Seagrass Leaf Research Articles

Seagrass Tolerance to Simulated Herbivory Along a Latitudinal Gradient: Predicting the Potential Effects of Tropicalisation

ABSTRACTThe polewards range expansion of tropical herbivorous fish into temperate latitudes is leading to overgrazing of marine habitats and community phase shifts in some regions. Here, we test the potential effects of increased herbivory on the temperate habitat‐forming seagrass Posidonia australis. We used a series of simulated herbivory experiments to predict the potential impacts of climate‐mediated increases in seagrass consumption along P. australis entire latitudinal range (~9° latitude) in eastern Australia (1700 km of coastline). We subjected treatment plots to two levels of simulated herbivory (10% or 80% of leaves clipped) and compared them to unclipped controls. We measured seagrass leaf growth rates and tissue chemical traits: carbohydrates in rhizomes, leaf phenolics, and nutrients (carbon, nitrogen, and C:N ratio) in leaves and rhizomes. At the warmest range‐edge population, we also tested how responses to increased herbivory may vary between summer and winter, or with repeated clipping events. Clipped shoots maintained growth rates similar to unclipped controls despite losing up to 80% of leaf biomass. This was consistent along the full latitudinal range and after repeated simulated herbivory at the northernmost location. One‐off clipping events impacted plant architecture, increasing the number of subdividing shoots. At the species range edge, leaves grew more in winter than in summer, and clipping tended to lower seagrass growth only in winter; however, higher levels of shoot subdivision were produced over summer than in winter. Plant chemical traits could not explain consistently the growth patterns observed despite some traits varying with latitude (e.g., leaf nitrogen content decreased with latitude and C:N ratio increased) and/or simulated herbivory. Synthesis: P. australis growth is not affected by increases in simulated herbivory and may be relatively resilient to future increases in seagrass consumption, suggesting that this species could be a relative ‘winner’ under future climate change conditions that lead to enhanced herbivory.

Seagrass growth at different turbidity levels

Seagrass beds are highly productive ecosystems and have ecosystem services in the coastal zone but are continuously declining globally. Mainly due to anthropogenic activities that reduce the amount of light entering the waters and reaching seagrasses, such as dredging, reclamation, sedimentation, and eutrophication. This research was conducted to determine the relationship between turbidity level and seagrass composition and morphology. The observations were conducted in November 2016 in Pangkep district on Sagara Island and Makassar City on Barrang Lompo and Lae-lae Island, South Sulawesi Province. The observation stations are determined based on the level of turbidity conditions. Sagara Island, Barrang Lompo, and Lae-lae are areas that are considered to represent turbid waters, by laying a line transect (roll meter) perpendicular to the coast and doing it 3 times. The data collected in this study related to seagrass conditions such as seagrass composition and seagrass morphometrics. Variables observed in seagrass leaf morphometric observations were leaf surface area and leaf width, leaf thickness, and distance between internodes. In addition, other data collected is environmental parameter data, such as currents, TSS, PAR, temperature, and salinity. One-way ANOVA analysis showed that there was a difference in turbidity levels between Barrang Lompo Island, Sagara Island, and Lae-lae Island with a significance value of 0.000. The types of seagrasses found on Barrang Lompo with light availability of 1735.23 µmol/m²/s are Enhalus acoroides, Cymodocea rotundata, Halodule uninervis, Halophila ovalis, Thalassia hemprichii. On P. Sagara with light availability of 170.01 µmol/m²/s, E. acoroides, T. hemprichii, Cymodocea serrulata, H. uninervis, and H. ovalis were found. Whereas in P. Lae-lae only E. acoroides was found. The results showed that the level of turbidity has an influence on seagrass morphometrics, namely leaf ratio and leaf thickness of E. acoroides species, and internode distance in H. uninervis species. Keywords:Availability of lightMorphometricsPARSeagrass bedsTurbidity

Trade‐Off Between Light Deprivation and Desiccation in Intertidal Seagrasses Due To Periodic Tidal Inundation and Exposure: Insights From a Data‐Calibrated Model

