Water column oxygenation by Posidonia oceanica seagrass meadows in coastal areas: A modelling approach.

BACKGROUND: Seagrass meadows are among the most abundant marine coastal ecosystems in the world. The wide variety of species, a worldwide distribution with overall high abundance, and especially their high productivity make them a plausible nature-based blue carbon solution to mitigate atmospheric CO2 levels. In the Mediterranean Basin, the endemic angiosperm Posidonia oceanica plays a remarkable role as a marine habitat provider in shallow waters through its vertical growth and as a carbon sink storing allochthonous carbon and biomass underneath the meadows. OBJECTIVES: Here, we assess the capacity of a pristine meadow to oxygenate the water column in the coastal area of the Balearic Islands through an evaluation of the metabolic rates in the benthic compartment as well as the resulting oxygen concentrations in the pelagic compartment. METHODS: Gross primary production (GPP), respiration (R), and net community production (NCP) are determined from dissolved oxygen (DO) measurements using two different calculation methods: a model developed for this purpose is used for data obtained from water column sensors and benthic multiparametric sensors, whereas the mass balance of measured DO is used to calculate the metabolic rates inside benthic chambers. RESULTS: The meadow at our study site was characterised as a net autotrophic ecosystem throughout the year. Oxygen productivity was significantly higher in the benthic compartment than in the water column and followed clear seasonal patterns, with enhanced productivity during spring. NOVELTY: This work shows the key role of a healthy Posidonia oceanica ecosystem as a water column oxygenator by comparing primary production using three different sampling strategies. The potential of the seagrass as climate change mitigator and its importance for the Mediterranean coasts should be considered in future coastal planning strategies.

How do soil processes control the provision of ecosystem services in coastal wetlands?

Coastal wetlands are known for their diverse ecosystems, yet their soil characteristics are often misunderstood and thought to be monotonous. These soils are frequently subjected to saline water saturation, leading to unique soil processes. However, the combination and intensity of these processes can vary considerably across different ecosystems. In this study, we hypothesize that these diverse soil processes not only govern the geochemical conditions in coastal ecosystems but also influence their ability to deliver ecosystem services. To test this hypothesis, we conducted soil analyses in mangroves, seagrass meadows, and hypersaline tidal flats along the Brazilian coast. We used key soil properties as indicators of soil processes and developed a conceptual model linking soil processes and soil-related ecosystem services in these environments. Under more anoxic conditions, the intense soil organic matter accumulation and sulfidization processes in mangroves evidence their significance in terms of climate regulation through organic carbon sequestration and contaminants immobilization. Similarly, pronounced sulfidization in seagrasses underscores their ability to immobilize contaminants. In contrast, hypersaline tidal flats soils exhibit increased intensities of salinization and calcification processes, leading to a high capacity for accumulating inorganic carbon as secondary carbonates (CaCO3), underscoring their role in climate regulation through inorganic carbon sequestration. Our findings show that contrary to previously thought coastal wetlands are far from monotonous, exhibiting significant variations in the types and intensities of soil processes, which in turn influence their capacity to deliver ecosystem services. This understanding is pivotal for guiding effective management strategies to enhance ecosystem services in coastal wetlands.

Succession of Bacteria Attached to Microplastics After Transferring from a Mariculture Area to a Seagrass Meadow.

Microplastics have been recognized as a novel niche for bacteria. However, studies have characterized the plastisphere microbial community in situ without exploring the microbial changes after transferring to other ecosystems. Here we focus on bacterial succession on typical microplastics (polypropylene and expanded polystyrene) and natural substrates (wood) after transferring from mariculture area to seagrass meadows system. Using high-throughput sequencing of 16 S rRNA, we found that alpha diversity significantly reduced after transferring and microplastics especially PP had significant separations on PCoA plots at different succession stages. The abundance and metabolic pathways of potential pathogen-associated microorganisms are significantly decreased. The relative abundance of xenobiotics biodegradation pathways was significantly lower and of energy metabolism pathways was significantly higher by comparing before and after transferring. Main environmental factors affecting microbial communities changed from nutrient characteristics to basic physicochemical properties after transferring. The succession times of the microbial communities of the three materials were different.

