Large-scale environmental signals in seagrass blue carbon stocks are hidden by high variability at local scales

Increasing focus on nature-based climate change mitigation and adaptation strategies has led to the recognition of seagrasses as globally significant organic carbon (Corg) stocks. However, estimates of carbon stocks have been generally confined to a few regions, with few African studies represented in global datasets. In addition, the extent to which biogeographical and environmental variation shape carbon stocks in marine vegetated environments remains uncertain. For South Africa, Zostera capensis is the dominant seagrass species with limited mapping and quantification of its Corg stocks. Here, we measured Z. capensis Corg stocks at six South African estuaries spanning ∼1800 km of the cool-temperate to subtropical marine environmental gradient. Targeting the intertidal zone of the upper and lower estuary reaches, we collected Z. capensis sediments to a depth of 50 cm and measured the Corg, with the median Corg stock estimated at 24.11 Mg C ha-1 (40.4 ± 53.02; mean ± SD). While this is lower than the global average, these data demonstrate that Z. capensis ecosystems are important contributors to blue carbon stocks in the region. Measured Corg stocks showed significant differences between sampling sites for estuaries; however, we did not detect significant differences between estuaries due to high intra-estuarine Corg variability. Examination of biogeographical regions, terrestrial and marine environmental variables as drivers of Corg variability revealed that annual mean sea surface temperature may explain variation in Corg stocks. Furthermore, we found evidence of signals of biogeographical regions and precipitation driving some of the variability in Corg stocks; however, this requires further investigation. Overall, our estimates for Z. capensis add to ongoing national and global efforts to quantify seagrass Corg stocks across environmental and biogeographic gradients to better determine their contributions as nature-based solutions to climate change.

Response of the toxic dinoflagellate Alexandrium minutum to exudates of the eelgrass Zostera marina

Biotic interactions are a key factor in the development of harmful algal blooms. Recently, a lower abundance of planktonic dinoflagellates has been reported in areas dominated by seagrass beds, suggesting a negative interaction between both groups of organisms. The interaction between planktonic dinoflagellates and marine phanerogams, as well as the way in which bacteria can affect this interaction, was studied in two experiments using a non-axenic culture of the toxic dinoflagellate Alexandrium minutum exposed to increasing additions of eelgrass (Zostera marina) exudates from old and young leaves and to the presence or absence of antibiotics. In these experiments, A. minutum abundance, growth rate and photosynthetic efficiency (Fv/Fm), as well as bacterial abundance, were measured every 48 h. Toxin concentration per cell was determined at the end of both experiments. Our results demonstrated that Z. marina exudates reduced A. minutum growth rate and, in one of the experiments, also the photosynthetic efficiency. These results are not an indirect effect mediated by the bacteria in the culture, although their growth modify the magnitude of the negative impact on the dinoflagellate growth rate. No clear pattern was observed in the variation of toxin production with the treatments.

Comparison of metals in eelgrass (Zostera marina L.) and the environment across the North Pacific Ocean: Environmental processes drive source delivery

Seagrass beds play a critical role in biodiversity maintenance, serving as nursery habitats for fisheries, and aiding in carbon and sediment sequestration in the ecosystem. These habitats receive dissolved and particulate material inputs, like nutrients and heavy metals, affecting both plant health and the ecosystem. Eelgrass (Zostera marina L.), sediments, and water were randomly collected at twenty sites along the temperate North Pacific coasts of Asia and North America to assess heavy metals concentrations (Cr, Cu, Zn, Cd, and Pb). This aimed to understand heavy metal distribution and accumulation patterns in eelgrass tissues, revealing crucial factors influencing metal accumulation. The sampling included various areas, from pristine marine reserves to human-influenced zones, covering industrial, agricultural, and aquaculture regions, enabling a thorough analysis. This study's uniqueness lies in comparing heavy metal distributions in eelgrass tissues with sediments, uncovering unique accumulation patterns. Aboveground eelgrass tissues mainly accumulated Cd, Zn, and Cu, while belowground tissues stored Cr and Pb. Aboveground eelgrass tissues proved reliable in indicating Cd and Pb concentrations in sediments. However, the correlation between Cu, Zn, and Cr in eelgrass tissues and environmental concentrations seemed less direct, requiring further investigation into factors affecting metal accumulation in seagrass. Human activities are probable major contributors to heavy metal presence in Asian marine environments, whereas oceanographic processes serve as primary metal sources in North American Pacific estuaries. Critical discoveries emphasize the necessity for ongoing research on phytotoxic thresholds and in-depth studies on the complex connections between seagrass physiology and environmental metal concentrations. Understanding these dynamics is crucial for evaluating the broader impact of heavy metal pollution on coastal ecosystems and developing effective conservation measures.

Marine heatwaves recurrence aggravates thermal stress in the surfgrass Phyllospadix scouleri

The surfgrass Phyllospadix scouleri grows in highly productive meadows along the Pacific coast of North America. This region has experienced increasingly severe marine heatwaves (MHWs) in recent years. Our study evaluated the impact of consecutive MHWs, simulated in mesocosms, on essential ecophysiological features of P. scouleri. Overall, our findings show that the plants' overall physiological status has been progressively declining. Interestingly, the indicators of physiological stress in photosynthesis only showed up once the initial heat exposure stopped (i.e., during the recovery period). The warming caused increased oxidative damage and a decrease in nitrate uptake rates. However, the levels of non-structural carbohydrates and relative growth rates were not affected. Our findings emphasize the significance of incorporating recovery periods in this type of study as they expose delayed stress responses. Furthermore, experiencing consecutive intense MHWs can harm surfgrasses over time, compromising the health of their meadows and the services they offer to the ecosystem.

Epiphytic and endophytic microbiome of the seagrass Zostera marina: Do they contribute to pathogen reduction in seawater?

Seagrass meadows provide crucial ecosystem services for coastal environments and were shown to reduce the abundance of waterborne pathogens linked to infections in humans and marine organisms in their vicinity. Among potential drivers, seagrass phenolics released into seawater have been linked to pathogen suppression, but the potential involvement of the seagrass microbiome has not been investigated. We hypothesized that the microbiome of the eelgrass Zostera marina, especially the leaf epiphytes that are at direct interface between the seagrass host and the surrounding seawater, inhibit waterborne pathogens thereby contributing to their removal. Using a culture-dependent approach, we isolated 88 bacteria and fungi associated with the surfaces and inner tissues of the eelgrass leaves (healthy and decaying) and the roots. We assessed the antibiotic activity of microbial extracts against a large panel of common aquatic, human (fecal) and plant pathogens, and mined the metabolome of the most active extracts. The healthy leaf epibiotic bacteria, particularly Streptomyces sp. strain 131, displayed broad-spectrum antibiotic activity superior to some control drugs. Gram-negative bacteria abundant on healthy leaf surfaces, and few endosphere-associated bacteria and fungi also displayed remarkable activities. UPLC-MS/MS-based untargeted metabolomics analyses showed rich specialized metabolite repertoires with low annotation rates, indicating the presence of many undescribed antimicrobials in the extracts. This study contributes to our understanding on microbial and chemical ecology of seagrasses, implying potential involvement of the seagrass microbiome in suppression of pathogens in seawater. Such effect is beneficial for the health of ocean and human, especially in the context of climate change that is expected to exacerbate all infectious diseases. It may also assist future seagrass conservation and management strategies.

Eelgrass (Zostera marina L.) populations are threatened by high sea-surface temperatures and impaired waters on Nantucket Island, USA

Eelgrass (Zostera marina L.) is a key foundation species that provides multiple ecosystem services to shallow coastal and estuarine systems in the Northern Hemisphere. It is estimated that, over the last century, up to 50 % of all Z. marina habitat has been lost along the east coast of the USA due to factors including light reduction, eutrophication, and physical disturbance. Warming sea surface temperatures are also believed to be exacerbating losses and the future of this ecosystem is unclear. Here, we assess Z. marina meadows on Nantucket, an island system located 50 km off-shore of Massachusetts, by using common indicators of seagrass plant health and environmental quality. Our results show that Z. marina meadows on Nantucket Island are thermally stressed and light-limited during parts of their peak growing season. This suggests that sea-surface temperatures are a pivotal factor, along with cultural eutrophication, in observed large-scale losses of Z. marina and that further degradation could be expected in the future as the climate continues to warm. Methods from this study may be used by managers as a guide to assess seagrass ecosystem status in degrading systems.

