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Susan Lynn Williams (1951–2018) was an exceptional marine ecologist whose research focused broadly on the ecology of benthic nearshore environments dominated by seagrasses, seaweeds, and coral reefs. She took an empirical approach founded in techniques of physiological ecology. Susan was committed to applying her research results to ocean management through outreach to decision-makers and resource managers. Susan’s career included research throughout the USA in tropical, temperate, and polar regions, but she specialized in tropical marine ecology. Susan’s scholarship, leadership, and friendship touched many people, leading to this multi-authored paper. Susan’s scholarship was multi-faceted, and she excelled in scientific discovery, integration of scientific results, application of science for conservation, and teaching, especially as a mentor to undergraduate and graduate students and postdoctoral scholars. Susan served in a variety of leadership positions throughout her career. She embodied all facets of leadership; leading by example, listening to others, committing to the “long haul,” maintaining trust, and creating a platform for all to shine. Susan was an important role model for women in science. Susan was also a loyal friend, maintaining friendships for many decades. Susan loved cooking and entertaining with friends. This paper provides an overview of the accomplishments of Susan in the broad categories of scholarship, leadership, and friendship.

Seagrass meadows are virtual blue carbon ecosystems facing a dramatic decline on a global scale. Tracking the status and trends of seagrass meadows, which is still pending on large scales, is an emerging priority for their conservation and restoration. Here, we develop a semi-automatic procedure to identify and map seagrass meadows. Using Sentinel-2 data in Google Earth Engine (GEE) platform, a map is generated, showing a total of 11113.4 ha of seagrass meadows in coastal China in 2020. There exists the largest extent (6450.3 ha) in the coast of the South China Sea, while the second area (3909.5 ha) is in the Yellow Sea-Bohai Sea, and the seagrass meadows in the East China Sea cover about 753.6 ha. Our results provide the baseline data of seagrass meadows distribution in coastal China, and this study can be of reference for mapping seagrass meadows on a broader or even global scale. [ABSTRACT FROM AUTHOR]

The capacity of Blue Carbon Ecosystems to act as carbon sinks is strongly influenced by the metabolism of soil-associated microbes, which ultimately determine how much carbon is accumulated or returned to the atmosphere. The rapid evolution of sequencing technologies has facilitated the generation of tremendous amounts of data on what taxa comprise belowground microbial assemblages, largely available as isolated datasets, offering an opportunity for synthesis research that informs progress on understanding Blue Carbon microbiomes. We identified questions that can be addressed with a synthesis approach, including the high variability across datasets, space, and time due to differing sampling techniques, ecosystem or vegetation specificity, and the relationship between microbiome community and edaphic properties, particularly soil carbon. To address these questions, we collated 34 16S rRNA amplicon sequencing datasets, including bulk soil or rhizosphere from seagrass, mangroves, and saltmarshes within publicly available repositories. We identified technical and theoretical challenges that precluded a synthesis of multiple studies with currently available data, and opportunities for addressing the knowledge gaps within Blue Carbon microbial ecology going forward. Here, we provide a standardisation toolbox that supports enacting tasks for the acquisition, management, and integration of Blue Carbon-associated sequencing data and metadata to potentially elucidate novel mechanisms behind Blue Carbon dynamics. [ABSTRACT FROM AUTHOR]