Self-organized sulfide-driven traveling pulses shape seagrass meadows.

Seagrasses provide multiple ecosystem services and act as intense carbon sinks in coastal regions around the globe but are threatened by multiple anthropogenic pressures, leading to enhanced seagrass mortality that reflects in the spatial self-organization of the meadows. Spontaneous spatial vegetation patterns appear in such different ecosystems as drylands, peatlands, salt marshes, or seagrass meadows, and the mechanisms behind this phenomenon are still an open question in many cases. Here, we report on the formation of vegetation traveling pulses creating complex spatiotemporal patterns and rings in Mediterranean seagrass meadows. We show that these structures emerge due to an excitable behavior resulting from the coupled dynamics of vegetation and porewater hydrogen sulfide, toxic to seagrass, in the sediment. The resulting spatiotemporal patterns resemble those formed in other physical, chemical, and biological excitable media, but on a much larger scale. Based on theory, we derive a model that reproduces the observed seascapes and predicts the annihilation of these circular structures as they collide, a distinctive feature of excitable pulses. We show also that the patterns in field images and the empirically resolved radial profiles of vegetation density and sediment sulfide concentration across the structures are consistent with predictions from the theoretical model, which shows these structures to have diagnostic value, acting as a harbinger of the terminal state of the seagrass meadows prior to their collapse.

Current distribution of Zostera seagrass meadows along the Bulgarian Black Sea coast (SW Black Sea, Bulgaria) (2010-2020).

BACKGROUND: The current distribution of Zostera spp. seagrass meadows along the Bulgarian Black Sea coast was studied. We used a combination of historical and recent observations of the habitat along the studied coastline. Remote sensing data (satellite images, sonar side-scans) was groundtruthed with georeferenced drop camera observations, scuba diving sampling and georeferenced scuba diving photo and video transects. NEW INFORMATION: Тhe total area of the habitat type 'MB548 - Black Sea seagrass meadows on lower infralittoral sands' (EUNIS habitat type list 2019) in the study area is 916.9 ha, of which only 17.9 ha are in man-made sheltered environments (harbours). All seagrass meadows identified in 1978-79 were also located during the current survey, despite the increased eutrophication pressure and overall degradation of benthic habitats in the western Black Sea during the 1980s and early 1990s.

Seagrass meadows mixed with calcareous algae have higher plant productivity and sedimentary blue carbon storage.

Seagrass meadows capture and store large amounts of carbon in the sediment beneath, thereby serving as efficient sinks of atmospheric CO2. Carbon sequestration levels may however differ greatly among meadows depending on, among other factors, the plant community composition. Tropical seagrass meadows are often intermixed with macroalgae, many of which are calcareous, which may compete with seagrass for nutrients, light, and space. While the photosynthetic CO2 uptake by both seagrasses and calcareous algae may increase the overall calcification in the system (by increasing the calcium carbonate saturation state, Ω), the calcification process of calcareous algae may lead to a release of CO2, thereby affecting both productivity and calcification, and eventually also the meadows' carbon storage. This study estimated how plant productivity, CaCO3 production, and sediment carbon levels were affected by plant community composition (seagrass and calcareous algae) in a tropical seagrass-dominated embayment (Zanzibar, Tanzania). Overall, the patterns of variability in productivity differed between the plant types, with net areal biomass productivity being highest in meadows containing both seagrass and calcareous algae. Low and moderate densities of calcareous algae enhanced seagrass biomass growth, while the presence of seagrass reduced the productivity of calcareous algae but increased their CaCO3 content. Sedimentary carbon levels were highest when seagrasses were mixed with low or moderate cover of calcareous algae. The findings show that plant community composition can be an important driver for ecosystem productivity and blue carbon sequestration.

Current and future carbon stocks in coastal wetlands within the Great Barrier Reef catchments.

