Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows: A Comparison of Four European Areas

Interconnectivity can be as important as habitat type in explaining carbon stocks in the coastal lagoons of arid regions

Coastal blue carbon ecosystems, typically comprising interconnected habitat mosaics, are globally important pathways of carbon sequestration and play a significant role in climate change regulation and mitigation. Current coastal management strategies often rely on simplified regional carbon stock estimates, that overlook the geographical variability and intricate ecological dynamics within these ecosystems. This study adopts a seascape ecology approach to evaluate the role of multiple seascape characteristics on carbon storage in two arid region coastal lagoons. We show that seascape location is the most influential driver of carbon stocks. Additionally, carbon isotopic variability, a proxy for connectivity, can be as influential as habitat type, particularly in the UAQ lagoon. This challenges the conventional reliance on data from individual habitat types (e.g., seagrass, mangrove, or tidal marsh) and highlights the context-dependency of carbon stocks. Moreover, the specific characteristics driving carbon stocks vary between seascapes: in Khor Faridah, connectivity to seagrass and mangrove habitats is crucial, while in the UAQ lagoon, sheltered and elevated areas are more influential. Our findings suggest that the interconnectivity between different habitat types, such as mangroves and saltmarshes, significantly enhances carbon storage. This is especially pronounced in large, sheltered mangrove habitat types within upper intertidal zones. Notably, small patches of mangroves, up to 10 ha, are associated with an approximate 10 % increase in carbon stocks. These results underscore the need for a more holistic, context-specific approach to designing nature-based solutions for coastal management and ecosystem restoration. By considering the specific characteristics and connectivity of seascape mosaics, we can more effectively enhance carbon stock potential in coastal ecosystems. This study contributes to a deeper spatially explicit understanding of the complex factors influencing carbon stocks in blue carbon ecosystems, highlighting the importance of tailored management strategies that reflect the unique ecological patterns of each seascape.

Seagrass decline weakens sediment organic carbon stability

Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as 'blue carbon'. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.

It's time to broaden what we consider a 'blue carbon ecosystem'

Photoautotrophic marine ecosystems can lock up organic carbon in their biomass and the associated organic sediments they trap over millennia and are thus regarded as blue carbon ecosystems. Because of the ability of marine ecosystems to lock up organic carbon for millennia, blue carbon is receiving much attention within the United Nations' 2030 Agenda for Sustainable Development as a nature-based solution (NBS) to climate change, but classically still focuses on seagrass meadows, mangrove forests, and tidal marshes. However, other coastal ecosystems could also be important for blue carbon storage, but remain largely neglected in both carbon cycling budgets and NBS strategic planning. Using a meta-analysis of 253 research publications, we identify other coastal ecosystems-including mud flats, fjords, coralline algal (rhodolith) beds, and some components or coral reef systems-with a strong capacity to act as blue carbon sinks in certain situations. Features that promote blue carbon burial within these 'non-classical' blue carbon ecosystems included: (1) balancing of carbon release by calcification via carbon uptake at the individual and ecosystem levels; (2) high rates of allochthonous organic carbon supply because of high particle trapping capacity; (3) high rates of carbon preservation and low remineralization rates; and (4) location in depositional environments. Some of these features are context-dependent, meaning that these ecosystems were blue carbon sinks in some locations, but not others. Therefore, we provide a universal framework that can evaluate the likelihood of a given ecosystem to behave as a blue carbon sink for a given context. Overall, this paper seeks to encourage consideration of non-classical blue carbon ecosystems within NBS strategies, allowing more complete blue carbon accounting.

Blue carbon storage in a tropical coastal estuary: Insights for conservation priorities

Seagrass ecosystems have been determined as necessary sinks in the global carbon cycle and contribute towards climate change mitigations. In the recent past, there has been an increase of studies focused on blue carbon opportunities provided by seagrasses but large knowledge gaps and uncertainties remain, particularly in tropical seagrass meadows in the South Asian regions. Therefore, the current study aims to quantify the organic carbon stocks in the seagrass meadows on the tropical estuary in southern coast of Sri Lanka and highlights the need of conserving seagrasses specially in the context of effective management of lagoons to achieve Sustainable Development Goals. Landsat 9 (OLI/TIRS) images were used to develop seagrass distribution maps for 2022 and the data were verified with ground truthing. Vegetation and soil samples were taken from eight sampling locations representing the Rekawa Lagoon. Aboveground biomass (AGB) and belowground biomass (BGB) were determined by multiplying the biomass with the carbon conversion factor whereas the loss-on-ignition (LOI) technique was applied to calculate the soil organic carbon. Results revealed that the soil core carbon content of the study site were ranged between 2.56 ± 0.29 to 3.04 ± 0.44 Mg C/ha. The calculated total carbon content of the 0.0324 km2 study area in Rekawa Lagoon was 10.21 Mg C, giving 87.06 % contribution from sediment organic carbon pool. This study provides insights for the conservation of these critical ecosystems and highlights the need of policy and action agendas for better management.