The effect of Zostera noltei recolonization on the sediment mercury vertical profiles of a recovering coastal lagoon

Mercury's extreme toxicity and persistence in the environment justifies a thorough evaluation of its dynamics in ecosystems. Aveiro Lagoon (Portugal) was for decades subject to mercury effluent discharges. A Nature-based Solution (NbS) involving Zostera noltei re-colonization is being tested as an active ecosystem restoration measure. To study the effect of Zostera noltei on the sediment contaminant biogeochemistry, seasonal (summer/winter) sediment, interstitial water and labile mercury vertical profiles were made in vegetated (Transplanted and Natural seagrass meadows) and non-vegetated sites (Bare-bottom area). While no significant differences (p > 0.05) were observed in the sedimentary phase, Zostera noltei presence reduced the reactive/labile mercury concentrations in the top sediment layers by up to 40% when compared to non-vegetated sediment, regardless of season. No differences were found between vegetated meadows, highlighting the fast recovery of the contaminant regulation ecosystem function provided by the plants after re-colonization and its potential for the rehabilitation of historically contaminated ecosystems.

Using seagrass as a nature-based solution: Short-term effects of Zostera noltei transplant in benthic communities of a European Atlantic coastal lagoon

Seagrass meadows provide several ecological functions that improve the overall ecological health of coastal systems and therefore, it is urgent to promote the restoration of such habitats. In Ria de Aveiro, a coastal lagoon in the Atlantic Coast of Portugal, a restoration initiative was responsible for transplanting the dwarf eelgrass Zostera noltei into a highly degraded area. This eelgrass was used as a nature-based solution (NbS) to mitigate some of the impacts of historical mercury contamination. Comparisons of key-species features (density and biomass), and some community-derived indicators (total density and biomass, species richness and Shannon-Wiener index) between the transplanted seagrass patch, their bare vicinities, and their counterpart habitats on the source area, provided signs of the effectiveness of the restoration action on the benthic communities' recovery. Indicators were higher within the restored meadow, and biomass derived indicators of the restored meadow were similar to the source meadow.

Using transplantation to restore seagrass meadows in a protected South African lagoon

Background

Seagrass meadows provide valuable ecosystem services but are threatened by global change pressures, and there is growing concern that the functions seagrasses perform within an ecosystem will be reduced or lost without intervention. Restoration has become an integral part of coastal management in response to major seagrass declines, but is often context dependent, requiring an assessment of methods to maximise restoration success. Here we investigate the use of different restoration strategies for the endangered Zostera capensis in South Africa.

Methods

We assessed restoration feasibility by establishing seagrass transplant plots based on different transplant source materials (diameter (ø) 10 cm cores and anchored individual shoots), planting patterns (line, dense, bullseye) and planting site (upper, upper-mid and mid-intertidal zones). Monitoring of area cover, shoot length, and macrofaunal diversity was conducted over 18 months.

Results

Mixed model analysis showed distinct effects of transplant material used, planting pattern and site on transplant survival and area cover. Significant declines in seagrass cover across all treatments was recorded post-transplantation (2 months), followed by a period of recovery. Of the transplants that persisted after 18 months of monitoring (~58% plots survived across all treatments), seagrass area cover increased (~112%) and in some cases expanded by over >400% cover, depending on type of transplant material, planting arrangement and site. Higher bioturbator pressure from sandprawns (Kraussillichirus kraussi) significantly reduced transplant survival and area cover. Transplant plots were colonised by invertebrates, including seagrass specialists, such as South Africa's most endangered marine invertebrate, the false-eelgrass limpet (Siphonaria compressa). For future seagrass restoration projects, transplanting cores was deemed the best method, showing higher long-term persistence and cover, however this approach is also resource intensive with potentially negative impacts on donor meadows at larger scales. There is a clear need for further research to address Z. capensis restoration scalability and improve long-term transplant persistence.

The role epiphytes play in particle capture of seagrass canopies

Seagrass epiphytic communities act as ecological indicators of the quality status of vegetated coastal environments. This study aims to determine the effect leaf epiphytes has on the sediment capture and distribution from outside sources. Thirteen laboratory experiments were conducted under a wave frequency of 0.5 Hz. Three epiphyte models were attached to a Zostera marina canopy of 100 plants/m2 density. The sediment deposited to the seabed, captured by the epiphytic leaf surface, and remaining in suspension within the canopy were quantified. This study demonstrated that the amount of epiphytes impacts on the sediment stocks. Zostera marina canopies with high epiphytic areas and long effective leaf heights may increase the sediment captured on the epiphyte surfaces. Also, reducing suspended sediment and increasing the deposition to the seabed, therefore enhancing the clarity of the water column. For largest epiphytic areas, a 34.5% increase of captured sediment mass is observed. The sediment trapped on the leaves can be 10 times greater for canopies with the highest epiphytic areas than those without epiphytes. Therefore, both the effective leaf length and the level of epiphytic colonization are found to determine the seagrass canopy ability at distributing sediment.

Vegetative fragment production as a means of propagule dispersal for tropical seagrass meadows

BACKGROUND AND AIMS

Long distance dispersal (LDD) contributes to the replenishment and recovery of tropical seagrass habitats exposed to disturbance, such as cyclones and infrastructure development. However, our current knowledge regarding the physical attributes of seagrass fragments that influence LDD predominantly stems from temperate species and regions. The goal of this paper is to measure seagrass fragment density and viability in two tropical species, assessing various factors influencing their distribution.

METHODS

We measured the density and viability of floating seagrass fragments for two tropical seagrass species (Zostera muelleri and Halodule uninervis) in two coastal seagrass meadows in the central Great Barrier Reef World Heritage Area, Australia. We assessed the effect of wind speed, wind direction, seagrass growing/senescent season, seagrass meadow density, meadow location and dugong foraging intensity on fragment density. We also measured seagrass fragment structure and fragment viability; i.e., potential to establish into a new plant.

KEY RESULTS

We found that seagrass meadow density, season, wind direction and wind speed influenced total fragment density, while season and wind speed influenced the density of viable fragments. Dugong foraging intensity did not influence fragment density. Our results indicate that wave action from winds combined with high seagrass meadow density increases seagrass fragment creation, and that more fragments are produced during the growing than the senescent season. Seagrass fragments classified as viable for Z. muelleri and H. uninervis had significantly more shoots and leaves than non-viable fragments. We collected 0.63 (±0.08 SE) floating viable fragments 100 m-2 in the growing season, and 0.13 (±0.03 SE) viable fragments 100 m-2 in the senescent season. Over a third (38%) of all fragments collected were viable.

CONCLUSION

There is likely to be a large number of viable seagrass fragments available for long distance dispersal. This study's outputs can inform dispersal and connectivity models that are used to direct seagrass ecosystem management and conservation strategies.

How does ocean acidification affect Zostera marina during a marine heatwave?

Extreme ocean events caused by global warming, such as marine heatwaves (MHWs) and ocean acidification (OA), are projected to intensify. A combination of extreme events may have severe consequences for marine ecosystems. Zostera marina was selected to understand how seagrass adapts to OA in extremely hot conditions. By combining morphology, transcriptomics, and metabolomics under mesoscale experimental conditions, we systematically investigated the response characteristics of Z. marina. Extremely high temperatures had a pronounced effect on growth, and the combined effect of OA mitigated the inhibitory effect of MHW. Both transcriptomic and metabolomic results showed that Z. marina resisted OA and MHW by upregulating the TCA cycle, glycolysis, amino acid metabolism, and relevant genes, as well as by activating the antioxidant system. The results of this study serve to improve our understanding of dual effects of factors of climate change on seagrass and may be used to direct future management and conservation efforts.

Assessment of pixel-oriented k-NN machine learning algorithm performance for the interannual remote sensing monitoring of eelgrass beds at the mouth of the Romaine

Eelgrass cover extent is among the most reliable indicators for measuring changes in coastal ecosystems. Eelgrass has colonized the mouth of the Romaine River and has become a part of environmental monitoring there since 2013. The presence of eelgrass in this area is an essential factor for the early detection of changes in the Romaine coastal ecosystem. This will act as a trigger for an appropriate environmental response to preserve ecosystem health. In this paper, a cost- and time-efficient workflow for such spatial monitoring is proposed using a pixel-oriented k-NN algorithm. It can then be applied to multiple modellers to efficiently map the eelgrass cover. Training data were collected to define key variables for segmentation and k-NN classification, providing greater edge detection for the presence of eelgrass. The study highlights that remote sensing and training data must be acquired under similar conditions, replicating methodologies for collecting data on the ground. Similar approaches must be used for the zonal statistic requirements of the monitoring area. This will allow a more accurate and reliable assessment of eelgrass beds over time. An overall accuracy of over 90% was achieved for eelgrass detection for each year of monitoring.