Anthropogenic impacts on marine ecosystems have led to a decline of biodiversity across the oceans, threatening invaluable ecosystem services on which we depend. Ecological temporal data to track changes in diversity are relatively rare, and the few long-term datasets that exist often only date back a few decades or less. Here, we use eDNA taken from dated sediment cores to investigate changes over approximately the last 100 years of metazoan communities in native (Cymodocea nodosa and Posidonia oceanica) and exotic (Halophila stipulacea) seagrass meadows within the eastern Mediterranean Sea, at two locations in Greece and two in Cyprus. Overall, metazoan communities showed a high turnover of taxa during the past century, where losses of individual taxa in a seagrass meadow were compensated by the arrival of new taxa, probably due to the arrival of exotic species introduced in the Mediterranean Sea from the Suez Canal or the Gibraltar Strait. Specifically, bony fishes (Class Actinopteri) and soft corals (Class Anthozoa) presented significantly higher richness in the past (before the 1980s) than in the most recent time periods (from 1980–2017) and some Cnidarian orders were solely found in the past, whereas sponges and Calanoids (Class Hexanauplia), an order of copepods, showed an increase in richness since the 1980s. Moreover, the Phyla Porifera, Nematoda and the Classes Staurozoa, Hydrozoa and Ophiuroidea were detected in P. oceanica meadows but not in C. nodosa and H. stipulacea, which led to P. oceanica meadows having twice the richness of other seagrasses. The greater richness resulted from the more complex habitat provided by P. oceanica. The combination of eDNA and sediment cores allowed us to reconstruct temporal patterns of metazoan community diversity and provides a novel approach to follow natural communities back in time in the absence of time series and baseline data. The ongoing loss of P. oceanica meadows, likely to be compounded with future warming, might lead to a major loss of biodiversity and the replacement by other seagrass species, whether native or exotic, does not compensate for the loss. [ABSTRACT FROM AUTHOR]

The introduction and establishment of exotic species often result in significant changes in recipient communities and their associated ecosystem services. However, usually the magnitude and direction of the changes are difficult to quantify because there is no pre‐introduction data. Specifically, little is known about the effect of marine exotic macrophytes on organic carbon sequestration and storage. Here, we combine dating sediment cores (210Pb) with sediment eDNA fingerprinting to reconstruct the chronology of pre‐ and post‐arrival of the Red Sea seagrass Halophila stipulacea spreading into the Eastern Mediterranean native seagrass meadows. We then compare sediment organic carbon storage and burial rates before and after the arrival of H. stipulacea and between exotic (H. stipulacea) and native (C. nodosa and P. oceanica) meadows since the time of arrival following a Before‐After‐Control‐Impact (BACI) approach. This analysis revealed that H. stipulacea arrived at the areas of study in Limassol (Cyprus) and West Crete (Greece) in the 1930s and 1970s, respectively. Average sediment organic carbon after the arrival of H. stipulacea to the sites increased in the exotic meadows twofold, from 8.4 ± 2.5 g Corg m−2 year−1 to 14.7 ± 3.6 g Corg m−2 year−1, and, since then, burial rates in the exotic seagrass meadows were higher than in native ones of Cymodocea nodosa and Posidonia oceanica. Carbon isotopic data indicated a 50% increase of the seagrass contribution to the total sediment Corg pool since the arrival of H. stipulacea. Our results demonstrate that the invasion of H. stipulacea may play an important role in maintaining the blue carbon sink capacity in the future warmer Mediterranean Sea, by developing new carbon sinks in bare sediments and colonizing areas previously occupied by the colder thermal affinity P. oceanica. [ABSTRACT FROM AUTHOR]