AbstractSome seagrass species thrive in shallow intertidal zones globally, adapting to periodic tidal inundation and exposure with distinctive physiological traits and offering crucial ecosystem services. However, predicting the responses of intertidal seagrasses to external stressors is hampered by the complexity of the dynamic and harsh environments they occupy. Intertidal seagrass growth models, especially those incorporating dynamic physiological responses, are scarce in the literature. Our study comprehensively collated relevant data from the literature to parameterize the relationship between air exposure, seagrass leaf water content and photosynthetic efficiency to inform new growth rate functions for generalizable intertidal seagrass growth models. We tested the applicability of these model formulations for scenarios with varying physiological process assumptions, seagrass species, tidal conditions, meadow elevations and water turbidity. We found that neglecting air‐exposed physiological responses (i.e., leaf water content loss and reduced photosynthetic efficiency) can substantially overestimate seagrass growth rates. We also observed a trade‐off between light deprivation and desiccation on intertidal seagrass growth under specific tidal ranges and turbidity conditions. This can yield an “optimal” elevation where overall stress of desiccation (increasing with meadow elevation) and light deprivation (decreasing with meadow elevation) are minimized. The predicted optimal elevation, that is, the most suitable habitat for intertidal seagrass, moves upward as water turbidity increases. Our study provides conceptual and quantitative guidance for ecological modelers to include air exposure responses of intertidal seagrasses in coastal ecosystem models. The model also helps to evaluate the viability of intertidal seagrass habitats and inform decisions on coastal ecosystem management under changing environmental conditions.

Nutrient loading weakens seagrass blue carbon potential by stimulating seagrass detritus carbon emission

Coastal nutrient loading has been linked to a decline in the capacity of seagrass ecosystems to sequester carbon (‘blue carbon’); however, the mechanisms are unclear. Here we investigated how nutrient loading can affect the contribution that seagrass plant material makes to blue carbon stocks by investigating plant quality-decomposition dynamics. Specifically, we used a combination of laboratory and field experiments to account for various changes in biogeochemical cycling from seagrass meadows, ranging from changes in leaf quality to CO2 fluxes. It was found that nutrient loading increased the ‘labile’ content of seagrass (i.e. increased levels of leaf nitrogen, phosphorus and soluble organic carbon (amino acid and soluble sugar content), and at the same time decreased levels of ‘recalcitrant’ carbon (i.e. materials that are harder for microbes to break down, such hemicellulose, cellulose and lignin contents). Nutrient-enriched leaves decomposed ∼ 18 % faster than non-enriched leaves (i.e. greater biomass loss from nutrient-affected seagrass), resulting in ∼ 80 % more CO2 emissions from nutrient-enriched seagrass. We also found that seagrass that naturally contained high levels of labile carbon at the start of the experiment were affected to a greater degree (i.e. higher CO2 emissions) by nutrients addition than seagrass that had high proportions of recalcitrant carbon to begin with. Overall, these findings suggest that nutrient loading can weaken the capacity of seagrass ecosystems to act as blue carbon sinks through its effect on seagrass leaf decomposability.

Potential Antibacterial Activity of Ethanol Extract of Seagrass (Thalassia hemprichii) From Depapre Jayapura against Propionibacterium acnes

Contains of Seagrass (Thalassia hemprichii) are flavonoid compounds, saponins, triterpenoids, alkaloids, steroids, and tannins. These compounds are known to have the potential as antibacterial. This study aimed to determine the inhibition of antibacterial activity and the most effective concentration inhibiting Propionibacterium acnes bacteria. The sample used in this study was seagrass leaves obtained from the Depapre District. Samples were extracted by the maceration method using 96% ethanol solvent. Then, the antibacterial test was carried out using the disk diffusion method. The medium used for bacterial isolation was Nutrient Agar (NA). This study used a completely randomized design (CRD) with concentrations of seagrass leaf extract 50, 100, 200, 400, 600, 800, and 1000 ppm, positive control (clindamycin), and negative control (sterile distilled water). The inhibition zone diameter was analyzed using the ANOVA test. A further test was carried out to find out the difference between the treatments of a significantly honest difference (HSD). The results showed that the inhibition zone diameter in Propionibacterium acnes ranged from 6–10 mm (medium) and 10–20 mm (strong). Based on the results, it was shown that the interaction between the concentration of Thalassia hemprichii and the type of bacteria had a significant effect (P<0.01) on the inhibition zone diameter formed.