Ensemble habitat suitability modeling for predicting optimal sites for eelgrass (Zostera marina) in the tidal lagoon ecosystem: Implications for restoration and conservation

Seagrass systems are in decline, mainly due to anthropogenic pressures and ongoing climate change. Implementing seagrass protection and restoration measures requires accurate assessment of suitable habitats. Commonly, such assessments have been performed using single-algorithm habitat suitability models, nearly always based on low environmental resolution information and short-term species data series. Here we address eelgrass (Zoostera marina) meadows' large-scale decline (>80%) in Shandong province (Yellow Sea, China) by developing an ensemble habitat model (EHM) to inform eelgrass conservation and restoration strategies in the Swan Lake (SL). For this, we applied a weighted EHM derived from ten single-algorithm models including profile, regression, classification, and machine learning methods to generate a high-resolution habitat suitability map. The EHM was constructed based on the predictive performances of each model, by combining a series of present-absent eelgrass datasets from recent years coupled with oceanographic and sediment data. The model was cross-validated with independent historical datasets, and a final habitat suitability map for conservation and restoration was generated. Our EHM scheme outperformed all single models in terms of habitat suitability, scoring ∼0.95 for both true statistic skill (TSS) and area under the curve (AUC) performance criteria. Machine learning methods outperformed profile, regression and classification methods. Regarding model explanatory variables, overall, topographic characteristics such as depth (DEP) and seafloor slope (SSL) are the most significant factors determining the distribution of eelgrass. The EHM predicted that the overlapping area was almost 90% of the current eelgrass habitat. Using results from our EHM, a LOESS regression model for the relationship of the habitat suitability to both the biomass and density of Z. marina outperformed better than the classic Ordinary Least Squares regression model. The EHM is a promising tool for supporting eelgrass protection and restoration areas in temperate lagoons as data availability improves.

Reactive persistence, spatial management, and conservation of metapopulations: An application to seagrass restoration

Assessing the conditions for persistence of spatially structured populations, especially those that are exploited by humans or threatened by global change, is of critical importance to inform management and conservation efforts. Observations for entire metapopulations are usually incomplete and rarely, if ever, sufficiently long to deduce population persistence from spatial patterns of abundance. Instead, insights based on metapopulation theory are often used for interpreting the demographic trajectories of real populations and for informing management decisions. The classical theoretical tool used to assess conditions for metapopulation persistence is the "invasibility criterion," which characterizes the asymptotic, or long-term, stability of a small colonizing population. Essentially, when the linear operator governing the metapopulation dynamics of an invasion event has a positive eigenvalue, recovery and resistance to extinction (resilience) are implied. The converse, however, is not necessarily the case-an invasion may grow over multiple generations, even when the eigenvalues indicate that extinction will eventually occur, a situation referred to here as "reactive persistence." For the management, restoration, and conservation of real metapopulations subject to continual disturbance, this transient behavior is often more relevant than the asymptotic behavior over long time scales. We develop the theoretical tools for assessing reactive persistence, demonstrating how the conditions for asymptotic and reactive persistence differ in both the patch-occupancy models suited to many terrestrial populations and those where local patch extinctions can be disregarded in the dynamics, often suited to marine species. After presenting the mathematical basis for generalizing the invasibility criterion to include reactive persistence, we illustrate how these concepts and tools can be applied in practice, using as a case study the population ecology and restoration of the seagrass Zostera muelleri (Irmisch ex Ascherson, 1867) in the Port of Gladstone in the Great Barrier Reef World Heritage Area Australia. It is shown how the analysis of the transient dynamics of the Z. muelleri metapopulation can be used to guide restoration efforts. Moreover, it is demonstrated that these reactive persistence concepts provide a more appropriate basis for site prioritization for restoration interventions than traditional stability analysis.

Disposable surgical masks affect the decomposition of Zostera muelleri

The coronavirus pandemic has caused a surge in the use of both disposable and re-usable mask pollution globally. It is important to understand the potential impact this influx of novel pollution has on key ecological processes, such as detrital dynamics. We aimed to understand the impact mask pollution has on the decomposition of a common coastal seagrass, Zostera muelleri. Using an outdoor mesocosm system with heater chiller units and a gas mixer, we were able to test the impact of both re-usable single-ply homemade cotton masks and disposable surgical masks on samples of Z. muelleri detritus under different environmental conditions. We found that disposable masks, but not re-usable masks, significantly increased decomposition of Z. muelleri detritus. This may be due to the increased surface area available for detritivorous microorganism colonisation, driving further decomposition. This could have negative ramifications for seagrass communities and adjacent ecosystems.

Computational estimation of sediment symbiotic bacterial structures of seagrasses overgrowing downstream of onshore aquaculture

Coastal seagrass meadows are essential in blue carbon and aquatic ecosystem services. However, this ecosystem has suffered severe eutrophication and destruction due to the expansion of aquaculture. Therefore, methods for the flourishing of seagrass are still being explored. Here, data from 49 public coastal surveys on the distribution of seagrass and seaweed around the onshore aquaculture facilities are revalidated, and an exceptional area where the seagrass Zostera marina thrives was found near the shore downstream of the onshore aquaculture facility. To evaluate the characteristics of the sediment for growing seagrass, physicochemical properties and bacterial ecological evaluations of the sediment were conducted. Evaluation of chemical properties in seagrass sediments confirmed a significant increase in total carbon and a decrease in zinc content. Association analysis and linear discriminant analysis refined bacterial candidates specified in seagrass overgrown- and nonovergrown-sediment. Energy landscape analysis indicated that the symbiotic bacterial groups of seagrass sediment were strongly affected by the distance close to the seagrass-growing aquaculture facility despite their bacterial population appearing to fluctuate seasonally. The bacterial population there showed an apparent decrease in the pathogen candidates belonging to the order Flavobacteriales. Moreover, structure equation modeling and a linear non-Gaussian acyclic model based on the machine learning data estimated an optimal sediment symbiotic bacterial group candidate for seagrass growth as follows: the Lachnospiraceae and Ruminococcaceae families as gut-inhabitant bacteria, Rhodobacteraceae as photosynthetic bacteria, and Desulfobulbaceae as cable bacteria modulating oxygen or nitrate reduction and oxidation of sulfide. These observations confer a novel perspective on the sediment symbiotic bacterial structures critical for blue carbon and low-pathogenic marine ecosystems in aquaculture.

Limited recovery following a massive seagrass decline in subarctic eastern Canada

Over the last few decades, there has been an increasing recognition for seagrasses' contribution to the functioning of nearshore ecosystems and climate change mitigation. Nevertheless, seagrass ecosystems have been deteriorating globally at an accelerating rate during recent decades. In 2017, research into the condition of eelgrass (Zostera marina) along the eastern coast of James Bay, Canada, was initiated in response to reports of eelgrass decline by the Cree First Nations of Eeyou Istchee. As part of this research, we compiled and analyzed two decades of eelgrass cover data and three decades of eelgrass monitoring data (biomass and density) to detect changes and assess possible environmental drivers. We detected a major decline in eelgrass condition between 1995 and 1999, which encompassed the entire east coast of James Bay. Surveys conducted in 2019 and 2020 indicated limited changes post-decline, for example, low eelgrass cover (<25%), low aboveground biomass, smaller shoots than before 1995, and marginally low densities persisted at most sites. Overall, the synthesized datasets show a 40% loss of eelgrass meadows with >50% cover in eastern James Bay since 1995, representing the largest scale eelgrass decline documented in eastern Canada since the massive die-off event that occurred in the 1930s along the North Atlantic coast. Using biomass data collected since 1982, but geographically limited to the sector of the coast near the regulated La Grande River, generalized additive modeling revealed eelgrass meadows are affected by local sea surface temperature, early ice breakup, and higher summer freshwater discharge. Our results caution against assuming subarctic seagrass ecosystems have avoided recent global declines or will benefit from ongoing climate warming.