The Red Sea is characterized by its high seawater temperature and salinity, and the resilience of its coastal ecosystems to global warming is of growing interest. This high salinity and temperature might also render the Red Sea a favorable ecosystem for calcification and therefore resistant to ocean acidification. However, there is a lack of survey data on the CO2 system of Red Sea coastal ecosystems. A 1‐year survey of the CO2 system was performed in a seagrass lagoon, a mangrove forest, and a coral reef in the central Red Sea, including fortnight seawater sampling and high‐frequency pHT monitoring. In the coral reef, the CO2 system mean and variability over the measurement period are within the range of other world's reefs with pHT, dissolved inorganic carbon (DIC), total alkalinity (TA), pCO2, and Ωarag of 8.016±0.077, 2061±58 μmol/kg, 2415±34 μmol/kg, 461±39 μatm, and 3.9±0.4, respectively. Here, comparisons with an offshore site highlight dominance of calcification and photosynthesis in summer‐autumn, and dissolution and heterotrophy in winter‐spring. In the seagrass meadow, the pHT, DIC, TA, pCO2, and Ωarag were 8.00±0.09, 1986±68 μmol/kg, 2352±49 μmol/kg, 411±66 μatm, and 4.0±0.3, respectively. The seagrass meadow TA and DIC were consistently lower than offshore water. The mangrove forest showed the highest amplitudes of variation, with pHT, DIC, TA, pCO2, and Ωarag, were 7.95±0.26, 2069±132 μmol/kg, 2438±91 μmol/kg, 493±178 μatm, and 4.1±0.6, respectively. We highlight the need for more research on sources and sinks of DIC and TA in coastal ecosystems. Plain Language Summary: Global warming and ocean acidification are consequences of increased CO2 emissions to the atmosphere by humankind and are major threats to marine ecosystems. The Red Sea waters are naturally warm and saline. The resilience of its coral reefs, mangrove forests, and seagrass meadows is of growing interest for the scientific community in the context of global warming. The high temperature and salinity might render the Red Sea quite resistant to ocean acidification as well as an environment chemically very favorable for calcification, notably by corals. Calcification is a process dampened by the acidity (pH) of water, which depends on the chemistry of CO2 in seawater. Warm and saline water naturally tend to have a more basic pH and then be less corrosive to calcareous skeletons. However, the chemistry of the CO2 and acidity baselines and variability in the Red Sea are poorly documented. We conducted a year‐round survey of the CO2 chemistry of seawater in a seagrass meadow, mangrove forest, and coral reef ecosystem, involving discrete water sampling and high‐frequency measurements. Key Points: Dominance of heterotrophy and dissolution in winter‐spring and autotrophy and calcification in summer‐autumn in the coral reefNegative anomaly of TA and DIC in the seagrass meadow relative to offshore waterThe mangrove forest exhibited the widest variations but no systematic anomaly of TA and DIC relative to offshore water [ABSTRACT FROM AUTHOR]

Seagrass meadows are important carbon sinks, and they are experiencing a global decline. Restoration of seagrass meadows provides a strategy to mitigate climate change while conserving these important ecosystems., We examined the long-term carbon sequestration expected for seagrass restoration programmes by developing a model that combined models of patch growth, patch survival in seagrass planting projects and estimates of seagrass CO2 sequestration per unit area for the five seagrass species commonly used in restoration programmes., The model results indicated that the cumulative C sequestered increased rapidly over time and with planting density to reach an asymptote at a planting density of 100 units ha−1 (or 6 m spacing between units). At this planting density, the modelled cumulative C sequestered ranges from 177 to over 1337 tons CO2 ha−1 after 50 years., The value corresponding to this carbon sequestration suggests that the costs of seagrass restoration programmes may be fully recovered by the total CO2 captured in societies with a carbon tax in place, providing additional ecosystem services derived from the role of seagrasses in providing ecosystem services, such as enhanced biodiversity., Synthesis and applications. Seagrass restoration programmes are economically viable strategies to mitigate climate change through carbon sequestration, particularly in subtropical and tropical island states where land-based options have a limited scope. [ABSTRACT FROM AUTHOR]

The build-up of sulphide concentrations in sediments, resulting from high inputs of organic matter and the mineralization through sulphate reduction, can be lethal to the benthos. Sulphate reduction is temperature dependent, thus global warming may contribute to even higher sulphide concentrations and benthos mortality. The seagrass Posidonia oceanica is very sensitive to sulphide stress. Hence, if concentrations build up with global warming, this key Mediterranean species could be seriously endangered. An 8-year monitoring of daily seawater temperature, the sulphur isotopic signatures of water ( δ 34 S water), sediment (δ 34 S CRS) and P. oceanica leaf tissue (δ 34 S leaves), along with total sulphur in leaves ( TS leaves) and annual net population growth along the coast of the Balearic archipelago (Western Mediterranean) allowed us to determine if warming triggers P. oceanica sulphide stress and constrains seagrass survival. From the isotopic S signatures, we estimated sulphide intrusion into the leaves ( F sulphide) and sulphur incorporation into the leaves from sedimentary sulphides ( SS leaves). We observed lower δ 34 S leaves, higher F sulphide and SS leaves coinciding with a 6-year period when two heat waves were recorded. Warming triggered sulphide stress as evidenced by the negative temperature dependence of δ 34 S leaves and the positive one of F sulphide, TS leaves and SS leaves. Lower P. oceanica net population growth rates were directly related to higher contents of TS leaves. At equivalent annual maximum sea surface water temperature ( SST max), deep meadows were less affected by sulphide intrusion than shallow ones. Thus, water depth acts as a protecting mechanism against sulphide intrusion. However, water depth would be insufficient to buffer seagrass sulphide stress triggered by Mediterranean seawater summer temperatures projected for the end of the 21st century even under scenarios of moderate greenhouse gas emissions, A1B. Mediterranean warming, therefore, is expected to enhance P. oceanica sulphide stress, and thus compromise the survival of this key habitat along its entire depth distribution range. [ABSTRACT FROM AUTHOR]