Assessment of blue carbon in seagrass meadows in Pulicat Lake, Andhra Pradesh, India

Coastal ecosystems such as seagrass, mangroves and saltmarshes sequester and store large quantities of carbon in the soil. Carbon sequestration potential of four seagrass species Halophila ovalis, Halodule pinifolia, Syringodium isoetifolium and Halodule uninervis in Pulicat Lake, Andhra Pradesh, India was assessed. The mean seagrass biomass within the seagrass beds in the lake ranged from 1.43 ± 0.06 to 2.01 ± 0.12 Mg/ ha and the total blue carbon storage in the seagrass meadows ranged from 0.49 ± 0.15 to 9.52 ± 0.15 Mg C ha-1 while stored carbon in the sediment ranged from 1.29 ± 0.04 to 11.94 ± 0.15 Mg C ha-1. Regression analysis showed a considerable correlation between seagrass carbon and sediment carbon. Among the environmental parameters analyzed, pH showed significant correlation with seagrass biomass and sediment carbon (P < 0.05). The present study demonstrates the carbon sequestration potential of seagrass ecosystem in the Pulicat Lake.

Anthropogenic influences on the sources and distribution of organic carbon, black carbon, and heavy metals in Daya Bay's surface sediments

The total organic carbon (TOC), total nitrogen (TN), black carbon (BC), δ13CTOC, δ15N, δ13CBC, grain size, and heavy metals of surface sediments collected from Daya Bay were determined to investigate the spatial distributions of these parameters and to evaluate the influences of human activities. Marine organic matter was found to constitute approximately 84.41 ± 7.70 % of these sediments on average. The western and northern regions of Daya Bay exhibited relatively fine grain sizes, weak hydrodynamic conditions, and high sedimentation rates, which favored the burial and preservation of organic matter. The high concentration of organic matter could be attributed to the influence of petroleum and aquaculture industries. Fossil fuels were the main source of BC. The enrichment factor (EF) and geo-accumulation index (Igeo) were used to evaluate the sources and pollution levels of heavy metals. The results revealed that the source and distribution of heavy metals were strongly influenced by human activities, resulting in moderate pollution levels across most regions of Daya Bay. A strong correlation was observed between the Igeo values of heavy metals and BC, TOC, TN, and mean particle grain size (Mz). This suggests that the ability of sediments in Daya Bay to enrich and adsorb heavy metals depends on the sediment grain size, the content and type of organic matter. Importantly, sediments in the inner bay of Daya Bay exhibited a greater capacity to impede the migration of heavy metals compared to those in the outer bay.

The influence of seagrass and its associated sediment on organic carbon storage: A case of Halodule uninervis and Syringodium isoetifolium meadows of Western India Ocean, Tanzania

Seagrass meadows are considered crucial natural carbon stocks. However, in Tanzania, few species have been assessed for their potential carbon stocks and variability in percentage organic carbon (%Corg) stocks. The study reports the contributions of seagrasses Halodule uninervis and Syringodium isoetifolium in carbon storage in WIO region. Findings revealed a significantly higher %Corg in seagrass meadows compared to unvegetated, confirming that seagrass heightens organic carbon storage. The seagrass carbon storage varied significantly among sites ranging from 4.05 ± 0.7% in Kaole to 0.62 ± 0.05% in Kunduchi. Syringodium isoetifolium meadows had higher organic carbon (p = 0.002) than H. uninervis. The partial least square analysis showed that below- and aboveground biomass and canopy height were positively correlated to %Corg. Sediment density and porosity were the vital predictor but negatively correlated with %Corg. The study showed a higher %Corg in the marine protected area, which could be linked to seagrass structural complexities and sediment porosity.