Impact of Atmospheric Correction on Classification and Quantification of Seagrass Density from WorldView-2 Imagery.

Mapping the seagrass distribution and density in the underwater landscape can improve global Blue Carbon estimates. However, atmospheric absorption and scattering introduce errors in space-based sensors' retrieval of sea surface reflectance, affecting seagrass presence, density, and above-ground carbon () estimates. This study assessed atmospheric correction's impact on mapping seagrass using WorldView-2 satellite imagery from Saint Joseph Bay, Saint George Sound, and Keaton Beach in Florida, USA. Coincident in situ measurements of water-leaving radiance (), optical properties, and seagrass leaf area index (LAI) were collected. Seagrass classification and the retrieval of LAI were compared after empirical line height (ELH) and dark-object subtraction (DOS) methods were used for atmospheric correction. DOS left residual brightness in the blue and green bands but had minimal impact on the seagrass classification accuracy. However, the brighter reflectance values reduced LAI retrievals by up to 50% compared to ELH-corrected images and ground-based observations. This study offers a potential correction for LAI underestimation due to incomplete atmospheric correction, enhancing the retrieval of seagrass density and above-ground Blue Carbon from WorldView-2 imagery without in situ observations for accurate atmospheric interference correction.

Stressor fluctuations alter mechanisms of seagrass community responses relative to static stressors

Ecosystems are increasingly affected by multiple anthropogenic stressors that contribute to habitat degradation and loss. Natural ecosystems are highly dynamic, yet multiple stressor experiments often ignore variability in stressor intensity and do not consider how effects could be mediated across trophic levels, with implications for models that underpin stressor management. Here, we investigated the in situ effects of changes in stressor intensity (i.e., fluctuations) and synchronicity (i.e., timing of fluctuations) on a seagrass community, applying the stressors reduced light and physical disturbance to the sediment. We used structural equation models (SEMs) to identify causal effects of dynamic multiple stressors on seagrass shoot density and leaf surface area, and abundance of associated crustaceans. Responses depended on whether stressor intensities fluctuated or remained static. Relative to static stressor exposure at the end of the experiment, shoot density, leaf surface area, and crustacean abundance all declined under in-phase (synchronous; 17, 33, and 30 % less, respectively) and out-of-phase (asynchronous; 11, 28, and 39 % less, respectively) fluctuating treatments. Static treatment increased seagrass leaf surface area and crustacean abundance relative to the control group. We hypothesised that crustacean responses are mediated by changes in seagrass; however, causal analysis found only weak evidence for a mediation effect via leaf surface area. Changes in crustacean abundance, therefore, were primarily a direct response to stressors. Our results suggest that the mechanisms underpinning stress responses change when stressors fluctuate. For instance, increased leaf surface area under static stress could be caused by seagrass acclimating to low light, whereas no response under fluctuating stressors suggests an acclimation response was not triggered. The SEMs also revealed that community responses to the stressors can be independent of one another. Therefore, models based on static experiments may be representing ecological mechanisms not observed in natural ecosystems, and underestimating the impacts of stressors on ecosystems.

Meadow trophic status regulates the nitrogen filter function of tropical seagrasses in seasonally eutrophic coastal waters

AbstractThe nutrient filter function is an important ecosystem service of seagrass meadows that mitigates the consequences of coastal eutrophication. In northeast Hainan in China, large seagrass areas were lost due to chronic eutrophication induced by untreated pond aquaculture effluents. However, in adjacent areas, seagrasses could survive due to seasonal exposure (i.e., not chronic) to eutrophication only. In a way, the conditions in these areas represent a transitional environment which allows investigating the effect of eutrophication on seagrass performance and their nitrogen uptake capacity. We tested how a 4‐week in situ nutrient enrichment affected inorganic nitrogen uptake rates of a multispecies seagrass meadow in eutrophic and non‐eutrophic seasons, in light and in darkness. All species maintained nitrogen uptake in the dark and preferred ammonium over nitrate. In the eutrophic season, the seagrass leaf biomass and growth were lower resulting in a lower nitrogen filter capacity. Among the species present, Cymodocea rotundata and Cymodocea serrulata covered 48% and 45%, respectively, of their daily nitrogen demand for leaf growth through leaf uptake from the water column, while it was only 30% for Thalassia hemprichii, the last remaining species in meadows degraded by eutrophication, as deduced from previous studies. It indicates that a multispecies seagrass meadow has a higher nitrogen filter capacity than a monospecific T. hemprichii meadow. By reducing seagrass diversity and, hence, the nitrogen filter function, eutrophication triggers a self‐reinforcing process. Once the nitrogen filtering capacity of a seagrass meadow is exhausted, further eutrophication and seagrass loss are expected.