Background The relationship between species diversity and components of ecosystem stability has been extensively studied, whilst the influence of the genetic component of biodiversity remains poorly understood. Here we manipulated both genotypic and allelic richness of the seagrass Zostera noltii, in order to explore their respective influences on the resistance of the experimental population to stress. Thus far intra-specific diversity was not taken into account in management plans, and restoration actions showed very low success. Information is therefore needed to understand the factors affecting resistance and resilience of populations. Results Our results show a positive influence of both allelic and genotypic richness on the resistance of meadows to environmental perturbations. They also show that at the low genotypic (i.e. clonal) richness levels used in prior experimental approaches, the effects of genotypic and allelic richness could not be disentangled and allelic richness was a likely hidden treatment explaining at least part of the effects hitherto attributed to genotypic richness. Conclusions Altogether, these results emphasize the need to acknowledge and take into account the interdependency of both genotypic and allelic richness in experimental designs attempting to estimate their importance alone or in combination. A positive influence of allelic richness on resistance to perturbations, and of allelic richness combined with genotypic richness on the recovery (resilience) of the experimental populations is supported by differential mortality results. These results, on the key species structuring of one of the most threatened coastal ecosystem worldwide, seagrass meadows, support the need to better take into account the distinct compartments of clonal and genetic diversity in management strategies, and in possible restoration plans in the future. [ABSTRACT FROM AUTHOR]

Abstract: Shoot meristematic activity of Mediterranean seagrass Posidonia oceanica has been assessed in eleven different meadows located around Balearic Islands (Spanish Mediterranean). Moreover, in six of them, the meristematic activity has been determined hourly (or every 2 or 3h, depending on the meadow) for at least 24h, with the aim of detecting a possible circadian rhythm in the % of dividing nuclei. Meristematic activity was inferred by applying flow cytometry techniques combined with DNA labeling to determine the percentage of nuclei in each phase of the cell cycle (i.e. G0+G1, S, G2). The percentage of nuclei in G2 phase of the cell cycle reflects the percentage of nuclei that are dividing in a specific moment. In the meristems of vertical shoots of P. oceanica the percentage of nuclei in G2 phase was on average 7±0.11%, and it ranged from 2% to 12% across the meadows studied. The average percentage of nuclei in the G2 phase in P. oceanica meristems is lower than reported for other plants. No circadian rhythms were detected in meristems of P. oceanica. The variability observed for meristematic activity across meadows suggests that it could be used as indicator of seagrass stress and, thus, to assess impacts to meristems before population declines could be observed. [Copyright &y& Elsevier]

Abstract: Uncertainty analyses allow the identification and quantification of the factors that contribute to the potential misclassification of the ecological status of water bodies, helping to improve the sampling design used in monitoring. Here we used a Posidonia oceanica multivariate index (POMI) bio-monitoring dataset covering a total of 81 sites distributed throughout 28 water bodies from the coast of Catalonia, Balearic Islands and Croatia to determine the levels of uncertainty associated with each region and how they change according to the quality status of water bodies. Overall, variability among sites (meadows) within water bodies was the factor that generated the greatest risk of misclassification among the three regions, within which the Balearic Islands had the lowest uncertainty, followed by Croatia and Catalonia. When water bodies classified in good/high quality were separated from those in moderate/poor status classes, we found that the latter displayed higher levels of uncertainty than the former. [Copyright &y& Elsevier]

The article presents a study which determines the effect of nutrient enrichment on five ecological services commissioned by a model of coastal system in Mobile Bay, Alabama. It states that eutrophication disrupts natural ecosystem and little effort has been made to address the increasing effects of the process. The result of the study points out the importance of creating management policies for anthropogenic eutrophication to minimize its effects.