Chronology of trace elements and radionuclides using sediment cores in Golden Horn Estuary, Sea of Marmara

Trace elements and radionuclides are substantial pollutants in marine environment since they are non-biodegradable and can be harmful even in minute concentrations. The Golden Horn estuary, where is an inlet of Bosphorus and two creeks, has been seriously polluted by untreated municipal and industrial dischargers for several decades. Since 1998, a large restoration and rehabilitation efforts have been undertaken in the estuary to mitigate the pollution. In the present study, four sediment cores were taken from the Golden Horn estuary to assess the historical accumulation of trace elements and radionuclides. Radiometric dating was implemented by ²¹⁰Pb and ¹³⁷Cs radionuclides and CRS model. Sedimentation rates were calculated in the range of 0.92-0.97 cm yr^-1 in the estuary. The distribution of radionuclides (⁴⁰K, ²²⁶Ra, and ²²⁸Ra) indicated some slight variations which ascribes to the geological characteristics of sediment along the cores. The concentrations of the anthropogenic elements were relatively higher in the intensive industrialization period. Their concentrations reduced in the latest 15-20 years thanks to the large-scale rehabilitation project in the estuary. The pollution indices, namely EF, Igeo, CF, and PLI showed that the concentrations of Cd, Cr, Cu, Pb, Sb, and Sn were above the world averages. Our results provide an insight on the long-term accumulation trends of trace element in the Golden Horn, which revealed that the estuary remains moderately polluted. We suggest that preventive countermeasures are much more important than post pollution remediation in the case of metallic pollution in the estuaries.

Sedimentary organic carbon and nitrogen stocks of intertidal seagrass meadows in a dynamic and impacted wetland: Effects of coastal infrastructure constructions and meadow establishment time

Seagrass meadows, through their large capacity to sequester and store organic carbon in their sediments, contribute to mitigate climatic change. However, these ecosystems have experienced large losses and degradation worldwide due to anthropogenic and natural impacts and they are among the most threatened ecosystems on Earth. When a meadow is impacted, the vegetation is partial- or completely lost, and the sediment is exposed to the atmosphere or water column, resulting in the erosion and remineralisation of the carbon stored. This paper addresses the effects of the construction of coastal infrastructures on sediment properties, organic carbon, and total nitrogen stocks of intertidal seagrass meadows, as well as the size of such stocks in relation to meadow establishing time (recently and old established meadows). Three intertidal seagrass meadows impacted by coastal constructions (with 0% seagrass cover at present) and three adjacent non-impacted old-established meadows (with 100% seagrass cover at present) were studied along with an area of bare sediment and two recent-established seagrass meadows. We observed that the non-impacted areas presented 3-fold higher percentage of mud and 1.5 times higher sedimentary organic carbon stock than impacted areas. Although the impacted area was relatively small (0.05-0.07 ha), coastal infrastructures caused a significant reduction of the sedimentary carbon stock, between 1.1 and 2.2 Mg OC, and a total loss of the carbon sequestration capacity of the impacted meadow. We also found that the organic carbon stock and total nitrogen stock of the recent-established meadow were 30% lower than those of the old-established ones, indicating that OC and TN accumulation within the meadows is a continuous process, which has important consequences for conservation and restoration actions. These results contribute to understanding the spatial variability of blue carbon and nitrogen stocks in coastal systems highly impacted by urban development.

A Blueprint for the Estimation of Seagrass Carbon Stock Using Remote Sensing-Enabled Proxies

Seagrass ecosystems sequester carbon at disproportionately high rates compared to terrestrial ecosystems and represent a powerful potential contributor to climate change mitigation and adaptation projects. However, at a local scale, rich heterogeneity in seagrass ecosystems may lead to variability in carbon sequestration. Differences in carbon sequestration rates, both within and between seagrass meadows, are related to a wide range of interrelated biophysical and environmental variables that are difficult to measure holistically using traditional field surveys. Improved methods for producing robust, spatially explicit estimates of seagrass carbon storage across large areas would be highly valuable, but must capture complex biophysical heterogeneity and variability to be accurate and useful. Here, we review the current and emerging literature on biophysical processes which shape carbon storage in seagrass beds, alongside studies that map seagrass characteristics using satellite remote sensing data, to create a blueprint for the development of remote sensing-enabled proxies for seagrass carbon stock and sequestration. Applications of satellite remote sensing included measuring seagrass meadow extent, estimating above-ground biomass, mapping species composition, quantifying patchiness and patch connectivity, determining broader landscape environmental contexts, and characterising seagrass life cycles. All of these characteristics may contribute to variability in seagrass carbon storage. As such, remote sensing methods are uniquely placed to enable proxy-based estimates of seagrass carbon stock by capturing their biophysical characteristics, in addition to the spatiotemporal heterogeneity and variability of these characteristics. Though the outlined approach is complex, it is suitable for accurately and efficiently producing a full picture of seagrass carbon stock. This review has drawn links between the processes of seagrass carbon sequestration and the capabilities of remote sensing to detect and characterise these processes. These links will facilitate the development of remote sensing-enabled proxies and support spatially explicit estimates of carbon stock, ensuring climate change mitigation and adaptation projects involving seagrass are accounted for with increased accuracy and reliability.

