass uptake rates. Anadyomene sp. and T. testudinum showed a similar ammonium surge uptake capacity, but the seagrass performed better than the seaweed in mixed incubations at high ammonium concentrations. T. testudinum was faster than Anadyomene sp. at taking up pulses of phosphate, but the uptake rates of the seagrass were significantly decreased in the presence of the seaweed. The takeover of T. testudinum by Anadyomene sp. at Biscayne Bay is likely dominated by light and cannot be simply explained by their single or mixed nutrient surge uptake rates, but the phosphate availability and the seagrass uptake inhibition by the seaweed may also play a key role in the process.

Composition of seagrass phyllosphere microbial communities suggests rapid environmental regulation of community structure.

Recent studies have revealed that seagrass blade surfaces, also known as the phyllosphere, are rich habitats for microbes; however, the primary drivers of composition and structure in these microbial communities are largely unknown. This study utilized a reciprocal transplant approach between two sites with different environmental conditions combined with 16S rRNA gene sequencing (iTag) to examine the relative influence of environmental conditions and host plant on phyllosphere community composition of the seagrass Thalassia testudinum. After 30 days, identity of phyllosphere microbial community members was more similar within the transplant sites than between despite differences in the source of host plant. Additionally, the diversity and evenness of these communities was significantly different between the two sites. These results indicated that local environmental conditions can be a primary driver in structuring seagrass phyllosphere microbial communities over relatively short time scales. Composition of microbial community members in this study also deviated from those in previous seagrass phyllosphere studies with a higher representation of candidate bacterial phyla and archaea than previously observed. The capacity for seagrass phyllosphere microbial communities to shift dramatically with environmental conditions, including ecosystem perturbations, could significantly affect seagrass-microbe interactions in ways that may influence the health of the seagrass host.

Landscape-scale variation in a sulfur-based sediment stress indicator for the seagrass Thalassia testudinum in Florida Bay, USA

Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correlated with reduced growth and has been linked to large-scale die-offs of Thalassia testudinum in Florida Bay, USA. In May of 2019, leaves from T. testudinum short shoots at 350 sites within 13 basins across Florida Bay were collected to investigate bay-wide variation in a novel, stable sulfur isotope-based indicator of hydrogen sulfide intrusion [(δ34Sleaf + 30)/total sulfur content]. The components of this sediment stress indicator (SSI), total sulfur content (% dry weight) and δ34S, were negatively correlated (R2 = -0.24), indicating greater sediment sulfide exposure in plants with higher sulfur content. Generalized additive model selection revealed that SSI was best predicted by a model which included T. testudinum cover, sediment depth, and Halodule wrightii cover (R2 = 0.24, weight = 0.48). Macrophyte communities dominated by dense T. testudinum climax communities and with deep sediments, which are characteristics associated with die-off, had the lowest SSI values, indicating greatest sulfide intrusion. Sites within the area of a recent (2015) seagrass die-off had significantly higher SSI values than nearby, non-die-off-affected sites (mean ± SE: 44.1 ± 1.5 vs. 40.2 ± 1.6, Wilcoxon p < 0.05). The recent die-off sites also had lower T. testudinum cover and higher H. wrightii cover than the non-die-off sites, indicating they were in the midst of recovery and at an early successional stage. Our findings indicate that SSI may provide a minimally destructive indicator of chronic sulfide intrusion in T. testudinum in Florida Bay.

Diversity patterns of free-living nematode assemblages in seagrass beds from the Cuban archipelago (Caribbean Sea).

Diversity patterns of free-living marine nematodes in tropical seagrass beds are understudied. Here, we describe the species richness and assemblage composition of nematodes in 13 seagrass sites covering the whole Cuban archipelago. Nematodes were collected from Thalassia testudinum seagrass beds and identified to species level. We provide a checklist of nematode species from seagrass beds. The species richness of nematode assemblages is high, with 215 species, 138 genus, 35 families, seven orders and two classes. That γ-diversity is higher than other studies and points to seagrass beds as diversity hotspots of free-living marine nematodes. Local species richness in seagrass bed sites is about 57 ± 17 species and broadly similar across the sites, despite the environmental heterogeneity. The geographical distance plays a weak, but significant, role on the decay of similarity likely affected by limited dispersal of nematodes. The pairwise similarity values, related to poor-coloniser nematodes, were twice more affected by the distance than those related to good-colonisers, possibly due to differential success of transport and settlement.

Measuring the complex shear modulus of seagrass with an iPhone: A COVID-19 experiment

The shear modulus of seagrass is an important, yet rarely reported, parameter used to study the resilience of seagrass communities to hydrodynamic disturbances, and to understand the propagation of acoustic waves through seagrass meadows. Two intact Thalassia testudinum seagrass bundles were sampled from Redfish Bay (RFB) and Lower Laguna Madre (LLM) in South Texas, transported under oxygenated salt water to Austin, Texas, where measurements were performed at the primary author's residence during the COVID-19 shelter-in-place order in May 2020. A subsample of ten seagrass blades from each bundle was chosen, and the average thickness and width of each blade were recorded. Next, an iPhone was suspended from a ledge by each blade and manually subjected to an initial angular displacement with the axis of rotation colinear to the blade length. Once released, the angular velocity of the underdamped system was measured by the iPhone's gyroscope. Finally, the angular frequency and decay rate were used to infer the complex shear modulus. The complex shear moduli reported for RFB and LLM blades are 71 + 3i MPa and 145 + 5i MPa, respectively. Results suggest epiphytic growth could both increase the loss factor and decrease the shear modulus of seagrass. [Work supported by ARL:UT IR&D.]