This paper provides a synthesis of the EU project MedVeg addressing the fate of nutrients released from fish farming in the Mediterranean with particular focus on the endemic seagrass Posidonia oceanica habitat. The objectives were to identify the main drivers of seagrass decline linked to fish farming and to provide sensitive indicators of environmental change, which can be used for monitoring purposes. The sedimentation of waste particles in the farm vicinities emerges as the main driver of benthic deterioration, such as accumulation of organic matter, sediment anoxia as well as seagrass decline. The effects of fish farming on P. oceanica meadows are diverse and complex and detected through various metrics and indicators. A safety distance of 400 m is suggested for management of P. oceanica near fish farms followed by establishment of permanent seagrass plots revisited annually for monitoring the health of the meadows. [Copyright &y& Elsevier]

Posidonia oceanica, a seagrass endemic to the Mediterranean forms extended and extremely persistent meadows. It is a clonal plant with an apparently irregular pattern of flowering events. An extensive bibliographic review allowed the reconstruction of past flowering events of this species around the Mediterranean, with a high degree of confidence for the last 30 years. The data series on annual flowering prevalence (FP, flowering records per total records) and flowering intensity (FI, fraction of flowering shoots) produced have been compared with four series on Sea Surface annual Temperature maxima (SSTmax) obtained for the NW Mediterranean (averaged from the local data series of l'Estartit and Villefranche: 1957–2005) and for the Eastern, Western basin and the whole Mediterranean sea (extracted from NCEP Reynolds interpolated SST maps: 1982–2005). Significant warming trends are detected in the Mediterranean SSTmax series, at a rate of (mean+SE) 0. 04±0. 01°C yr−1 ( R2=0. 24, P<0. 01, N=24 years), in the Eastern basin series (0. 06±0. 01°C yr−1, R2=0. 43, P<0. 001, N=24 years) and in the long SSTmax series of the NW Mediterranean (0. 02±0. 01 C yr−1, R2=0. 12, P<0. 02, N=49 years). The magnitudes of the SSTmax anomalies around the absolute warming trend do not increase with time in any SSTmax series. Peaks of FP and FI in the Mediterranean seem to occur each 9–11 years, and coincide with peaks of annual SSTmax. Annual FP and FI increase with the residuals of annual SSTmax warming trend in all Mediterranean basins (FPMED: R2=0. 27, P<0. 01, N=23; FPNW: R2=0. 34, P<0. 01, N=31; FPE: R2=0. 20; P<0. 10, N=23). An outstanding event of P. Oceanica flowering across the Mediterranean has been registered in Autumn 2003; 1 month after the highest annual SSTmax recorded in the series. The hypothesis of flowering induction by thermal stress as the possible cause of this relationship is discussed, as well as the potential use of P. Oceanica flowering record as early indicator of biological change induced by global sea warming in Mediterranean marine ecosystems. [ABSTRACT FROM AUTHOR]

The extent of clonality within populations strongly influences their spatial genetic structure (SGS), yet this is hardly ever thoroughly analysed. We employed spatial autocorrelation analysis to study effects of sexual and clonal reproduction on dispersal of the dioecious seagrass Cymodocea nodosa. Analyses were performed both at genet level (i.e. excluding clonal repeats) and at ramet level. Clonal structure was characterized by the clonal subrange, a spatial measure of the linear limits where clonality still affects SGS. We show that the clonal subrange is equivalent to the distance where the probability of clonal identity approaches zero. This combined approach was applied to two meadows with different levels of disturbance, Cadiz (stable) and Alfacs (disturbed). Genotypic richness, the proportion of the sample representing distinct genotypes, was moderate (0.38 Cadiz, 0.46 Alfacs) mostly due to dominance of a few clones. Expected heterozygosities were comparable to those found in other clonal plants. SGS analyses at the genet level revealed extremely restricted gene dispersal in Cadiz ( Sp = 0.052, a statistic reflecting the decrease of pairwise kinship with distance), the strongest SGS found for seagrass species, comparable only to values for selfing herbaceous land plants. At Cadiz the clonal subrange extended across shorter distances (20–25 m) than in Alfacs (30–35 m). Comparisons of sexual and vegetative components of gene dispersal suggest that, as a dispersal vector within meadows, clonal spread is at least as important as sexual reproduction. The restricted dispersal and SGS pattern in both meadows indicates that the species follows a repeated seedling recruitment strategy. [ABSTRACT FROM AUTHOR]