Impacts of land-use change and urban development on carbon sequestration in tropical seagrass meadow sediments

Seagrass meadows store significant carbon stocks at a global scale, but land-use change and other anthropogenic activities can alter the natural process of organic carbon (C_org) accumulation. Here, we assessed the carbon accumulation history of two seagrass meadows in Zanzibar (Tanzania) that have experienced different degrees of disturbance. The meadow at Stone Town has been highly exposed to urban development during the 20th century, while the Mbweni meadow is located in an area with relatively low impacts but historical clearing of adjacent mangroves. The results showed that the two sites had similar sedimentary C_org accumulation rates (22-25 g m^-2 yr^-1) since the 1940s, while during the last two decades (∼1998 until 2018) they exhibited 24-30% higher accumulation of C_org, which was linked to shifts in C_org sources. The increase in the δ¹³C isotopic signature of sedimentary C_org (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and C_org accumulation capacity of the meadow after the relocation of a major sewage outlet in the mid-1990s. In contrast, the decrease in the δ¹³C signatures of sedimentary C_org in the Mbweni meadow since the early 2010s was likely linked to increased C_org run-off of mangrove/terrestrial material following mangrove deforestation. This study exemplifies two different pathways by which land-based human activities can alter the carbon storage capacity of seagrass meadows (i.e. sewage waste management and mangrove deforestation) and showcases opportunities for management of vegetated coastal C_org sinks.

Variability in blue carbon storage related to biogeochemical factors in seagrass meadows off the coast of the Korean peninsula

Coastal vegetated habitats such as mangroves, salt marshes, and seagrasses, referred to as blue carbon ecosystems, play an important role in climate change mitigation by an effective CO₂ capture from atmosphere and water columns and long-term organic carbon (C_org) storage in sediments. Although seagrass meadows are considered intense carbon sinks, information on regional variability in seagrass blue carbon stock and factors influencing its capacity still remain sparse. In the present study, seagrass blue carbon storage by measuring C_org stocks in sediments and living seagrass biomass, and carbon accumulation rates (CARs) in seagrass meadows were estimated along the Korean coast. Factors affecting variability in C_org stocks were also analyzed using partial least squares (PLS) regression and principal component analysis (PCA). Projected C_org stocks in sediment, extrapolated to a depth 1 m, exhibited substantial variability among sites, ranging from 49.91 to 125.71 Mg C ha^-1. The majority of C_org (96-99%) was stored in sediments, whereas the contribution of living biomass was minor. PLS regression and PCA indicated that C_org stocks in seagrass meadows are strongly associated with sediment characteristics such as dry bulk density and water and mud content. Among seagrass traits, above- to below-ground biomass ratio was significantly related to the quantity of C_org stocks in seagrass meadows. Because of the high spatial variability in C_org stocks and CARs, local and regional differences in seagrass blue carbon storage should be considered to accurately assess the climate change mitigation potential of seagrass ecosystems.

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.

Blue carbon storage in Fijian seagrass meadows: First insights into carbon, nitrogen and phosphorus content from a tropical southwest Pacific Island

Seagrasses provide vital ecosystem services which include the accumulation and storage of carbon and nutrients in sediments and biomass. Despite their importance in climate change mitigation and adaptation, seagrass ecosystems have been poorly studied, particularly in the Pacific. Therefore, the present study assessed variability in sedimentary and vegetative C, N and P storage in three monospecific seagrass meadows (Halophila ovalis, Halodule pinifolia and Halodule uninervis), reporting baseline data for the first time. Sediment C_org stocks ranged from 31 to 47 Mg C ha^-1 and varied (p < 0.001) between seagrass meadows, unvegetated areas and locations. Comparison of N and P storage between vegetated meadows and unvegetated areas revealed differences (p < 0.05); implying seagrass meadows function as C, N and P sinks. Differences in species composition and environmental conditions, may play a key role in estimating C, N and P stocks, which are valuable data for conservation and monitoring of seagrass ecosystems.

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.