Australia's Great Barrier Reef (GBR) catchments include some of the world's most intact coastal wetlands comprising diverse mangrove, seagrass and tidal marsh ecosystems. Although these ecosystems are highly efficient at storing carbon in marine sediments, their soil organic carbon (SOC) stocks and the potential changes resulting from climate impacts, including sea level rise are not well understood. For the first time, we estimated SOC stocks and their drivers within the range of coastal wetlands of GBR catchments using boosted regression trees (i.e. a machine learning approach and ensemble method for modelling the relationship between response and explanatory variables) and identified the potential changes in future stocks due to sea level rise. We found levels of SOC stocks of mangrove and seagrass meadows have different drivers, with climatic variables such as temperature, rainfall and solar radiation, showing significant contributions in accounting for variation in SOC stocks in mangroves. In contrast, soil type accounted for most of the variability in seagrass meadows. Total SOC stock in the GBR catchments, including mangroves, seagrass meadows and tidal marshes, is approximately 137 Tg C, which represents 9%-13% of Australia's total SOC stock while encompassing only 4%-6% of the total extent of Australian coastal wetlands. In a global context, this could represent 0.5%-1.4% of global SOC stock. Our study suggests that landward migration due to projected sea level rise has the potential to enhance carbon accumulation with total carbon gains between 0.16 and 0.46 Tg C and provides an opportunity for future restoration to enhance blue carbon.

A new species of Alpheus Fabricius, 1798 (Decapoda: Caridea: Alpheidae) from the Caribbean coast of Panama.

Alpheus viserion sp. n. is described based on the material from Bocas del Toro archipelago on the Caribbean coast of Panama. The new species is morphologically closest to three members of the speciose A. armillatus H. Milne-Edwards, 1837 species complex, viz. A. carlae Anker, 2012, A. angulosus McClure, 2002, and A. tenuis Kim Abele, 1988, differing from them, as well as from all the other species currently included in this complex, by a suite of morphological characters and a diagnostic colour pattern. With the description of yet another new shrimp species from Bocas del Toro, the authors hope to contribute to the awareness that this archipelago represents one of the most biologically diverse places in the Caribbean Sea and to encourage the preservation of the remaining natural habitats of this unique area.

What drives putative bacterial pathogens removal within seagrass meadows?

To analyze the mechanism of bacterial pathogen removal in seagrass meadows, we compared bacterial pathogens abundance in trapped particles in different seagrass meadows under different intensities of human activities. We compared the particle deposition rates and abundances of bacterial pathogen in Thalassia hemprichii, Enhalus acoroides stands and adjacent unvegetated patches. The bacterial pathogens abundance was much higher in E. acoroides than in adjacent unvegetated patches, however, the trapped particles under T. hemprichii were lower than in nearby unvegetated areas with the exception of the pristine seagrass meadow. These results indicate that seagrass, at least E. acoroides, can remove bacterial pathogens by trapping particles. What is unknown, nevertheless, is how the trapped bacterial pathogens are removed by T. hemprichii. We put forward that antibacterial chemical compounds release from seagrass was stimulated by stress from human activities for inhibition of bacterial pathogen. This putative mechanism needs to be explored in future studies.

Oil spill + COVID-19: A disastrous year for Brazilian seagrass conservation.

The COVID-19 pandemic has been the greatest global public health threat of the 21st century. Additionally, it has been challenging for the Brazilian shores that were recently (2019/2020) affected by the most extensive oil spill in the tropical oceans. Monitoring programs and studies about the economic, social and ecological consequences of the oil disaster were being carried out when the COVID-19 (coronavirus disease 2019) pandemic was declared, which has heavily affected Brazil. For Brazilian seagrasses conservation, this scenario is especially challenging. An estimated area of +325 km2 seagrass meadows was affected by the 2019 oil spill. However, this area is undoubtedly underestimated since seagrasses have not yet been adequately mapped along the 9000 km-long Brazilian coast. In addition to scientific budget cuts, the flexibilization of public and environmental policies in recent years and absence of systematic field surveys due to COVID-19 has increased the underestimation of affected seagrass areas and ecosystem service losses due to the oil spill. Efforts to understand and solve the oil spill crisis were forced to stop (or slow down) due to COVID-19 and the economic crisis, leaving ecosystems and society without answers or conditions to identify the source(s) that was/were responsible for this spill, mitigate the damage to poor communities, promote adequate impact assessment or restoration plans, or properly monitor the environment. Our results highlight that pandemic and large-scale environmental disasters may have had a synergistic effect on the economy (e.g., artisanal fisheries and tourism), public health and ecology, mainly due to government inaction, social inequality and poorly studied tropical ecosystems. The results of this study also demonstrate the need to analyze the short- and long-term impacts of the combined effects (oil spill + COVID-19) on the recovery of the economy and coastal ecosystems.