Advances in understanding multilevel responses of seagrasses to hypersalinity

Human- and nature-induced hypersaline conditions in coastal systems can lead to profound alterations of the structure and vitality of seagrass meadows and their socio-ecological benefits. In the last two decades, recent research efforts (>50 publications) have contributed significantly to unravel the physiological basis underlying the seagrass-hypersalinity interactions, although most (∼70%) are limited to few species (e.g. Posidonia oceanica, Zostera marina, Thalassia testudinum, Cymodocea nodosa). Variables related to photosynthesis and carbon metabolism are among the most prevalent in the literature, although other key metabolic processes such as plant water relations and responses at molecular (i.e. gene expression) and ultrastructure level are attracting attention. This review emphasises all these latest insights, offering an integrative perspective on the interplay among biological responses across different functional levels (from molecular to clonal structure), and their interaction with biotic/abiotic factors including those related to climate change. Other issues such as the role of salinity in driving the evolutionary trajectory of seagrasses, their acclimation mechanisms to withstand salinity increases or even the adaptive properties of populations that have historically lived under hypersaline conditions are also included. The pivotal role of the costs and limits of phenotypic plasticity in the successful acclimation of marine plants to hypersalinity is also discussed. Finally, some lines of research are proposed to fill the remaining knowledge gaps.

The influence of decreased salinity levels on the survival, growth and physiology of eelgrass Zostera marina

Low salinity generally promotes the growth and propagation of temperate seagrasses, but the appropriate range is unclear. We subjected shoots of eelgrass Zostera marina to different salinity levels [10, 15, 20, 25, 30 PSU (control)] for 6 weeks under controlled laboratory conditions. We measured eelgrass responses in terms of survivorship, growth, productivity, leaf pigmentation and carbohydrate concentrations. Survival analysis combined with growth assessment suggested that the optimal salinity range for the propagation of Z. marina shoots was 18-21 PSU. Structural equation model (SEM) analysis indicated that the promotion effect of decreased salinity levels on the survival and growth of Z. marina shoots mainly depended on the increase in chlorophyll content and the accumulation and synthesis of nonstructural carbohydrates. The carotenoid content and soluble sugar content of the aboveground tissues of Z. marina shoots exposed to 20 PSU were 1.1 and 1.6 times higher than those of shoots under the control, respectively. The results will provide valuable data that could prove helpful in the development of efficient artificial propagation technology for Z. marina shoots.

A Digital Twin modelling framework for the assessment of seagrass Nature Based Solutions against storm surges

In this paper we demonstrate a novel framework for assessing nature-based solutions (NBSs) in coastal zones using a new suite of numerical models that provide a virtual "replica" of the natural environment. We design experiments that use a Digital Twin strategy to establish the wave, sea level and current attenuation due to seagrass NBSs. This Digital Twin modelling framework allows us to answer "what if" scenario questions such as: (i) are indigenous seagrass meadows able to reduce the energy of storm surges, and if so how? (ii) what are the best seagrass types and their landscaping for optimal wave and current attenuation? An important result of the study is to show that the landscaping of seagrasses is an important design choice and that seagrass does not directly attenuate the sea level but the current amplitudes. This framework reveals the link between seagrass NBS and the components of the disruptive potential of storm surges (waves and sea level) and opens up new avenues for future studies.

A Cytotoxic Heterodimeric Cyclic Diarylheptanoid with a Rearranged Benzene Ring from the Seagrass Zostera marina

The widespread seagrass Zostera marina contains a new diarylheptanoid heterodimer, zosterabisphenone C (1), featuring an unprecedented rearrangement of one of its benzene rings to a cyclopentenecarbonyl unit. The planar structure and absolute configuration of zosterabisphenone C were elucidated by a combination of spectroscopic (MS, ECD, and low-temperature NMR) and computational (DFT-NMR and DFT-ECD) evidence. Consistent with the previously isolated zosterabisphenones, compound 1 was selectively cytotoxic against HCT 116 adenocarcinoma colon cancer cells, reducing their viability by 73% at 10 μM (IC₅₀ of 7.6 ± 1.1 μM). The biosynthetic origin of zosterabisphenone C (1) from an oxidative rearrangement of zosterabisphenone A (4) is proposed.

Phenolic fingerprints of the Pacific seagrass Phyllospadixtorreyi - Structural characterization and quantification of undescribed flavonoid sulfates

Four undescribed flavonoid sulfates were isolated from Phyllospadix torreyi S. Watson foliar tissue. In addition, nine known flavonoid sulfates and three phenolic acids were isolated from the same extract, of which seven had never been reported for the genus Phyllospadix. Structural elucidation of individual phenolics was assigned using complementary informations from their spectral evidence (HPLC-DAD, LC-MS, NMR, and UV) and chemical behavior. The inter-annual variation in phenolic concentrations was determined by quantitative HPLC-DAD over a three-year period. The results showed a relative constancy of phenolic content over time and the high prevalence of flavonoid disulfates (70-90% of the total flavonoids detected). All samples were found dominated by the unreported nepetin 7, 3'-disulfate and 5-methoxyluteolin 7, 3'-disulfate, followed by luteolin 7, 3'-disulfate. Considering the economic potential of flavonoid sulfates in the pharmaceutical and nutraceutical segments, a sample of detrital leaves was also analyzed. The same phenolic pattern was found and the concentration of the individuals, although lower than in fresh material, makes this abundant biomass of interest for dietary and pharmaceutical applications.

Simulated megaherbivore grazing as a driver of seagrass flowering

Seagrass meadows are an important habitat for Testudines (sea turtles) and Sirenia (dugong and manatee) megaherbivores. Megaherbivores can influence the structuring of seagrass meadows; for example, foraging patterns have been found to relate to seagrass phenological strategy. However, as these observations are derived from uncontrolled field studies, it is unclear whether grazing drives such changes or if the changes are related to other factors (e.g., temperature, tidal depth, light). In the present study, a mesocosm experiment was designed to test the impacts of grazing on metrics of flowering of Zostera muelleri over two consecutive flowering seasons. Prior to each flowering season, plants were cropped to 3 cm and 1 cm lengths to represent turtle and dugong grazing, respectively. This study measured the timing of flowering, the number of flowering shoots, the height of the flowering shoot, and the number of spathes (sheathing bracts containing seeds) per flowering shoot in each replicate (n = 5) weekly. Cropping had no significant influence on the timing of flowering (i.e., number of days to first and peak flowering) indicating that it is not a trigger for flowering. However, cropping significantly reduced the maximum density of flowering shoots and spathes, which was proposed to be due to resource allocation differences between clonal growth and flower production. A reduction in the flowering ratio was observed in both cropped plant groups and the relatively high density and the ratio of flowering observed in the 1 cm group indicate that the plant was adapting to cope with stress. Morphology of flowering (i.e., the maximum height of flowering shoot and the maximum number of spathes per flowering shoot) was not significantly affected by cropping and these two variables were strongly correlated. The results suggest that cropping can influence the overall flowering densities in a season but not the timing of flowering. This study demonstrated that cropping prior to the flowering season can reduce the expected production of spathes in seed nurseries and suggests it may be beneficial to consider megaherbivores in seed-based restoration activities.

A network analysis of connected biophysical pathways to advice eelgrass (Zostera marina) restoration

The North Sea and the Baltic Sea, including Danish coastal waters, have experienced a drastic decline in eelgrass Zostera marina coverage during the past century. Around 1900, eelgrass meadows covered about 6700 km² of Danish coastal waters while the current potential distribution area is only about one third of this. In some areas, the potential distribution area is far from realized, and restoration efforts are needed to assist recovery. Such efforts are challenging, and resource-demanding and careful site selection is, therefore, important. In the present study, we aim to identify the connectivity of eelgrass populations as a basis for guiding site selection for restoration. We developed a coupled biophysical model to study eelgrass dispersal in the Kattegat. Partly submerged particles simulated the dispersal of reproductive eelgrass shoots containing seeds during the flowering season July-September. We then used network analysis to identify the potential connectivity between populations. We evaluated connectivity based on In-strength, Betweenness and Eigenvector centrality metrics and identified key areas in the Kattegat such as the central part of Aalborg Bay, to be considered to restore the network of Z. marina patches. The study proves the potentials of combining hydrodynamic models and network analysis to support marine conservation and planning, and highlights the importance of collaboration between ecologists, oceanographers, and practitioners in this endeavour.