Fine sediment effects on seagrasses: A global review, quantitative synthesis and multi-stressor model

This review collates research into fine sediment as a stressor of seagrass and emphasizes the multiple modes of action of this contaminant. The article is based on a bibliographic database search that identified 201 articles describing sediment impacts on seagrasses. Articles were classified by one of three non-exclusive modes of action: 1) light reduction; 2) smothering (burial), and 3) effects via rhizosphere physico-chemistry. Most citations (104) investigated multi-mode impacts of sediments, but the most frequently investigated single mode was light reduction (57 citations), followed by substrate rhizosphere chemistry (31) then smothering effects (6). Mud with high organic content is particularly problematic and smaller seagrasses are particularly vulnerable. Research gaps include polyphasic approaches, and studies of interactions between smothering, rhizosphere biogeochemistry and light climate. Identifying the thresholds of seagrass health indicators under mud stress should benefit coastal resource management, enabling improved decision-making and implementation of protective actions.

Factors Influencing Carbon Stocks and Accumulation Rates in Eelgrass Meadows Across New England, USA

Increasing the protection of coastal vegetated ecosystems has been suggested as one strategy to compensate for increasing carbon dioxide (CO₂) in the atmosphere as the capacity of these habitats to sequester and store carbon exceeds that of terrestrial habitats. Seagrasses are a group of foundation species that grow in shallow coastal and estuarine systems and have an exceptional ability to sequester and store large quantities of carbon in biomass and, particularly, in sediments. However, carbon stocks (C_org stocks) and carbon accumulation rates (C_org accumulation) in seagrass meadows are highly variable both spatially and temporally, making it difficult to extrapolate this strategy to areas where information is lacking. In this study, C_org stocks and C_org accumulation were determined at 11 eelgrass meadows across New England, representing a range of eutrophication and exposure conditions. In addition, the environmental factors and structural characteristics of meadows related to variation in C_org stocks were identified. The objectives were accomplished by assessing stable isotopes of δ¹³C and δ¹⁵N as well as %C and %N in plant tissues and sediments, measuring grain size and ²¹⁰Pb of sediment cores, and through assessing site exposure. Variability in C_org stocks in seagrass meadows is well predicted using commonly measured environmental variables such as grain size distribution. This study allows incorporation of data and insights for the northwest Atlantic, where few studies on carbon sequestration by seagrasses have been conducted.

The influence of hydrodynamic exposure on carbon storage and nutrient retention in eelgrass (Zostera marina L.) meadows on the Swedish Skagerrak coast

Cold-temperate seagrass (Zostera marina) meadows provide several important ecosystem services, including trapping and storage of sedimentary organic carbon and nutrients. However, seagrass meadows are rapidly decreasing worldwide and there is a pressing need for protective management of the meadows and the organic matter sinks they create. Their carbon and nutrient storage potential must be properly evaluated, both at present situation and under future climate change impacts. In this study, we assessed the effect of wave exposure on sedimentary carbon and nitrogen accumulation using existing data from 53 Z. marina meadows at the Swedish west coast. We found that meadows with higher hydrodynamic exposure had larger absolute organic carbon and nitrogen stocks (at 0-25 cm depth). This can be explained by a hydrodynamically induced sediment compaction in more exposed sites, resulting in increased sediment density and higher accumulation (per unit volume) of sedimentary organic carbon and nitrogen. With higher sediment density, the erosion threshold is assumed to increase, and as climate change-induced storms are predicted to be more common, we suggest that wave exposed meadows can be more resilient toward storms and might therefore be even more important as carbon- and nutrient sinks in the future.

Estuarine submerged aquatic vegetation habitat provides organic carbon storage across a shifting landscape

Submerged aquatic vegetation (SAV) thrives across the estuarine salinity gradient providing valuable ecosystem services. Within the saline portion of estuaries, seagrass areas are frequently cited as hotspots for their role in capturing and retaining organic carbon (C_org). Non-seagrass SAV, located in the fresh to brackish estuarine areas, may also retain significant soil C_org, yet their role remains unquantified. Given rapidly occurring landscape and salinity changes due to human and natural disturbances, landscape level carbon pool estimates from estuarine SAV habitat blue carbon estimates are needed. We assessed C_org stocks in SAV habitat soils from estuarine freshwater to saline habitats (interior deltaic) to saline barrier islands (Chandeleur Island) within the Mississippi River Delta Plain (MRDP), Louisiana, USA. SAV habitats contain C_org stocks equivalent to those reported for other estuarine vegetation types (seagrass, salt marsh, mangrove). Interior deltaic SAV C_org stocks (231.6 ± 19.5 Mg C_org ha^-1) were similar across the salinity gradient, and significantly higher than at barrier island sites (56.6 ± 10.4 Mg C_org ha^-1). Within the MRDP, shallow water SAV habitat covers up to an estimated 28,000 ha, indicating that soil C_org storage is potentially 6.4 ± 0.1 Tg representing an unaccounted C_org pool. Extrapolated across Louisiana, and the Gulf of Mexico, this represents a major unaccounted pool of soil C_org. As marshes continue to erode, the ability of coastal SAV habitat to offset some of the lost carbon sequestration may be valuable. Our estimates of C_org sequestration rates indicated that conversion of eroding marsh to potential SAV habitat may help to offset the reduction of C_org sequestration rates. Across Louisiana, we estimated SAV to offset this loss by as much as 79,000 Mg C yr^-1 between the 1960s and 2000s.