Unexpected slow recovery of seagrass leaf epiphytes after the impact of a summer heat wave and concomitant mucilage bloom

The epiphytes of seagrass leaves constitute a peculiar community, comprised of a number of species specialized for this living substrate. Several studies report on the response of epiphytes to different pressures but no information exists about the effects of summer heatwaves, which have become frequent events in the last decades. This paper represents the first attempt to investigate the change in the leaf epiphyte community of the Mediterranean seagrass Posidonia oceanica due to the heatwave occurred in summer 2003. Thanks to a series of data collected seasonally between 2002 and 2006, and punctual data in the summers of 2014 and 2019, we assessed the change over time in the leaf epiphyte community. Temperature data trends were analysed through linear regression, while multivariate analyses (i.e., nMDS and SIMPER) were applied to cover data in order to assess changes over time in the epiphyte community. As a whole, the two most abundant taxa were the crustose coralline alga Hydrolithon and the encrusting bryozoan Electra posidoniae, which displayed the highest average cover values in summer (around 19%) and spring (around 9%), respectively. Epiphytes proved to be sensitive to temperature highs, displaying different effects on cover, biomass, diversity and community composition. Cover and biomass exhibited a dramatic reduction (more than 60%) after the disturbance. In particular, Hydrolithon more than halved, while E. posidoniae dropped sevenfold during summer 2003. While the former recovered comparatively quickly, the latter, as well as the whole community composition, apparently required 16 years to return to a condition similar to that of 2002.

Composition and Density of Peryphyton on the leaves Thalassia hemprichii and Cymodocea rotundata at Panjang Island, Jepara

Seagrass as a flowering plant (Angiosperms) that can live in the marine environment well. One type of seagrass whose presence is dominant in Indonesian waters is the species Thalassia hemprichii and Cymodocea rotundata. Periphyton is an organism that has a close relationship with seagrass and has a role in seagrass as an increase in primary productivity. The purpose of this study was to determine the genus composition and density of periphyton on seagrass leaves Thalassia hemprichii and Cymodocea rotundata on Panjang Island, Jepara. The method carried out is seagrass leaf field data collection using the purposive sampling method with squared line transects. Seagrass leaf samples were taken and periphyton identification observations were made in the laboratory and then data processing analysis. Periphyton results were obtained from 10 classes, 39 genera and 1 type of zooplankton. Seagrass leaf periphyton density in both species is highest in old leaves and lowest in medium seagrass leaves. The periphyton diversity index of two seagrass species belongs to the medium category, the uniformity index belongs to the high category, and the dominance index shows that no genus dominates. The index value can indicate that the waters in the environment are still in stable condition.

ANALISIS KADAR KARBOHIDRAT PADA TEH CELUP BERBAHAN DASAR DAUN LAMUN (Enhalus acoroides)

Background: Seagrass is the only flowering plant (angiosperm) that has true root and leaf rhizomes that live submerged in the sea. There are 60 species of seagrass scattered in the world's coastal waters, one type of seagrass that can be used by humans is the seagrass Enhalus acoroides. The economic advantage of Enhalus acoroides leaves can be used as a food ingredient because it has a complete and relatively high nutritional content. Enhalus acoroides leaves can be used as raw material for herbal teas. Herbal teas have a high carbohydrate content, for example in Camellia sinensis tea of ​​0.286%, carbohydrates are one of the chemicals that are needed by the human body. Methods: This study was a descriptive study to determine the value of carbohydrate content in seagrass (Enhalus acoroides) leaf teabags. The research was carried out in March 2022. Sampling of seagrass leaves was carried out in Suli Village, Salahutu District, Central Maluku Regency. The stage of making seagrass teabags was carried out at the Basic Biology Laboratory of FKIP Pattimura University. And the analysis of carbohydrate content was carried out at the Biochemistry Laboratory of the Faculty of Mathematics and Natural Sciences, Pattimura University, Ambon with the type of experimental research. Results: Based on the observations that have been made, the results showed that the carbohydrate content of TL U1 was 5.148%, TL U2 was 5.211%l, TL U3 was 5.165%. With an average carbohydrate of 5.174%. Conclusion: Seagrass leaf teabags (Enhalus acoroides) have a high carbohydrate content and can be a new product that is rich in nutrients.