Indicators of nutrient enrichment in coastal ecosystems of the northern Mexican Caribbean

We measured dissolved inorganic nitrogen (DIN = NH4+ + NOx [= NO3− + NO2−]), soluble reactive phosphorous (SRP), and ecological indicators (chlorophyll-a concentration, coverage of submerged aquatic vegetation, and %N, C:N and δ15N values in the seagrass Thalassia testudinum) to evaluate the effects of nutrient enrichment at three shallow coastal sites on the northern part of the Mexican Caribbean (Puerto Morelos, Puerto Juárez, and Isla Blanca) in two contrasting (cold-front or dry) seasons of the year. High nutrient concentrations prevailed in both seasons but were significantly higher during the cold-front season, probably as a result of sediment resuspension and groundwater discharges. The highest δ15N values were recorded in Puerto Juárez in both seasons (cold-front, 7.58‰; dry, 7.35‰). The highest macroalgae coverage and NH4+ concentration were also recorded in this site. These results, together with the high correlation between δ15N and NH4+ (r = 0.94) and DIN (r = 0.88) concentrations, suggest wastewater contamination in Puerto Juárez due to its proximity to the city of Cancún. In contrast, the lowest mean δ15N (−1.35‰) and nutrient enrichment effects (i.e., highest coverage of T. testudinum and lowest macroalgae coverage) were observed in Isla Blanca, a site with no urban settlements. The low δ15N values recorded along the Puerto Morelos coastal transect suggest that a significant fraction of the nitrogen assimilated by T. testudinum may come from either exudates or the decomposition of drifting Sargassum stranded in the coast, as the δ15N values recorded in Sargassum (−0.64‰) were similar to those found in the seagrass (−0.83‰). The correlation between the DIN:SRP ratio and chlorophyll-a concentration in the study area may indicate N-limitation of phytoplankton growth, despite the high DIN concentrations. Beyond the differences observed between sites – which may indicate site-specific, long-term nutrient inputs – the indicators examined evidence environmental deterioration of the study area, likely due to a shift in the nutrient regime (away from oligotrophic), particularly in Puerto Juárez, the site with the heaviest load of urban and tourism activities.

Facilitation of a tropical seagrass by a chemosymbiotic bivalve increases with environmental stress

Abstract Facilitation of foundation species is critical to the structure, function and persistence of ecosystems. Understanding the dependence of the strength of this facilitation on environmental conditions is important for informed ecosystem management and for predicting the impacts of global change. In coastal seagrass habitats, chemosymbiotic lucinid bivalves can facilitate seagrasses by decreasing potentially toxic levels of sulphide in sediment porewater. However, variation in the strength of lucinid–seagrass facilitation with environmental context has not been experimentally investigated. We tested the hypothesis that the presence of the tiger lucine Codakia orbicularis becomes more important to the growth and survival of the seagrass Thalassia testudinum under decreased light availability and increased sulphide stress. In a mesocosm experiment, we reduced average ambient‐light to T. testudinum by 64% and/or increased sediment porewater sulphide concentrations by ~200% and compared growth and tissue chemistry of T. testudinum with and without C. orbicularis. We found that T. testudinum was better able to maintain growth under shading and sulphide stress when C. orbicularis was present. C. orbicularis strongly decreased sediment porewater sulphide, an effect that minimized sulphur build‐up in seagrass tissue and was likely achieved through bioirrigation as well as chemoautotrophy. The relative effects of C. orbicularis on T. testudinum growth were strongest in the presence of environmental stressors. Synthesis. The strength of lucinid–seagrass facilitation increases under environmental conditions that hinder the ability of seagrass to detoxify sulphide. Our results provide evidence of a potential mechanism by which the spatiotemporal association between lucinids and seagrasses is maintained and support the incorporation of interspecific facilitation into conservation and restoration strategies for foundation species in the face of increasing anthropogenic impact and global change.

INCREMENTO DE NITRÓGENO EN <em>Thalassia testudinum</em> POR EL APORTE DE NITRÓGENO DISUELTO INORGÁNICO DE AGUAS RESIDUALES DURANTE PRECIPITACIONES EXTRAORDINARIAS