Marine fish farming is increasing rapidly in the Mediterranean and in contrast to the Atlantic the coastal zone in the Mediterranean is characterized by clear waters with high transparency. This allows benthic primary producers such as the slow-growing seagrass Posidonia oceanica to grow at large depths at locations suitable for fish farming and generating a conflict between the conservation of these meadows and the growth of aquaculture operations in the Mediterranean. In this paper we review the current knowledge on environmental interactions between fish farming and benthic primary producers with particular focus on P. oceanica, as this seagrass is a key component along Mediterranean coasts. The recovery times of P. oceanica are very long, in the order of centuries, and losses of this species are thus considered to be irreversible at managerial time scales. [Copyright &y& Elsevier]

Blue Carbon (BC) strategy refers to the approaches that mitigate and adapt to climate change through the conservation and restoration of seagrass, saltmarsh and mangrove ecosystems and, in some BC programs, also through the expansion of seaweed aquaculture. The major losses of coastal habitats in combination with the commitments of China under the Paris Agreement provide unique opportunity and necessity to develop a strong Chinese BC program. Here, we (1) characterize China's BC habitats, examine their changes since 1950 along with the drivers of changes; (2) consider the expansion of seaweed aquaculture and how this may be managed to become an emerging BC resource in China, along with the engineering solutions required to enhance its potential; and (3) provide the rationale and elements for BC program in China. We find China currently has 1326–2149 km2 wild and 2–15 km2 created mangrove, saltmarsh and seagrass habitats, while 9236–10059 km2 (77–87%) has been lost since 1950, mainly due to land reclamation. The current area of farmed seaweed habitat is 1252–1265 km2, which is close to the area of wild mangrove, saltmarsh and seagrass habitats. We conclude that BC strategies have potentials yet to be fully developed in China, particularly through climate change adaptation benefits such as coastal protection and eco-environmental co-benefits of seaweed farming such as habitat creation for fish and other biota, alleviation of eutrophication, hypoxia and acidification, and the generation of direct and value added products with lower environmental impact relative to land-based production. On this basis, we provide a roadmap for BC strategies adjusted to the unique characteristics and capacities of China. Blue Carbon (BC) plays important roles in climate change mitigation/adaptation. In this paper, we 1) characterize China's BC habitats and examine their changes since 1950 along with the drivers of changes; 2) consider the expansion of seaweed aquaculture and how this may be managed to become an emerging BC resource in China, along with the engineering solutions required to enhance its potential; 3) provide the rationale and elements for BC program in China; and 4) draw a roadmap for BC strategies adjusted to the unique characteristics and capacities of China.Porphyra farm in Dongtou, Wenzhou city, Zhejiang province, China. Image 1 • China's Blue Carbon (BC) habitats, changes and the drivers of changes since 1950. • Potential BC resources, seaweed aquaculture expansion with engineering solutions. • BC Benefits for climate adaptation and co-benefits for environment and economy. • Great potential of China's BC strategies through climate change adaptation benefits. • A roadmap for BC strategies adjusted to the China's characteristics and capacities. [ABSTRACT FROM AUTHOR]