Multiple declines and recoveries of Adriatic seagrass meadows over forty years of investigation.

This paper investigated the long-term changes (from 1973 to 2013) of the seagrass meadows of Zostera marina, Zostera noltei and Cymodocea nodosa in the Adriatic Sea subjected to multiple pressures. We examined the changes of the meadows by means of field data collection, observations and analysis of aerial photography to identify the most important drivers of habitat loss. The major decline of seagrass extension observed from 1973 to 1989, was primarily driven by urban development, and by the increase of the blue tourism. From 1989 to 2007 seagrass habitats progressively recovered due to the decrease of urbanization, but from 2007 to 2013 a further significant loss of seagrass meadows was apparently driven by thermal anomalies coupled with an increasing anthropogenic pressure. Our long-term analysis provides evidence that the rates of seagrass loss are faster than the recovery rates (i.e., -4.5 loss rate vs +2.5% recovery rate per year).

Co-occurrence patterns and the large-scale spatial structure of benthic communities in seagrass meadows and bare sand.

BACKGROUND: Species distribution models are commonly used tools to describe diversity patterns and support conservation measures. There is a wide range of approaches to developing SDMs, each highlighting different characteristics of both the data and the ecology of the species or assemblages represented by the data. Yet, signals of species co-occurrences in community data are usually ignored, due to the assumption that such structuring roles of species co-occurrences are limited to small spatial scales and require experimental studies to be detected. Here, our aim is to explore associations among marine sandy-bottom sediment inhabitants and test for the structuring effect of seagrass on co-occurrences among these species across a New Zealand intertidal sandflat, using a joint species distribution model (JSDM). RESULTS: We ran a JSDM on a total of 27 macrobenthic species co-occurring in 300,000 m2 of sandflat. These species represented all major taxonomic groups, i.e. polychaetes, bivalves and crustaceans, collected in 400 sampling locations. A number of significant co-occurrences due to shared habitat preferences were present in vegetated areas, where negative and positive correlations were approximately equally common. A few species, among them the gastropods Cominella glandiformis and Notoacmea scapha, co-occurred randomly with other seagrass benthic inhabitants. Residual correlations were less apparent and mostly positive. In bare sand flats shared habitat preferences resulted in many significant co-occurrences of benthic species. Moreover, many negative and positive residual patterns between benthic species remained after accounting for habitat preferences. Some species occurring in both habitats showed similarities in their correlations, such as the polychaete Aglaophamus macroura, which shared habitat preferences with many other benthic species in both habitats, yet no residual correlations remained in either habitat. CONCLUSIONS: Firstly, analyses based on a latent variable approach to joint distributions stressed the structuring role of species co-occurrences beyond experimental scales. Secondly, results showed context dependent interactions, highlighted by species having more interconnected networks in New Zealand bare sediment sandflats than in seagrass meadows. These findings stress the critical importance of natural history to modelling, as well as incorporating ecological reality in SDMs.

Seagrass losses since mid-20th century fuelled CO2 emissions from soil carbon stocks.