An examination of seed germination and seedling growth of Zostera marina for planting-time selection in Rongcheng Bay, Shandong Peninsula, China

This study firstly quantified the responses of seeds of Zostera marina to different planting times (22 September, 5 October, 23 October, 7 November and 20 November in 2015) through a field seed-planting experiment over a two year period. The suitable seed planting time required by the seeds of Z. marina was evaluated. The seedling establishment rate of Z. marina subjected to different planting times ranged from 7% to 55%, with the higher values attained on the treatments of 22 September and 5 October. New plant patches from seed were fully developed and well maintained on the planting time of 22 September, 5 October and 23 October after 2 years following planting. The shoot density under the three treatments ranged from 62 shoots per replicate to 72 shoots per replicate with an average of 67 shoots per replicate in September 2017. According to the propagation assessment and growth analysis, we found that the planting time from mid-September to mid-October may be the optimal time to plant seeds of Z. marina in our experimental site. Our results demonstrate that seed planting time has an important effect on the effectiveness of eelgrass restoration and provide data that could prove helpful in the development of successful eelgrass restoration.

Local adaptation in a marine foundation species: Implications for resilience to future global change

Environmental change is multidimensional, with local anthropogenic stressors and global climate change interacting to differentially impact populations throughout a species' geographic range. Within species, the spatial distribution of phenotypic variation and its causes (i.e., local adaptation or plasticity) will determine species' adaptive capacity to respond to a changing environment. However, comparatively less is known about the spatial scale of adaptive differentiation among populations and how patterns of local adaptation might drive vulnerability to global change stressors. To test whether fine-scale (2-12 km) mosaics of environmental stress can cause adaptive differentiation in a marine foundation species, eelgrass (Zostera marina), we conducted a three-way reciprocal transplant experiment spanning the length of Tomales Bay, CA. Our results revealed strong home-site advantage in growth and survival for all three populations. In subsequent common garden experiments and feeding assays, we showed that countergradients in temperature, light availability, and grazing pressure from an introduced herbivore contribute to differential performance among populations consistent with local adaptation. Our findings highlight how local-scale mosaics in environmental stressors can increase phenotypic variation among neighboring populations, potentially increasing species resilience to future global change. More specifically, we identified a range-center eelgrass population that is pre-adapted to extremely warm temperatures similar to those experienced by low-latitude range-edge populations of eelgrass, demonstrating how reservoirs of heat-tolerant phenotypes may already exist throughout a species range. Future work on predicting species resilience to global change should incorporate potential buffering effects of local-scale population differentiation and promote a phenotypic management approach to species conservation.

Loss of surficial sedimentary carbon stocks in seagrass meadows subjected to intensive clam harvesting

Seagrass carbon stocks are vulnerable to physical disturbance. We assessed the effect of clam harvesting on the organic carbon (C_org) stocks in surface sediments in four intertidal Zostera noltei meadows on the Iberian Atlantic coast (Spain and Portugal), by comparing undisturbed and harvested areas. We also monitored the spatial cover of the meadows throughout the growing season. Sedimentary C_org content and C_org stocks were about four times lower in intensively harvested areas than in control areas, but there were not differences between areas with low harvesting pressure and control areas. Reductions of 53-85% in sedimentary C_org stocks of Z. noltei meadows were caused by intensive clam harvesting. The effect of intensive clam harvesting on C_org stocks increased throughout the growing season, but the area covered by the seagrass increased from 21 to 37%, suggesting rapid recovery of seagrass canopies and potential recovery of sedimentary C_org stocks.

Adaptive intertidal seed-based seagrass restoration in the Dutch Wadden Sea

Seagrasses form the foundation of many coastal ecosystems but are rapidly declining on a global scale. The Dutch Wadden Sea once supported extensive subtidal seagrass meadows that have all disappeared. Here, we report on the setbacks and successes of intertidal seed-based restoration experiments in the Dutch Wadden Sea between 2014-2017. Our main goals were to 1) optimize plant densities, and 2) reduce seed losses. To achieve our goals, we conducted research-based, adaptive seagrass (Zostera marina) restoration, adjusting methods yearly based on previous results. We applied various seeding methods in three subsequent years-from Buoy Deployed Seeding (BuDS), and 'BuDS-in-frame' in fall, to a newly developed 'Dispenser Injection Seeding' (DIS) method. Our adaptive experimental approach revealed high seed losses between seeding and seedling establishment of the BuDS methods (>99.9%), which we mitigated by controlled harvest and storage of seeds throughout fall and winter, followed by DIS-seeding in spring. These iterative innovations resulted in 83 times higher plant densities in the field (0.012 to 1.00 plants m-2) and a small reduction in seed loss (99.94 to 99.75%) between 2015-2017. Although these developments have not yet resulted in self-sustaining seagrass populations, we are one step closer towards upscaling seagrass restoration in the Dutch Wadden Sea. Our outcomes suggest that an iterative, research-based restoration approach that focuses on technological advancement of precision-seeding may result in advancing knowledge and improved seed-based seagrass restoration successes.

Enhancing the resilience of Zostera noltei seagrass meadows against Arenicola spp. bio-invasion: A decision-making approach

Seagrass meadows provide important and valuable ecosystem services. They are affected by several natural and human-induced stressors, but a combination of natural recovery and management actions have recently inverted the worldwide reduction. The main objectives of this study were to provide science-based knowledge on ecology and restoration, framed on environmental-related policies. By coupling the general guidelines with practical experience, obtained from sequential in situ experiments carried out for several months in a show-case study area, this study provides guidelines useful for restoration practitioners. A decision-making approach is proposed to answer the following questions: 1) What is the best Zostera noltei transplanting method? 2) What is the best technique to reduce the bioturbation activity of Arenicola spp.?, 3) Do bioturbation reduction techniques affect the survival rate of Z. noltei transplants?, and finally, 4) What are the key steps to maximize the success of a Z. noltei transplant and increase the species' resilience? Having a Portuguese coastal lagoon as show-case (Mira Channel, Ria de Aveiro), different transplant and restoration methodologies were tested (i.e. metal frames, nails, bamboo sticks, shoots inserted unanchored into the sediment, and intact units of sediment with seagrasses, named as SODs) to assure low environmental impact on donor meadows, high survival rate of transplanted shoots and the recovery of fragmented or lost meadows. Moreover, to potentially reverse a degraded Arenicola spp. colonized seagrass habitat, different types of natural membranes were tested. Results showed that the best transplanting method is the use of SODs as the self-facilitation process of Z. noltei is enhanced, while being the least invasive for the donor population. The use of a natural membrane can significantly decrease the bioturbation stress caused by Arenicola spp., with jute membrane being the best option, given its cost-handling-benefit trade-offs. Enhancing the success of seagrass restoration requires the implementation of effective measures by environmental restoration practitioners. We defined a three-step process to improve the resilience of Z. noltei. This stepwise approach consists on 1) Characterization of the donor population, 2) Identification of the constraints and implementation of measures to prevent them, and 3) Scale-up the restoration plan. The application of this stepwise approach in intertidal coastal and estuarine systems management will, therefore, facilitate the success of Z. noltei restoration plans.

Eelgrass (Zostera marina) and its epiphytic bacteria facilitate the sinking of microplastics in the seawater

Marine microplastics have received considerable attention as a global environmental issue. However, despite the constant accumulation of microplastics in the ocean, their transport processes and mechanisms remain poorly understood. This study investigated microplastics in the sediments of seagrass meadows and nearby regions without seagrass along the Shandong coast and found that the sediment in the seagrass meadows was a sink for microplastics. Subsequently, we evaluated the influence of eelgrass (Zostera marina), a common coastal seagrass, on the sedimentation of suspended polystyrene microplastics. The results showed that 0.5, 1.0, and 2.0 g/L eelgrass leaves decreased the abundance of microplastics in seawater in a dose-dependent manner over a period of 3-48 h under shaking conditions at 120 rpm at 22 °C. After 48 h of shaking, microplastic abundances in the 0.5, 1.0, and 2.0 g/L eelgrass groups significantly decreased by 46.9%, 53.1%, and 88.4%, respectively. Microplastics can adhere to eelgrass leaves and form biofilms, which promoted the formation of white floc that traps the suspended microplastics, causing them to sink. Furthermore, two epiphytic bacteria (Vibrio and Exiguobacterium) isolated from the eelgrass leaves decreased the abundances of suspended microplastics by 95.7% and 94.5%, respectively, in 48 h by accelerating the formation of biofilms on the microplastics. Therefore, eelgrass and its epiphytic bacteria facilitated the sinking of microplastics and increased the accumulation of microplastics in the sediments of seagrass meadows in coastal regions.