Las lagunas de arrecife del Caribe mexicano son ecosistemas oligotróficos, donde las descargas de agua residuales (DAR) asociadas a la actividad turística, en algunos casos son vertidas a los acuíferos subterráneos, suministrando nitrógeno inorgánico disuelto (NID) a las mismas. El aporte de NID puede ser exacerbado en años con precipitación extraordinaria (2013 con > 2500 mm) en el Caribe mexicano. Por su parte, el pasto marino Thalassia testudinum se caracteriza por un contenido de nitrógeno < 2 % en peso seco. Si el flujo de NID se incrementa debido a precipitaciones excepcionales, es de esperar que el contenido de nitrógeno en T. testudinum aumente con respecto a la precipitación promedio histórica (1200 mm) de los últimos 30 años. En el presente trabajo se cuantificó el contenido de nitrógeno en T. testudinum mediante un analizador elemental, para determinar cambios asociados con las DAR durante la época seca y lluviosa entre los años 2010 a 2016 en la Bahía de Akumal. En los años de 2010, 2012, 2015 y 2016 el contenido de nitrógeno fue similar entre épocas y en los años 2013 y 2014 fue mayor en la época de lluvias con respecto a la época seca. Esto sugiere que, en periodos de precipitaciones extraordinarias el flujo de NID a las lagunas costeras del Caribe incrementa haciendo disponible este nitrógeno para T. testudinum.

Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean

Some benthic dinoflagellates produce toxins that can affect other organisms including humans, and their proliferation seems to be related to the environmental variability. For this reason, the present study aims to compare the structural variation of potentially toxic dinoflagellates associated with the seagrass Thalassia testudinum from two nearby systems, with different environmental characteristics in Colombian Caribbean, corresponding to a brackish water coastal lagoon and an adjacent bay. Between January 2014 and December 2015, leaves of T. testudinum were collected monthly to obtain the dinoflagellates. Salinity, temperature, dissolved oxygen, pH, nutrients and total suspended solids (TSS) were measured, and precipitation data and the Oceanic Niño Index (ONI) were obtained. Dinoflagellates were detached from the leaves, morphologically identified by analyzing their thecal plates arrangements, and quantified using a Sedgewick-Rafter chamber. The information was analyzed using standard statistics and regression models. Fourteen species of potentially toxic epiphytic dinoflagellate belonging to four genera were recorded, being Prorocentrum the most representative in number of species. The maximum density, dominated by P. lima, were found in Bahía Chengue during the rainy season of 2014 (18452 and 20109 cells g−1 w.w.), with salinity of 35.50, high temperatures (>29.60 °C), dissolved oxygen >6 mg L−1, pH close to 8 and TSS >85 mg L−1. Densities at the Lagoon were lower than 80 cells g−1 w.w. with the highest values of Prorocentrum sp.1 under different environmental conditions. With the statistical relationships between the most abundant species and the main environmental variables, fundamental niche models were proposed in which cells could proliferate. The degree of risk to human health due to the presence of these potentially toxic epiphytic dinoflagellates will not be resolved until their toxicity discarded.

Simulated green turtle grazing affects benthic infauna abundance and community composition but not diversity in a Thalassia testudinum seagrass meadow

Seagrass meadows host diverse invertebrate faunal communities. Infaunal organisms residing in the sediments of meadows play important roles in the functioning of these ecosystems, such as the breakdown of organic matter. Disturbance to the benthic environment through grazing by megaherbivores, such as dugongs (Dugong dugon), can reduce infauna abundance within localized areas in a meadow. However, it is not known how removal of the aboveground canopy of a meadow without severe physical disturbance to the benthic habitat, such as through grazing by green turtles (Chelonia mydas), affects seagrass meadow infaunal communities. Increasing green turtle abundance will likely lead to greater areas of grazed seagrass with implications for the invertebrate infaunal communities within meadows. We experimentally simulated green turtle grazing for 16 months in a Thalassia testudinum seagrass meadow in The Bahamas to test effects of grazing on meadow infaunal communities. Total abundance of the infaunal community was reduced by 59% within six months of simulated grazing and thereafter remained lower throughout the experiment. Six out of eleven individual infaunal groups present (e.g. nematodes) also decreased in abundance following simulated grazing, but temporal abundance dynamics varied among groups. Simulated grazing had no effect on Simpson's Diversity Index of the infaunal community inhabiting the meadow at any point during the 16-month experiment. Though diversity was not affected, relative abundance of individual groups varied over time, and simulated grazing led to a significant change in infaunal community composition. These results demonstrate how green turtle grazing may affect the infaunal communities of shallow seagrass meadows with potential implications for the ecosystem services provided by these important habitats.

Toward the Ultrasonic Sensing of Organic Carbon in Seagrass‐Bearing Sediments

AbstractTen percent of all organic carbon (Corg) absorbed by the ocean each year is stored in seagrass‐bearing sediments. The preservation of these carbon stores is considered a vital method to mitigate climate change. Seagrass‐bearing sediments have been correlated with sediment geophysical properties yet have not been related to sediment acoustic properties. For this purpose, sediment cores were collected from a Thalassia testudinum seagrass meadow in South Texas, USA, where geophysical, acoustical, and Corg properties were measured. It is hypothesized that when deposits of Corg adsorb onto mineral surfaces and are stored in pore spaces, compliant layers between grain contacts and the formation of an organic‐rich suspension reduce sediment stiffness. Results from this seagrass meadow demonstrated a strong correlation between sediment P wave modulus and Corg and show promise toward the development of an in situ ultrasonic sediment probe to more rapidly quantify and monitor seagrass carbon stores.