Seagrass meadows store globally significant organic carbon (Corg ) stocks which, if disturbed, can lead to CO2 emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO2 emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil Corg erosion following eutrophication-driven seagrass loss in Cockburn Sound (23 km2 between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher Corg stocks in 50 cm thick soils (4.5 ± 0.7 kg Corg /m2 ) than previously vegetated counterparts (0.5 ± 0.1 kg Corg /m2 ). In deeper areas (>5 m), however, soil Corg stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg Corg /m2 , respectively). The soil Corg sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g Corg  m-2  year-1 , respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil Corg but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil Corg erosion. We estimate, from field studies and satellite imagery, that soil Corg erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06-0.14 Tg CO2-eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11-21 Tg CO2 -eq since the 1950s, increasing cumulative CO2 emissions from land-use change in Australia by 1.1%-2.3% per annum. The patterns described serve as a baseline to estimate potential CO2 emissions following disturbance of seagrass meadows.

Nutrient loading diminishes the dissolved organic carbon drawdown capacity of seagrass ecosystems.

Seawater dissolved organic carbon (DOC) in seagrass meadows is gaining attention for its role in carbon sequestration. Abundant refractory compounds in DOC are exported by seagrass meadows to the deep sea, thereby contributing to long-term carbon drawdown. DOC lability and bacterioplankton communities are key determining factors in this carbon sequestration process, and it has been hypothesized that these may be affected by nutrient loading - however, scientific evidence is so far weak. Here, we studied the response of DOC composition and bacterioplankton communities to nutrient loading in seagrass meadows of the South China Sea. We found that increasing nutrient loads enhanced nitrogen and phosphorus concentrations in DOC, which promoted algae blooms (i.e. epiphyte, phytoplankton and macroalgae) in seagrass meadows, and presumably increased the lability of DOC and its bioavailability to microbes. Also, the relative abundance of K-strategist bacterioplankton communities with the potential to degrade refractory compounds (Acidimicrobiia, Verrucomicrobiales and Micrococcales) increased in the seagrass meadows exposed to high nutrient loads. These results suggest that high nutrient loading can enhance labile DOC composition, and thus increase refractory DOC remineralization rate, thereby weakening the DOC contribution potential of seagrass meadows to long-term carbon sequestration.

Secondary dispersal of seagrass seeds in complex microtopographies.

Motivated by observational and experimental evidence, a theoretical model is proposed to relate the secondary dispersal of seagrass seeds with the complexity of microtopography in natural environments. Complexity is encoded in terms of the Hurst exponent of a fractal description of the microtopographical geometry. The percentage of a seafloor transect where secondary dispersal of seagrass seeds occurs, is quantified in terms of the mainstream velocity, bottom complexity and properties of the seeds. Theoretical expressions are validated considering the cases of Zostera marina and Posidonia oceanica seeds and using computational fluid dynamics (CFD). A total of 200 CFD simulations with different bottom complexities and flow conditions, were done for each seagrass genus to validate the theoretical model. Numerical results agree with theoretical predictions. This finding provides a management tool to assess the degree of seed retention in seed-based restoration areas.

Macroalgae bloom decay decreases the sediment organic carbon sequestration potential in tropical seagrass meadows of the South China Sea.

Seagrass meadows are experiencing worldwide declines mainly because of nutrient enrichment, which always result in macroalgae bloom and consequently periodic collapse and decomposition. However, effects of macroalgae decay on the sediment organic carbon (SOC) sequestration capacity remain unknown. Depending on the macroalgae biomass in eutrophic seagrass meadows of South China Sea, we carried out a laboratory chamber experiment to investigate the sediment labile organic carbon (OC) compositions and the influencing SOC transformation enzyme activity variations of seagrass meadows in response to common macroalgae bloom species (Cladophora spp.) decomposition. Although the dehydrogenase and ß-glucosidase activities were not affected by macroalgae decomposition, the macroalgae decomposition significantly elevated the salt-extractable carbon (SEC) content, SEC/SOC, levels of invertase and polyphenol oxidase activities, and the CO2 release. Overall, this study indicates that macroalgae decomposition stimulates the SOC transformation, and therefore, it is not benefit for SOC sequestration within seagrass meadows of the South China Sea.

Relative sea-level change regulates organic carbon accumulation in coastal habitats.