High variability of Blue Carbon storage in seagrass meadows at the estuary scale

Seagrass meadows are considered important natural carbon sinks due to their capacity to store organic carbon (C_org) in sediments. However, the spatial heterogeneity of carbon storage in seagrass sediments needs to be better understood to improve accuracy of Blue Carbon assessments, particularly when strong gradients are present. We performed an intensive coring study within a sub-tropical estuary to assess the spatial variability in sedimentary C_org associated with seagrasses, and to identify the key factors promoting this variability. We found a strong spatial pattern within the estuary, from 52.16 mg C_org cm^-3 in seagrass meadows in the upper parts, declining to 1.06 mg C_org cm^-3 in seagrass meadows at the estuary mouth, despite a general gradient of increasing seagrass cover and seagrass habitat extent in the opposite direction. The sedimentary C_org underneath seagrass meadows came principally from allochthonous (non-seagrass) sources (~70-90 %), while the contribution of seagrasses was low (~10-30 %) throughout the entire estuary. Our results showed that C_org stored in sediments of seagrass meadows can be highly variable within an estuary, attributed largely to accumulation of fine sediments and inputs of allochthonous sources. Local features and the existence of spatial gradients must be considered in Blue Carbon estimates in coastal ecosystems.

Relevance of carbon burial and storage in two contrasting blue carbon ecosystems of a north-east Pacific coastal lagoon

Coastal vegetated ecosystems constitute very productive habitats, characterized by efficient C_org sequestration and long-term preservation in sediments, so they play an important role in climate change mitigation. The temporal evolution of C_org content, stocks and burial rates were evaluated in seagrass and salt marsh habitats in San Quintin Bay (northeast Pacific, Mexico) by using ²¹⁰Pb-dated sediment cores. Salt marsh cores were characterized by fine-grained sediments, higher salinities, lower terrigenous input and lower mass accumulation rates (MAR: 0.01-0.03 g cm^-2 yr^-1) than seagrass cores (MAR: 0.02-3.21 g cm^-2 yr^-1). Accumulation rates in both habitats steadily increased throughout the past century most likely because of soil erosion promoted by land use changes in the surroundings. The C_org stocks were highest in salt marsh cores (12.2-53.6 Mg ha^-1 at 10 cm depth; 259-320 Mg ha^-1 at 1 m depth) than in seagrass cores (5.7-14.4 Mg ha^-1, and 80-98, Mg ha^-1, respectively), whereas C_org burial rates were considerably lower in salt marsh (13-60 g m^-2 yr^-1) than in seagrass (9-144 g m^-2 yr^-1) habitats, and the temporal variations observed in C_org burial rates were mostly driven by changes in the accumulation rates. The overall C_org stock (485 ± 51 Gg C) for both habitats together was comparable to the carbon emissions of a major city nearby. Our results highlight the need to protect these environments as relevant carbon reservoirs.

Variation of carbon contents in eelgrass ( Zostera marina) sediments implied from depth profiles

Seagrass meadows are able to store significant amounts of organic carbon in their underlying sediment, but global estimates are uncertain partly owing to spatio-temporal heterogeneity between and within areas and species. In order to provide robust estimates, there is a need to better understand the fate of, and mechanisms behind, organic carbon storage. In this observational study, we analyse a suite of biotic and abiotic parameters in sediment cores from 47 different eelgrass ( Zostera marina) beds spanning the distributional range of the Northern Hemisphere. Depth profiles of particulate organic carbon (POC) revealed three patterns of vertical distribution where POC either increased, decreased or showed no pattern with sediment depth. These categories exhibited distinct profiles of δ¹³C and C:N ratios, where high POC profiles had a proportionally larger storage of eelgrass-derived material whereas low POC profiles were dominated by phytoplanktonic and macroalgal material. However, high POC did not always translate into high carbon density. Nevertheless, this large-scale dataset provides evidence that the variability in organic matter source in response to natural and anthropogenic environmental changes affects the potential role of eelgrass beds as POC sinks, particularly where eelgrass decline is observed.