Rapid enhancement of multiple ecosystem services following the restoration of a coastal foundation species

The global decline of marine foundation species (kelp forests, mangroves, salt marshes, and seagrasses) has contributed to the degradation of the coastal zone and threatens the loss of critical ecosystem services and functions. Restoration of marine foundation species has had variable success, especially for seagrasses, where a majority of restoration efforts have failed. While most seagrass restorations track structural attributes over time, rarely do restorations assess the suite of ecological functions that may be affected by restoration. Here we report on the results of two small-scale experimental seagrass restoration efforts in a central California estuary where we transplanted 117 0.25-m² plots (2,340 shoots) of the seagrass species Zostera marina. We quantified restoration success relative to persistent reference beds, and in comparison to unrestored, unvegetated areas. Within three years, our restored plots expanded ~8,500%, from a total initial area of 29 to 2,513 m² . The restored beds rapidly began to resemble the reference beds in (1) seagrass structural attributes (canopy height, shoot density, biomass), (2) ecological functions (macrofaunal species richness and abundance, epifaunal species richness, nursery function), and (3) biogeochemical functions (modulation of water quality). We also developed a multifunctionality index to assess cumulative functional performance, which revealed restored plots are intermediate between reference and unvegetated habitats, illustrating how rapidly multiple functions recovered over a short time period. Our comprehensive study is one of few published studies to quantify how seagrass restoration can enhance both biological and biogeochemical functions. Our study serves as a model for quantifying ecosystem services associated with the restoration of a foundation species and demonstrates the potential for rapid functional recovery that can be achieved through targeted restoration of fast-growing foundation species under suitable conditions.

Organophosphate pesticides sequestered in tissues of a seagrass species - Zostera capensis from a polluted watershed

Organophosphate pesticides (OPPs) are persistent in the environment, but little information is available on their bioaccumulation in seagrass. In this study, the seagrass - Zostera capensis was collected from Swartkops Estuary in South Africa to investigate the bioaccumulation of OPPs from contaminated sediments and the water column. This plant was chosen because it grows abundantly in the estuary's intertidal zone, making it a viable phytoremediator in the urban environment. Extraction was performed by the QuEChERS method followed by GC-MS analysis. The mean concentration of ∑OPPs ranged from 0.01 to 0.03 μg/L for surface water; 6.20-13.35 μg/kg dw for deep-rooted sediments; 18.79-37.75 μg/kg dw for leaf tissues and 12.14-39.80 μg/kg dw for root tissues of Z. capensis. The biota-sediment accumulation factors (BSAFs) were greater than one, indicating the potential for Z. capensis to bioaccumulate and intercept the targeted pesticides. A weak insignificant correlation observed between log BSAFs and log Kow indicates that the bioaccumulation of OPPs in tissues of Z. capensis were not dependent on the Kow. Eight of the selected pesticides had root-leaf translocation factors (TFr-l) greater than 1, indicating that Z. capensis can transport these chemicals from roots to leaves. The results from this study implies that this plant species can clean up OPP contamination in the environment.

Ecophysiological responses and self-protective canopy effects of surfgrass (Phyllospadix torreyi) in the intertidal

Intertidal seagrasses are subjected to desiccation and direct solar radiation during low tides. It is assumed that the canopy structure can self-protect the underlying shoots during these events, although there is no evidence on a physiological basis. The physiological responses of the surfgrass Phyllospadix torreyi were examined when emerged during low tide, on i) shoots overlaying the canopy, and ii) shoots sheltered within the canopy. Leaf water potential and water content decreased in external leaves after emersion, and the higher concentration of organic osmolytes reflected osmoregulation. Additionally, these shoots also exhibited a drastic reduction in carbohydrates after re-immersion, likely from cellular damage. Lipid peroxidation and antioxidant activity increments were also detected, while photosynthetic efficiency strongly diminished from direct exposure to solar radiation. Conversely, the sheltered shoots did not dehydrate and solely accumulated some oxidative stress, likely resulting from the warming of the canopy. In conclusion, the leaf canopy structure buffers physiological stress in the sheltered shoots, thus acting as a self-protective mechanism to cope with emersion.

Comparative studies on the response of Zostera marina leaves and roots to ammonium stress and effects on nitrogen metabolism

Coastal eutrophication has resulted in the rapid loss and deterioration of seagrass beds worldwide. The high concentration of ammonium in eutrophic aquatic environments has been invoked as the main cause. In this study, leaves and roots of the seagrass Zostera marina were treated with simulated eutrophic seawater with elevated ammonium concentrations. The tolerance to ammonium stress and mechanism of nitrogen metabolism detoxification in different tissues were investigated. The results showed that high ammonium stress significantly affected the growth of leaves and had a negative effect on photosynthesis. The root activity of Z. marina was not inhibited at ammonium concentrations of ≤100 mg/L, indicating that the roots exhibited tolerance to ammonium stress. Increasing ammonium concentrations led to a higher increase of ammonium and free amino acid (FAA) contents in leaves than in roots. However, nitrogen storage decreased in Z. marina leaves after high ammonium treatments. The enzyme activity and gene expression of glutamine synthetase (GS) in roots were significantly higher than in the leaves even under ammonium stress. Meanwhile, ammonium stress increased the enzyme activities and gene expression of glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) in roots, which suggested that the roots had a strong ability to assimilate ammonium under ammonium stress. In contrast, although the GOGAT and GDH activity and gene expression in the leaves were initially increased, they significantly decreased when the ammonium concentration exceeded 100 mg/L. These results indicated that the concentration of 100 mg/L might be a threshold marking a transition from tolerance to toxicity for the leaves. Our study demonstrates that Z. marina leaves could be prone to higher damage than roots because the mechanism of ammonium assimilation in leaves is more susceptible to ammonium toxicity.

The super typhoon Lekima (2019) resulted in massive losses in large seagrass (Zostera japonica) meadows, soil organic carbon and nitrogen pools in the intertidal Yellow River Delta, China

Seagrass meadows are key ecosystems, and they are among the most threatened habitats on the planet. Increased numbers of extreme climate events, such as hurricanes and marine heatwaves have caused severe damage to global seagrass meadows. The largest Zostera japonica meadows in China are located in the Yellow River Delta. It had a distribution area of 1031.8 ha prior to August 2019 when the Yellow River Delta was severely impacted by the passage of typhoon Lekima. In this study, we compared field data collected before and after the typhoon to determine its impact on seagrass beds in the Yellow River Delta. The super typhoon caused dramatic changes in Z. japonica in the Yellow River Delta, resulting in a greater than 100-fold decrease in distribution area, a greater than 35% loss of soil organic carbon, and a greater than 65% loss of soil total nitrogen in the top 35 cm sediments. Owing to the lack of seeds and overwintering shoots, as well as the small remaining distribution area, recovery was impossible, even though environmental factors were still suitable for species growth. Thus, restoration efforts are required for seagrass meadow recovery. Additionally, the long-term monitoring of this meadow will provide new information on the ecosystem's status and will be useful for future protection.

Small-scale patches of detritus as habitat for invertebrates within a Zostera noltei meadow

Seagrass detritus can attract numerous invertebrates as it provides food and substrate within the meadow or in adjacent environments. Nonetheless, several factors could modify the invertebrate response to this habitat. In this study, we tested if epifaunal colonisation of Zostera noltei detritus was related to substrate availability rather than food and whether colonising assemblages were similar according to the meadow structural complexity. Litterbags filled with natural or artificial detritus were deployed within an eelgrass meadow in a Mediterranean coastal lagoon (Thau lagoon, France). Colonisation appeared to be driven by the presence of detritus, with similar assemblages in natural and artificial substrate, but with more individuals than the empty bags, used as controls. There were also no differences according to habitat complexity. These findings show that detritus, acting as a faunal magnet, plays an important role in maintaining biodiversity, as epifauna is a critical trophic link between primary producers and consumers.

Seagrass beds reveal high abundance of microplastic in sediments: A case study in the Baltic Sea

Microplastic (MPL) contamination in the marine environment is extensively studied yet little is known about the extent of MPL abundance in seagrass beds. The aim of this study was to evaluate MPL accumulation in coastal seagrass (Zostera marina) beds in the Baltic Sea, Estonia. Surface water was sampled by pumping using 40 μm plankton net, and sediments by trowel. MPL was extracted with NaCl, identified by microscopy and ATR-FTIR on selected samples. Surface water in the seagrass beds had 0.04-1.2 (median 0.14) MPL/L, similar to other areas of the Baltic Sea. Sediments had 0-1817 (median 208) MPL/kg (dwt), much higher than previously recorded from adjacent unvegetated and offshore sediments, thereby suggesting a strong ability of the sediments in seagrass beds to retain MPL. Of identified MPL, blue fibres were dominant in both the sampled media. Sediment characterization showed a correlation between MPL counts with poorly sorted sediments.