Because coastal habitats store large amounts of organic carbon (Corg ), the conservation and restoration of these habitats are considered to be important measures for mitigating global climate change. Although future sea-level rise is predicted to change the characteristics of these habitats, its impact on their rate of Corg sequestration is highly uncertain. Here we used historical depositional records to show that relative sea-level (RSL) changes regulated Corg accumulation rates in boreal contiguous seagrass-saltmarsh habitats. Age-depth modeling and geological and biogeochemical approaches indicated that Corg accumulation rates varied as a function of changes in depositional environments and habitat relocations. In particular, Corg accumulation rates were enhanced in subtidal seagrass meadows during times of RSL rise, which were caused by postseismic land subsidence and climate change. Our findings identify historical analogs for the future impact of RSL rise driven by global climate change on rates of Corg sequestration in coastal habitats.

Effects of nutrient loading on sediment bacterial and pathogen communities within seagrass meadows.

Eutrophication can play a significant role in seagrass decline and habitat loss. Microorganisms in seagrass sediments are essential to many important ecosystem processes, including nutrient cycling and seagrass ecosystem health. However, current knowledge of the bacterial communities, both beneficial and detrimental, within seagrass meadows in response to nutrient loading is limited. We studied the response of sediment bacterial and pathogen communities to nutrient enrichment on a tropical seagrass meadow in Xincun Bay, South China Sea. The bacterial taxonomic groups across all sites were dominated by the Gammaproteobacteria and Firmicutes. Sites nearest to the nutrient source and with the highest NH4+ and PO43- content had approximately double the relative abundance of putative denitrifiers Vibrionales, Alteromonadales, and Pseudomonadales. Additionally, the relative abundance of potential pathogen groups, especially Vibrio spp. and Pseudoalteromonas spp., was approximately 2-fold greater at the sites with the highest nutrient loads compared to sites further from the source. These results suggest that proximity to sources of nutrient pollution increases the occurrence of potential bacterial pathogens that could affect fishes, invertebrates and humans. This study shows that nutrient enrichment does elicit shifts in bacterial community diversity and likely their function in local biogeochemical cycling and as a potential source of infectious diseases within seagrass meadows.

Perception of faunal circadian rhythms depends on sampling technique.

Ecologists aim at disentangling how species vary in abundance through spatial and temporal scales, using a range of sampling techniques. Here, we investigated the circadian rhythm of seagrass-associated decapod crustaceans through three sampling techniques. Specifically, we compared the abundance, biomass and structure of seagrass-associated decapod assemblages between the day and night using a hand net, an airlift pump and baited traps. At night, the hand-net consistently collected a larger total abundance and biomass of decapods, what resulted in significant diel differences, which were detected for the total biomass, but not for the total abundance, when decapods were sampled through an airlift pump. Traps, however, collected a larger total abundance, but not total biomass, of decapods during the night. In summary, our perception of faunal diel rhythms is notably influence by the way organisms are sampled.

Shading and simulated grazing increase the sulphide pool and methane emission in a tropical seagrass meadow.

Though seagrass meadows are among the most productive habitats in the world, contributing substantially to long-term carbon storage, studies of the effects of critical disturbances on the fate of carbon sequestered in the sediment and biomass of these meadows are scarce. In a manipulative in situ experiment, we studied the effects of successive loss of seagrass biomass as a result of shading and simulated grazing at two intensity levels on sulphide (H2S) content and methane (CH4) emission in a tropical seagrass meadow in Zanzibar (Tanzania). In all disturbed treatments, we found a several-fold increase in both the sulphide concentration of the sediment pore-water and the methane emissions from the sediment surface (except for CH4 emissions in the low-shading treatment). This could be due to the ongoing degradation of belowground biomass shed by the seagrass plants, supporting the production of both sulphate-reducing bacteria and methanogens, possibly exacerbated by the loss of downwards oxygen transport via seagrass plants. The worldwide rapid loss of seagrass areas due to anthropogenic activities may therefore have significant effects on carbon sink-source relationships within coastal seas.