Plant and sediment properties in seagrass meadows from two Mediterranean CO2 vents: Implications for carbon storage capacity of acidified oceans

Assessing the status of important carbon sinks such as seagrass meadows is of primary importance when dealing with potential climate change mitigation strategies. This study examined plant and sediment properties in seagrass meadows (Cymodocea nodosa (Ucria) Asch.) from two high pCO₂-low pH Mediterranean vent systems, located at Milos (Greece) and Vulcano (Italy) Islands, providing insights on carbon storage potential in future acidified oceans. Contrary to what has been suggested, carbon content (both inorganic and organic) and its surficial accumulation decreased at high pCO₂-low pH in comparison with controls. The decrease in inorganic carbon may result from the higher solubility of carbonates due to the more acidic conditions. At Vulcano, the seagrass properties (e.g., leaf area, biomass) appeared negatively affected by environmental conditions at high pCO₂-low pH conditions and this may have had a detrimental effect on the organic carbon content and accumulation. At Milos, organic carbon decreased at high pCO₂-low pH conditions, despite the increase in seagrass aboveground biomass, leaf length and area, probably as a consequence of site-specific features, which need further investigation and may include both biotic and abiotic factors (e.g., oligotrophic conditions, decreased sedimentation rate and input of allochthonous material). Results suggest that, in contrast to previous predictions based exclusively on the expected positive response of seagrasses to ocean acidification, carbon storage capacity of the seagrass C. nodosa may not increase at high pCO₂-low pH conditions. This study emphasizes the need to investigate further the potential alteration in the climate mitigation service delivered by seagrass meadows in acidified oceans.

Physiological responses of eelgrass (Zostera marina) to ambient stresses such as herbicide, insufficient light, and high water temperature

This study aimed to elucidate the biological responses of eelgrass (Zostera marina) to artificially induced stresses such as herbicide (Irgarol 1051, Irg) exposure, insufficient light, and high water temperature (27 ± 1.0 °C) by evaluating growth inhibition, photosynthetic activity, and metabolomic profiles. After 14 days, all treatments inhibited growth, but photosynthetic activity was only reduced in the Irg-exposed group. In the Irg-exposed and insufficient light groups, the metabolomic profiles were characterized by decreased levels of sugar (sucrose) and increased levels of amino acids such as glutamine, glycine, and leucine. Biochemical and ultrastructural analyses revealed that the loss of sugar-derived metabolic energy was compensated for by energy generated during autophagic protein degradation. Furthermore, the level of myo-inositol, which has various biological roles and participates in several cellular processes such as cell wall synthesis, stress response, and mineral nutrient storage, was markedly increased in the Irg-exposed and insufficient light groups. A combination of metabolomic analysis with other analyses such as measurement of photosynthetic activity might further elucidate the response of eelgrass to ambient stresses in the natural environment.

Superficial sedimentary stocks and sources of carbon and nitrogen in coastal vegetated assemblages along a flow gradient

Coastal vegetated ecosystems are major organic carbon (OC) and total nitrogen (TN) sinks, but the mechanisms that regulate their spatial variability need to be better understood. Here we assessed how superficial sedimentary OC and TN within intertidal vegetated assemblages (saltmarsh and seagrass) vary along a flow gradient, which is a major driver of sediment grain size, and thus of organic matter (OM) content. A significant relationship between flow current velocity and OC and TN stocks in the seagrass was found, but not in the saltmarsh. OC and TN stocks of the saltmarsh were larger than the seagrass, even though that habitat experiences shorter hydroperiods. Mixing models revealed that OM sources also varied along the flow gradient within the seagrass, but not in the saltmarsh, showing increasing contributions of microphytobenthos (17–32%) and decreasing contributions of POM (45–35%). As well, OM sources varied vertically as microphytobenthos contribution was highest at the higher intertidal saltmarsh (48%), but not POM (39%). Macroalgae, seagrass and saltmarsh showed low contributions. Local trade-offs between flow current velocities, hydroperiod and structural complexity of vegetation must be considered, at both horizontal and vertical (elevation) spatial dimensions, for better estimates of blue carbon and nitrogen in coastal ecosystems.