Effects of habitat complexity on the abundance and diversity of seagrass leaf meiofauna communities in tropical Kenyan seagrass meadows

We examined the abundance and diversity of seagrass leaf meiofauna, with special emphasis on harpacticoid copepods (Crustacea), in Thalassodendron ciliatum seagrass meadows differing in structural attributes (complexity) in terms of shoot density, canopy, and epiphytic microalgae as well as environmental variables (nutrients). Higher biomass of epiphytic microalgae was observed in study sites with lower shoot density and canopy height. Further, our results showed that seagrass leaf meiofauna assemblages (including harpacticoid copepods) were positively correlated with epiphytic microalgae and water nutrients but negatively correlated to seagrass variables (seagrass cover, shoot density, and canopy height). Our study revealed that food resources were the major determinant of the abundance and diversity of seagrass leaf meiofauna assemblages in these meadows. In contrast to our expectations, meiofauna abundance was negatively associated with seagrass density and canopy height. Thus, the findings indicate that variation in seagrass habitats may, in turn, influence the distribution, community structure, and functioning of epiphytic communities. We suggest that habitat quality depends on the substrate’s longevity supporting epiphytic microalgal biomass that serves as a food source for seagrass leaf meiofauna communities.

Effective medium modeling of acoustic propagation in a seagrass meadow

Seagrasses are foundation species in many coastal ecosystems, but these environments are declining globally due to climate change and other anthropogenic impacts. Ballard et al. [J. Acoust. Soc. Am. 147, 2020] established the efficacy of acoustic remote sensing techniques for seagrass monitoring by exploiting acoustic sensitivity to gas bubbles produced by photosynthesis and gas channels within the seagrass leaves. However, the effects of seagrass on acoustic propagation are not understood with sufficient quantitative detail, and an improved model describing propagation through a mixture of seagrass leaves, free gas bubbles, and seawater is needed to aid in integrating acoustic methods into conservation efforts. This talk provides an overview of developments in the modeling of acoustic propagation through a Thalassia testudinum meadow using a homogeneous effective medium approach to represent the seagrass leaves and seawater. The model accounts for the complex microstructure of seagrass leaves including the encapsulated gas channels and the elastic properties of the seagrass tissue. The model is intended for use in geoacoustic inference algorithms for the overall goal of providing estimates of seagrass productivity and biomass. Candidate effective medium models include 2D cylindrical seagrass leaves and a micromechanical model of a seagrass leaf cross-section. [Work supported by NSF.]

Drivers of variation in seagrass-associated amphipods across biogeographical areas

Amphipods are one of the dominant epifaunal groups in seagrass meadows. However, our understanding of the biogeographical patterns in the distribution of these small crustaceans is limited. In this study, we investigated such patterns and the potential drivers in twelve Cymodocea nodosa meadows within four distinctive biogeographical areas across 2000 Km and 13° of latitude in two ocean basins (Mediterranean Sea and Atlantic Ocean). We found that species abundances in the assemblage of seagrass-associated amphipods differed among areas following a pattern largely explained by seagrass leaf area and epiphyte biomass, while the variation pattern in species presence/absence was determined by seagrass density and epiphyte biomass. Seagrass leaf area was also the most important determinant of greater amphipod total density and species richness, while amphipod density also increased with algal cover. Overall, our results evidenced that biogeographical patterns of variation in amphipod assemblages are mainly influenced by components of the habitat structure, which covary with environmental conditions, finding that structurally more complex meadows harboring higher abundance and richness of amphipods associated.