Diversity, distribution and conservation of seagrass in coastal waters of the Liaodong Peninsula, North Yellow Sea, northern China: Implications for seagrass conservation

Seagrass beds are highly productive coastal ecosystems that are widely distributed along temperate and tropical coastlines globally. Although seagrass distribution and diversity have been widely reported on a global scale, there have been few reports on seagrass distribution and diversity in northern China, especially for coastal waters of the Liaodong Peninsula in the North Yellow Sea. In the present study, we investigated the distribution and diversity of seagrass in coastal waters of the Liaodong Peninsula in the North Yellow Sea, northern China. Field surveys of seagrass wrack were conducted along shorelines, to identify whether seagrass beds occurred in nearby waters, and sonar methods were then used to collect data relating to seagrass bed extent. Also, we analyzed the major threats facing seagrass beds. The results of the study revealed that four species (Zostera marina L., Z. japonica Aschers. & Graebn., Z. caespitosa M., and Phyllospadix iwatensis M.) were found in study area, covering a total area of 1253.47 ha. Seagrass bed area significantly decreased with increasing water depth, and most seagrass was recorded at depths of 2-5 m. Due to the steep slope of the seabed, seagrass beds exhibited a zonal distribution in most of the study areas. In addition, the amount of seagrass wrack along shorelines could be used to infer the size and distance of seagrass beds. Human activities, such as clam harvesting, land reclamation, coastal aquaculture pose a threat to the seagrass beds. This study provides new information to fill knowledge gaps regarding seagrass distribution in northern China and it provides a baseline for further monitoring of these seagrass beds.

Growth of Zostera japonica in different sediment habitats of the Yellow River estuary in China

The estuarine delta of the Yellow River is a region of strong land-ocean-human interactions that undergoes a unique evolutionary process. The delta is formed by deposition of large quantities of sediments carried by the Yellow River, especially during the annual water and sediment regulation period; more than one-third of the total annual sediments is deposited to the estuary area. The seagrass Zostera japonica is located at the forefront of the Yellow River delta. To study the impact of the different sediment environments on the Z. japonica growth, its growth and water quality and sediment parameters were measured on the northern and southern sides of the estuary from April to October in 2019. The action of wind and tides have re-suspended and dispersed sediments over time, producing shores on the southern delta characterized by nutrient-enriched clays and shores on the northern delta characterized by coarser sands and silts with poor nutrients. During the monitoring period, the concentrations of TC, TN, and TP in the root-zone sediments at the southern site were 1.56%, 0.04%, and 0.06%, respectively, whereas they were 0.69%, 0.007%, and 0.06%, respectively, at the northern site. Sufficient nutrients supported the growth of Z. japonica at the southern site, while poor nutrition limited the continuous growth of Z. japonica at the northern site. In July, the plant height, biomass, and shoot density of Z. japonica at the southern site reached the maximum values of 23.6 cm, 0.14 g/shoot, and 3245 shoots/m2, respectively, whereas they were 16.4 cm, 0.06 g/shoot, and 2740 shoots/m2, respectively, at the northern site. The sediment grain size and their nutrients contributed to different growth patterns of Z. japonica at the southern and northern sites. Our research could provide important implication for the conservation of Z. japonica habitats in the Yellow River estuary in China.

Coast-wide evidence of low pH amelioration by seagrass ecosystems

Global-scale ocean acidification has spurred interest in the capacity of seagrass ecosystems to increase seawater pH within crucial shoreline habitats through photosynthetic activity. However, the dynamic variability of the coastal carbonate system has impeded generalization into whether seagrass aerobic metabolism ameliorates low pH on physiologically and ecologically relevant timescales. Here we present results of the most extensive study to date of pH modulation by seagrasses, spanning seven meadows (Zostera marina) and 1000 km of U.S. west coast over 6 years. Amelioration by seagrass ecosystems compared to non-vegetated areas occurred 65% of the time (mean increase 0.07 ± 0.008 SE). Events of continuous elevation in pH within seagrass ecosystems, indicating amelioration of low pH, were longer and of greater magnitude than opposing cases of reduced pH or exacerbation. Sustained elevations in pH of >0.1, comparable to a 30% decrease in [H⁺ ], were not restricted only to daylight hours but instead persisted for up to 21 days. Maximal pH elevations occurred in spring and summer during the seagrass growth season, with a tendency for stronger effects in higher latitude meadows. These results indicate that seagrass meadows can locally alleviate low pH conditions for extended periods of time with important implications for the conservation and management of coastal ecosystems.

Variable contributions of seafloor communities to ecosystem metabolism across a gradient of habitat-forming species

The contributions of habitat-forming species to the biodiversity and ecosystem processes of marine and terrestrial ecosystems are widely recognized. Aquatic plants are considered foundation species in shallow ecosystems, as they maintain biodiversity and sustain many ecosystem functions such as primary production and respiration. Despite the increasing amount of biodiversity-ecosystem functioning experiments in seagrass habitats, the effects of benthic variability on ecosystem functioning are rarely investigated across spatially variable aquatic plant habitats. Here, we quantitatively link seasonal variability in seafloor metabolism (i.e. gross primary production and community respiration) with major benthic community components (i.e. microphytobenthos, aquatic plants and macrofauna) across a structural complexity gradient of habitat-forming species (in terms of shoot density and biomass), ranging from bare sand, to a sparse mixture of plants to a dense monospecific seagrass meadow. The increasing complexity gradient enhanced the magnitude of the relationships between benthic community and seafloor metabolism. The daily average seafloor metabolism per season at the bare site was similar to the sparse site, highlighting the role of microphytobenthos for seafloor metabolism in shallow unvegetated sediments. The contribution of the associated macrofauna to the seafloor respiration was similar to the aquatic plant community contribution. Infauna was the main macrofaunal component significantly explaining the seasonal variability of seafloor respiration. However, benthic community-metabolism relationships were stronger within the plant community than within the macrofauna community (i.e. steepest slopes and lowest p-values). Understanding these relationships are a priority since climate change and biodiversity loss are reducing habitat complexity around the world, jeopardizing valuable ecosystem functions and services.

Modeling eelgrass spatial response to nutrient abatement measures in a changing climate

For many coastal areas including the Baltic Sea, ambitious nutrient abatement goals have been set to curb eutrophication, but benefits of such measures were normally not studied in light of anticipated climate change. To project the likely responses of nutrient abatement on eelgrass (Zostera marina), we coupled a species distribution model with a biogeochemical model, obtaining future water turbidity, and a wave model for predicting the future hydrodynamics in the coastal area. Using this, eelgrass distribution was modeled for different combinations of nutrient scenarios and future wind fields. We are the first to demonstrate that while under a business as usual scenario overall eelgrass area will not recover, nutrient reductions that fulfill the Helsinki Commission's Baltic Sea Action Plan (BSAP) are likely to lead to a substantial areal expansion of eelgrass coverage, primarily at the current distribution's lower depth limits, thereby overcompensating losses in shallow areas caused by a stormier climate.

Century-long records reveal shifting challenges to seagrass recovery

Global losses over the 20th century placed seagrass ecosystems among the most threatened ecosystems in the world, with eutrophication, and associated deterioration of the submarine light environment identified as the main driver. Growing appreciation of the ecological and societal benefits of healthy seagrass meadows has stimulated efforts to protect and restore them, largely focused on reducing nutrient input to coastal waters. Here we analyze a unique data set spanning 135 years on eelgrass (Zostera marina), the dominant seagrass of the northern hemisphere. We show that meadows in the Western Baltic Sea exhibited major declines relative to historic (1890-1910) reference due to the wasting disease in the 1930s followed by eutrophication peaking in the 1980s, but have only shown modest improvement despite major eutrophication mitigation, halving nitrogen input since the 1980s. Across the past century, we identified generally shallower colonization depths of eelgrass for a given submarine light penetration and, hence, increased apparent light requirements. This suggests that eelgrass recovery is limited by additional stressors. Our study indicates that bottom trawling and intense recent warming (0.5°C per decade, 1985-2018), which impact on deeper and shallower meadows, respectively, suppress eelgrass from fully recovering from eutrophication. Warming is most severe in shallow turbid waters, while clear-water areas offer eelgrass refugia from warming in deeper, cooler waters; but trawling can prevent eelgrass from reaching these refugia. Efforts to reduce nutrient input and thereby improve water clarity have been instrumental in avoiding a catastrophic loss of eelgrass ecosystems. However, local-scale future management must, in addition, reduce bottom trawling to facilitate eelgrass reaching deeper, cooler refugia, and increase resilience toward realized and further warming. Warming needs to be limited by meeting global climate change mitigation goals.