The recurrent lack of isolation by distance reported at regional scale in seagrass species was recently suggested to stem from stochastic events of large-scale dispersal. We explored the usefulness of phylogenetic information contained in microsatellite loci to test this hypothesis by using the Genetic Diversity Spectrum (GDS) on databases containing, respectively, 7 and 9 microsatellites genotypes for 1541 sampling units of Posidonia oceanica and 1647 of Cymodocea nodosa. The simultaneous increase of microsatellite and geographic distances that emerges reveals a coherent pattern of isolation by distance in contrast to the chaotic pattern previously described using allele frequencies, in particular, for the long-lived P. oceanica. These results suggest that the lack of isolation by distance, rather than the resulting from rare events of large-scale dispersal, reflects at least for some species a stronger influence of mutation over migration at the scale of the distribution range. The global distribution of genetic polymorphism may, therefore, result predominantly from ancient events of step-by-step (re)colonization followed by local recruitment and clonal growth, rather than contemporary gene flow. The analysis of GDS appears useful to unravel the evolutionary forces influencing the dynamics and evolution at distinct temporal and spatial scales by accounting for phylogenetic information borne by microsatellites, under an appropriate mutation model. This finding adds nuance to the generalization of the influence of large-scale dispersal on the dynamics of seagrasses. [ABSTRACT FROM PUBLISHER]

We evaluated the photosynthetic performance of Posidonia oceanica during short-term laboratory exposures to ambient and elevated temperatures (24–25 °C and 29–30 °C) warming and p CO 2 (380, 750 and 1000 ppm p CO 2 ) under normal and low light conditions (200 and 40 μmol photons m −2 s −1 respectively). Plant growth was measured at the low light regime and showed a negative response to warming. Light was a critical factor for photosynthetic performance, although we found no evidence of compensation of photosynthetic quantum efficiency in high light. Relative Electron Rate Transport (rETR max ) was higher in plants incubated in high light, but not affected by p CO 2 or temperature. The saturation irradiance (I k ) was negatively affected by temperature. We conclude that elevated CO 2 does not enhance photosynthetic activity and growth, in the short term for P. oceanica , while temperature has a direct negative effect on growth. Low light availability also negatively affected photosynthetic performance during the short experimental period examined here. Therefore increasing concentrations of CO 2 may not compensate for predicted future conditions of warmer water and higher turbidity for seagrass meadows. [ABSTRACT FROM AUTHOR]

Blue carbon ecosystems, particularly seagrass meadows, provide a myriad of critical ecosystem services through their role in supporting biodiversity, protecting shorelines, and sequestering and storing carbon. Despite their increased recognition for advancing global marine conservation efforts, our knowledge of the location and spatial extent of seagrass ecosystems globally remains relatively poor, due to the logistical and operational challenges of accurately mapping seafloor habitat using remote sensing techniques. Here we develop and validate an aerial LiDAR approach for mapping tropical seagrass blue carbon habitat in The Bahamas, by integrating full waveform bathymetric LiDAR with multi-spectral imaging. The results demonstrate accuracies between 92 and 98% for predicting seagrass types, over an extensive (e.g., >1000 km2) study area whilst utilizing robust ground-truthing techniques. These findings demonstrate the value of the aerial LiDAR approach used here for ongoing bathymetric surveys of seagrass habitat in The Bahamas and throughout similar ecosystems in the wider Caribbean, which should provide key insights for coastal management and conservation efforts. • Innovations in the mapping of blue carbon ecosystems are needed to properly their conservation potential • We developed a full waveform bathymetric LiDAR and multi-spectral imagining solution for mapping tropical seagrass meadows. • We focused our efforts in the most extensive seagrass ecosystem on Earth, located in The Bahamas. • Results suggest unprecedented accuracies in identifying all four seafloor class types, ranging from 92 to 98% for seagrass. • These findings provide a new solution for the high-resolution mapping of expansive and extensive seagrass habitat, which may be confidently expanded to other regions and blue carbon ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: The available information from experimental and descriptive studies on the effects of sediment burial and erosion on seagrasses was compiled to synthesize the information regarding the species-specific impacts and to relate them to plant characteristics. Burial thresholds (i.e. the burial levels causing 50% and 100% shoot mortality) and mortality-burial curves were estimated for the 15 seagrass species where the effects of experimental burial have been tested. All the species investigated reached 50% shoot mortality at burial levels ranging from 2cm (Halophila ovalis) to 19.5cm (Posidonia australis). P. australis was the most tolerant seagrass species to burial, while Thalassia testudinum was the most tolerant species to erosion. The relationships among plant size, growth, biomass and density with burial thresholds were examined. There were significant relationships between the burial thresholds and the shoot mass, the rhizome diameter, the aboveground biomass, the horizontal rhizome elongation and the leaf length of seagrass species. The leaf size and the rhizome diameter are the best predictors of the capacity of seagrasses to withstand burial. The burial thresholds estimated for seagrass species were in many cases in agreement with the burial impacts described by field observations (bioturbation), while in some cases was related to the species long-term colonization capacity (dune migration). Most human-induced impacts result in important changes of the sedimentary environment, with permanent negative effects on seagrass meadows (regression and complete destruction), whereas natural events, whether extreme (hurricane) or regular (dune migration), allow the recovery and/or adaptation of seagrasses to the burial/erosion sediment dynamics. The extent of the effects of burial and erosion on seagrasses is species-specific and strongly size-dependent. [Copyright &y& Elsevier]