Leaf Senescence of the Seagrass Cymodocea nodosa in Cádiz Bay, Southern Spain

Leaf decay in seagrasses is enhanced in some seasons since large green senescent beach-cast seagrass leaves are frequently recorded during autumn and winter seasons. Here, we explore if senescence is operating in seagrass leaf decay or if hydrodynamic stress is responsible for the seasonal leaf abscission. A seasonal study on the temperate seagrass Cymodocea nodosa was carried out in four locations with contrasting hydrodynamic regimes. The morphological, biomechanical and material properties of C. nodosa were measured. The force required to break the ligule was always lower than that required to break the blade. This could be considered an adaptive strategy to reduce acute drag forces and thus lessen the chance of plant uprooting. The absolute force needed to dislodge the blade at the ligule level varied with season and location, with the lowest forces recorded in autumn. This may indicate that senescence is operating in this species. On the other hand, the minimum estimated failure velocities for leaf abscission were also recorded in autumn. Consequently, this may cause the premature shedding of leaves in this season before the senescence process has finished and can probably explain the occurrence of green beach-cast seagrass leaves usually found during autumn and winter.

Nitrate threshold of tropical seagrass susceptibility to herbivory

Plant-herbivore interaction, susceptible to nutrient enrichment, is fundamental for maintaining a healthy and stable ecosystem. Hence, identifying nutrient thresholds may enable better management and monitoring of ecosystems. However, little information is available for the nutrient threshold for the susceptibility of seagrass, the only marine angiosperm, to herbivory. In present study, we investigated how nitrate enrichment altered the traits of dominant tropical seagrass Thalassia hemprichii at the physiological level (nutritional quality, chemical deterrents, and structural characteristics), changing its susceptibility to the herbivory of Cerithidea rhizophorarum. A set of multiple-choice feeding assays were conducted using nitrate-enriched seagrass pre-cultivated in the lab for 60 days. The present study found that the effect of nitrate enrichment on seagrass susceptibility to herbivory was non-linear, with a threshold effect. Low nitrate enrichment chemically and structurally defended seagrass, subsequently reducing herbivory, while continuously increasing nutrient enrichment crossing the threshold enhanced seagrass susceptibility to herbivory. Meanwhile, seagrass leaf total phenol and free fatty acid contents were the two dominant factors determining the herbivory preference of C. rhizophorarum. Overall, since anthropogenic nitrogen enrichment is increasing in intensity and frequency, our findings are significant for conservation and emphasize the importance of coastal eutrophication control.

Image analysis reveals environmental influences on the seagrass-epiphyte dynamic relationship for Thalassia testudinum in the northwestern Gulf of Mexico

Spatiotemporal patterns in seagrass-epiphyte dynamics for Thalassia testudinum in the northwestern Gulf of Mexico were evaluated through biomass measurements and scanned-image based metrics to investigate the potentially harmful impact of excessive epiphyte accumulations on seagrass condition. Image analysis with Spectral Angle Mapper algorithms distinguished epiphyte and uncovered seagrass leaf pixels to generate a normalized metric of leaf area coverage (epiphyte pixels/total leaf pixels). Imaging metrics were compared to biomass-based metrics seasonally, among three locations with different environmental conditions (depth, salinity, temperature and nutrient levels inferred from sediment porewater measurements) near Redfish Bay, Texas, USA. Image analysis, in conjunction with biomass measures, provides enhanced insight into the seagrass-epiphyte dynamic relationship and how it varies with environmental conditions. Compared with the biomass and morphological measures, image analysis may be more informative as an indicator of environmental changes. Variation in linear regressions of epiphyte biomass vs. epiphyte area (pixels) suggested changes in the thickness and/or density of accumulated epiphytes across environmental contexts and seasons. Two different epiphyte colonization patterns were presented based on the correlation between the normalized metrics of epiphyte load and epiphyte leaf coverage. The epiphyte load was highest at low temperatures and locations with elevated DIN:P ratio in sediment porewater. Conversely, the mean leaf coverage by epiphytes stayed relatively constant (± 10%) across seasons but differed by location (25% ~55% in this case), suggesting that leaf growth in this study is regulated to maintain the proportion of uncolonized leaf surface and that epiphyte coverage plays a role in its regulation.

Characterizing the acoustic response of Thalassia testudinum leaves using resonator measurements and finite element modeling.