Microplastic retention by marine vegetated canopies: Simulations with seagrass meadows in a hydraulic flume

Marine canopies formed by seagrass and other coastal vegetated ecosystems could act as sinks of microplastics for being efficient particle traps. Here we investigated for the first time the occurrence of microplastic retention by marine canopies in a hydraulic flume under unidirectional flow velocities from 2 to 30 cm s^-1. We used as model canopy-forming species the seagrass Zostera marina with four canopy shoot density (0, 50, 100, 200 shoots m^-2), and we used as microplastic particles industrial pristine pellets with specific densities from 0.90 to 1.34 g cm^-3 (polypropylene PP; polystyrene PS; polyamide 6 PA; and polyethylene terephthalate PET). Overall, microplastics particles transported with the flow were retained in the seagrass canopies but not in bare sand. While seagrass canopies retained floating microplastics (PP) only at low velocities (<12 cm s^-1) due to a barrier created by the canopy touching the water surface, the retention of sinking particles (PS, PA, PET) occurred across a wider range of flow velocities. Our simulations revealed that less dense sinking particles (PS) might escape from the canopy at high velocities, while denser sinking particles can be trapped in scouring areas created by erosive processes around the eelgrass shoots. Our results show that marine canopies might act as potential barriers or sinks for microplastics at certain bio-physical conditions, with the probability of retention generally increasing with the seagrass shoot density and polymer specific density and decreasing with the flow velocity. We conclude that seagrass meadows, and other aquatic canopy-forming ecosystems, should be prioritized habitats in assessment of microplastic exposure and impact on coastal areas since they may accumulate high concentration of microplastic particles that could affect associated fauna.

Purification of malachite green as a model biocidal agent from aqueous system by using a natural widespread coastal biowaste (Zostera marina)

The present paper aimed to perform an environmentally friendly and effective study on the purification of biocidal material using bioremediation technique, and in this context, a natural widespread coastal biowaste (Zostera marina) was applied to remove a model biocide from aqueous system. Herein, malachite green was selected as a common agent to evaluate the biosorption efficiency of waste biomaterial. The bioremediation properties of biosorbent were studied in a controlled batch experiment system by the optimization practice of operating parameters like biosorbent quantity, medium pH, time, pollutant concentration and temperature, and kinetic, thermodynamic, equilibrium, and characterization operations. The optimum operating conditions were considered as 10 mg, 4, 6 h, 15 mg L^-1, and 25 °C, respectively. Elovich and Langmuir were found to be the best-fitted models, describing the experimental biosorption data. Thermodynamic study revealed a favorable nature of the cleanup process. The characterization analysis indicated the presence of various functional groups on the layered biosorbent surface involved on the pollutant treatment. The untreated biosorbent showed a good biocide purification performance with a value of 97.584 mg g^-1, and it could thus be employed as an eco-friendly and cost-effective cleaning agent in environmental bioremediation studies.

Plant morphology and seed germination responses of seagrass (Zostera japonica) to water depth and light availability in Ailian Bay, northern China

Poor water quality and light reduction owing to anthropogenic impacts are the most widespread causes behind marine submerged angiosperm (seagrass) declines, worldwide. Seagrasses could respond to sustained environmental stresses, such as increasing water depth and light reduction, through morphological changes, particularly shoot density and/or biomass reductions. The seagrass Zostera japonica Asch. and Graebn. has been introduced to the Pacific Coast of North America, but it is widely threatened in its native northwestern Pacific Coast range alongside the east coast of China. The main aims of this study were to determine: 1) the depth limit of Z. japonica growth in its native range, and 2) how light availability affects the growth and recruitment of Z. japonica. To achieve these aims, we investigated the temporal responses of Z. japonica shoots and seeds from an intertidal donor site, Swan Lake, to light availability at water depths ranging from 1 to 6 m using in situ suspended cultures deployed in the experimental site, Ailian Bay, off the coast of Weihai City, China. The results showed that the transplanted Z. japonica shoots and seeds could survive for the duration of their annual growth cycle, permanently underwater, at a depth ≤2 m. There was a significant inverse relationship between water depth and time to complete shoot loss, despite temporally varying water clarity levels. Due to the local turbidity of the waters in Ailian Bay, a depth of 2 m yielded sufficient light deprivation (5%-37% surface irradiance) to negatively affect the seagrass shoot density. Our results suggest that this intertidal species can potentially persist in shallow subtidal areas following transplantation with shoots and seeds. The findings may also serve as useful information for local seagrass distribution limits, and will facilitate their habitat establishment and restoration efforts.

Sonar and in situ surveys of eelgrass distribution, reproductive effort, and sexual recruitment contribution in a eutrophic bay with intensive human activities: Implication for seagrass conservation

Seagrass beds are recognized as pivotal and among the most vulnerable coastal marine ecosystems globally. The eelgrass Zostera marina L. is the most widely distributed seagrass species and dominates the temperate northern hemisphere. However, an alarming decline in seagrass has been occurring worldwide due to multiple stressors. Seagrass meadow degradation is particularly serious in the Bohai Sea, in temperate China; however, large areas (> 500 ha) of seagrass meadows and population recruitment have rarely been reported in this area. In the present study, we report on a large eelgrass bed in a eutrophic bay of the Bohai Sea. Sonar and field survey methods were used to investigate the distribution of seagrass and its population recruitment. We also analyzed the major threats to this large seagrass bed. Results showed that a large Z. marina bed with an area of 694.36 ha occurred in this area of the Bohai Sea, with a peripheral area of ~25 km². Seagrass canopy height and plant coverage had a significant correlation with water depth. Asexual reproduction principally occurred in autumn and played a dominant role in population recruitment in vegetated areas, where no seedlings successfully colonized. In contrast, a considerable number of seedlings survived in the seagrass meadow gaps, and thus played a critical role in the recruitment in these areas. The maximum reproductive shoot densities were about 100 and 70 shoots m^-2 at sampling site (S)-1 and S-2 in 2018, respectively, which was about two times more than in 2019 (50 and 20 reproductive shoots m^-2 at S-1 and S-2, respectively). The potential seed output per unit area in 2019 was about 1020 seeds m^-2 at S-1 and 830 seeds m^-2 at S-2, and the seed output in the study area was at a low level compared with global values. Overall, high spring and summer water temperature appeared to induce sexual reproduction of Z. marina in the study area, including reproductive effort, reproductive investment, and seedling development. Furthermore, eelgrass height, aboveground biomass, and density were significantly related to water temperature. Among the potential threatening factors to seagrass in this area, the activities of clam harvesting were intense with daily clam catches >2000 kg, leading to patchy seagrass meadows, especially in the fringe areas. The seagrass bed was also threatened by marine pollution (nutrient loading) and land reclamation. Therefore, the protection and restoration of this seagrass bed are strongly recommended. Our study will provide fundamental information for the conservation and management strategies of large eelgrass beds in the Bohai Sea.

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).

Seagrass vegetation affect the vertical organization of microbial communities in sediment

Seagrasses represent high primary productivity and provide important ecosystem services to the marine environment. Seagrass-associated microbial communities are playing essential ecological functional roles in biogeochemical cycles. However, little is known about the effect of seagrass vegetation on microbial communities in sediment. In the present study, the sediment cores of seagrass bed (dominated by Zostera japonica and Zostera marine) and degradation area in Swan Lake (China) were sampled; then, biogeochemical parameters were analyzed, and microbial community composition was investigated by using high-throughput sequencing of the 16S rRNA gene. The results showed that the presence of seagrass could lead to a decrease in the richness and diversity of the microbial community. In the vertical direction, a pronounced shift from Proteobacteria-dominated upper layers to Chloroflexi and Crenarchaeota-dominated deep layers in all sediment cores were observed. Besides, Bathyarchaeia is more abundant at degradation area, while Vibrionaceae, Sulfurovum and Lokiarchaeial overrepresent at the seagrass bed area. Vibrionaceae was abundant in the rhizosphere of Z. marina and Z. japonica, and the proportions reached 84.45% and 63.89%, respectively. This enrichment of Vibrio spp. may be caused by the macrobenthic species near the seagrass rhizosphere, and these Vibrio spp. reduced the diversity and stability of microbial community, which may lead to the degradation of seagrass. This study would provide clues for the distribution patterns and niche preferences of seagrass microbiome. The conservation strategy of seagrass would be further elucidated from the perspective of the microbiome.