Abstract: In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6μM) to those growing over higher sulfide conditions (33.5μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10μmols H2SL−1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea. [Copyright &y& Elsevier]

Seasonality and depth zonation of two common intertidal northern Vietnamese seagrass species, Halophila ovalis and Zostera japonica, were investigated in a 50 m long intertidal transect from 1999 till 2001. Z. japonica occurred in the middle intertidal (1.2 m above mean chart datum to mean sea level at 2.1 m above chart datum). H. ovalis occurred deeper in the intertidal (between 1.0 and 1.4 m above chart datum). Based on seasonally different sensitivity to reciprocal transplants, it is concluded that this zonation is most likely due to different tolerances to low light availability (less in Z. japonica) and desiccation (less in H. ovalis). Both species reached maximum shoot densities in September–October with total biomasses around 70 g DW m−2. Flowering in Z. japonica occurred in April only, whilst H. ovalis flowered in November after the rainy season and again in April. Seasonality in density and biomass was apparent and similar between the two species. It was largely coupled to the rainy season, which brings spates of turbid water during May–August, thereby possibly limiting light availability and hence growth. [Copyright &y& Elsevier]

• Seagrass epiphyte carbonate load was examined along a latitudinal and Ω gradient. • Epiphyte carbonate content, load and production decreased as latitude increased. • Epiphyte carbonate content, load and production increased as Ω increased. • These trends are similar to those described in other carbonate producer communities. Seagrass meadows play a significant role in the formation of carbonate sediments, serving as a substrate for carbonate-producing epiphyte communities. The magnitude of the epiphyte load depends on plant structural and physiological parameters, related to the time available for epiphyte colonization. Yet, the carbonate accumulation is likely to also depend on the carbonate saturation state of seawater (Ω) that tends to decrease as latitude increases due to decreasing temperature and salinity. A decrease in carbonate accumulation with increasing latitude has already been demonstrated for other carbonate producing communities. The aim of this study was to assess whether there was any correlation between latitude and the epiphyte carbonate load and net carbonate production rate on seagrass leaves. Shoots from 8 different meadows of the Zostera genus distributed across a broad latitudinal range (27 °S to up to 64 °N) were sampled along with measurements of temperature and Ω. The Ω within meadows significantly decreased as latitude increased and temperature decreased. The mean carbonate content and load on seagrass leaves ranged from 17% DW to 36% DW and 0.4–2.3 mg CO 3 cm−2, respectively, and the associated mean carbonate net production rate varied from 0.007 to 0.9 mg CO 3 cm−2 d-1. Mean carbonate load and net production rates decreased from subtropical and tropical, warmer regions towards subpolar latitudes, consistent with the decrease in Ω. These results point to a latitudinal variation in the contribution of seagrass to the accumulation of carbonates in their sediments which affect important processes occurring in seagrass meadows, such as nutrient cycling, carbon sequestration and sediment accretion. [ABSTRACT FROM AUTHOR]