Seagrasses play an important role in coastal ecosystems and serve as important marine carbon stores. Acoustic monitoring techniques exploit the sensitivity of underwater sound to bubbles, which are produced as a byproduct of photosynthesis and present within the seagrass tissue. To make accurate assessments of seagrass biomass and productivity, a model is needed to describe acoustic propagation through the seagrass meadow that includes the effects of gas contained within the seagrass leaves. For this purpose, a new seagrass leaf model is described for Thalassia testudinum that consists of a comparatively rigid epidermis that composes the outer shell of the leaf and comparatively compliant aerenchyma that surrounds the gas channels on the interior of the leaf. With the bulk modulus and density of the seagrass tissue determined by previous work, this study focused on characterizing the shear moduli of the epidermis and aerenchyma. These properties were determined through a combination of dynamic mechanical analysis and acoustic resonator measurements coupled with microscopic imagery and finite element modeling. The shear moduli varied as a function of length along the leaves with values of 100 and 1.8 MPa at the basal end and 900 and 3.7 MPa at the apical end for the epidermis and aerenchyma, respectively.

Response of tropical seagrass palatability based on nutritional quality, chemical deterrents and physical defence to ammonium stress and its subsequent effect on herbivory

Seagrass-herbivore interactions play a principal role in regulating the structure and function of coastal food webs, which were affected by nutrient enrichment. Seawater nutrient enrichment might change seagrass palatability by altering seagrass physical and chemical traits, consequently modulating herbivory patterns, but this remains elusive. In this study, the dominant tropical seagrass Thalassia hemprichii was cultured in different ammonium concentrations to examine the response of seagrass nutritional quality, deterrent secondary metabolites, and leaf toughness, as well as the subsequent effect of the changed physical (e.g., leaf toughness) and chemical traits (e.g., nitrogen content; total phenol) on the grazing activity of the herbivorous snail Cerithidea rhizophorarum. Ammonium enrichment enhanced seagrass nutritional quality and decreased physical defence. Low ammonium enrichment increased total phenol content, while high ammonium enrichment reduced it. Both low and high ammonium enrichment enhanced the grazing intensity of C. rhizophorarum on seagrass. Interestingly, nutritional quality mostly determined the herbivory preference of C. rhizophorarum on the intact seagrass having physical structure, with a chemical deterrent (total phenol) playing a secondary role. In contrast, chemical deterrent mainly determined the grazing intensity on agar seagrass food which was made artificially to exclude physical structure. This indicated that seagrass leaf physical structure might hinder phenol compounds from deterring herbivores. Overall, the results presented here demonstrate that ammonium enrichment remarkably increased seagrass palatability and subsequently induced higher susceptibility to herbivory, which might induce seagrass loss.

Seagrass (Zostera marina) transplant experiment reveals core microbiota and resistance to environmental change.

Zostera marina (seagrass) is a coastal marine angiosperm that sustains a diverse and productive ecosystem. Seagrass-associated microbiota support host health, yet the ecological processes that maintain biodiversity and stability of the seagrass leaf microbiota are poorly understood. We tested two hypotheses: (1) Microbes select seagrass leaves as habitat such that they consistently host distinct microbiota and/or core taxa in comparison to nearby substrates, and (2) seagrass leaf microbiota are stable once established and are resistant to change when transplanted to a novel environment. We reciprocally transplanted replicate seagrass shoots (natural and surface sterilized/dead tissue treatments) among four meadows with different environmental conditions and deployed artificial seagrass treatments in all four meadows. At the end of the 5-day experiment, the established microbiota on natural seagrass partially turned over to resemble microbial communities in the novel meadow, and all experimental treatments hosted distinct surface microbiota. We consistently found that natural and sterilized/dead seagrass hosted more methanol-utilizing bacteria compared to artificial seagrass and water, suggesting that seagrass core microbiota are shaped by taxa that metabolize seagrass exudates coupled with minor roles for host microbial defence and/or host-directed recruitment. We found evidence that the local environment strongly influenced the seagrass leaf microbiota in natural meadows and that transplant location explained more variation than experimental treatment. Transplanting resulted in high turnover and variability of the seagrass leaf microbiota, suggesting that it is flexibly assembled in a wide array of environmental conditions which may contribute to resilience of seagrass in future climate change scenarios.