Carbon stocks and accumulation rates in Red Sea seagrass meadows

Seagrasses play an important role in climate change mitigation and adaptation, acting as natural CO₂ sinks and buffering the impacts of rising sea level. However, global estimates of organic carbon (C_org) stocks, accumulation rates and seafloor elevation rates in seagrasses are limited to a few regions, thus potentially biasing global estimates. Here we assessed the extent of soil C_org stocks and accumulation rates in seagrass meadows (Thalassia hemprichii, Enhalus acoroides, Halophila stipulacea, Thalassodendrum ciliatum and Halodule uninervis) from Saudi Arabia. We estimated that seagrasses store 3.4 ± 0.3 kg C_org m⁻² in 1 m-thick soil deposits, accumulated at 6.8 ± 1.7 g C_org m⁻² yr⁻¹ over the last 500 to 2,000 years. The extreme conditions in the Red Sea, such as nutrient limitation reducing seagrass growth rates and high temperature increasing soil respiration rates, may explain their relative low C_org storage compared to temperate meadows. Differences in soil C_org storage among habitats (i.e. location and species composition) are mainly related to the contribution of seagrass detritus to the soil C_org pool, fluxes of C_org from adjacent mangrove and tidal marsh ecosystems into seagrass meadows, and the amount of fine sediment particles. Seagrasses sequester annually around 0.8% of CO₂ emissions from fossil-fuels by Saudi Arabia, while buffering the impacts of sea level rise. This study contributes data from understudied regions to a growing dataset on seagrass carbon stocks and sequestration rates and further evidences that even small seagrass species store C_org in coastal areas.

Variability of UK seagrass sediment carbon: Implications for blue carbon estimates and marine conservation management

Seagrass meadows provide a multitude of ecosystem services, including a capacity to sequester carbon dioxide (CO₂) within their sediments. Seagrass research in the UK is lacking and there is no published data on sediment carbon (C) within UK seagrass meadows. We sampled 13 Zostera marina meadows along the southwest coast of the UK to assess the variability in their sedimentary organic carbon (OC) stocks. The study sites were considered representative of sub-tidal Z. marina meadows in the UK, spanning a gradient of sheltered to exposed sites, varying in formation, size and density, but found along the same latitudinal gradient. OC stocks (C_stocks) integrated across 100cm depth profiles were similar among all sites (98.01 ± 2.15 to 140.24 ± 10.27 Mg C ha^-1), apart from at Drakes Island, which recorded an unusually high C_stock (380.07 ± 17.51 Mg C ha^-1) compared to the rest of the region. The total standing stock of C in the top 100cm of the surveyed seagrass meadows was 66,337 t C, or the equivalent of 10,512 individual UK people’s CO² emissions per year. This figure is particularly significant relative to the seagrass area, which totalled 549.79 ha. Using estimates of seagrass cover throughout the UK and recent UK C trading values we approximate that the monetary value of the UK’s seagrass standing C stock is between £2.6 million and £5.3 million. The C stock of the UK’s seagrass meadows represent one of the largest documented C stocks within Europe and are, therefore, of important ecosystem service value. The research raises questions concerning the reliability of using global or regional data as a proxy for local seagrass C stock estimates and adds to a growing body of literature that is looking to understand the mechanisms of seagrass C storage. When taken with the fact that seagrass meadows are an important habitat for commercially important and endangered species in the UK, along with their declining health and cover, this research supports the need for more robust conservation strategies for UK seagrass habitats.

Habitat characteristics provide insights of carbon storage in seagrass meadows

Seagrass meadows provide multiple ecosystem services, yet they are among the most threatened ecosystems on earth. Because of their role as carbon sinks, protection and restoration of seagrass meadows contribute to climate change mitigation. Blue Carbon strategies aim to enhance CO₂ sequestration and avoid greenhouse gasses emissions through the management of coastal vegetated ecosystems, including seagrass meadows. The implementation of Blue Carbon strategies requires a good understanding of the habitat characteristics that influence C_org sequestration. Here, we review the existing knowledge on Blue Carbon research in seagrass meadows to identify the key habitat characteristics that influence C_org sequestration in seagrass meadows, those factors that threaten this function and those with unclear effects. We demonstrate that not all seagrass habitats have the same potential, identify research priorities and describe the implications of the results found for the implementation and development of efficient Blue Carbon strategies based on seagrass meadows.

Identification of Two Novel Amalgaviruses in the Common Eelgrass (Zostera marina) and in Silico Analysis of the Amalgavirus +1 Programmed Ribosomal Frameshifting Sites

The genome sequences of two novel monopartite RNA viruses were identified in a common eelgrass (Zostera marina) transcriptome dataset. Sequence comparison and phylogenetic analyses revealed that these two novel viruses belong to the genus Amalgavirus in the family Amalgaviridae. They were named Zostera marina amalgavirus 1 (ZmAV1) and Zostera marina amalgavirus 2 (ZmAV2). Genomes of both ZmAV1 and ZmAV2 contain two overlapping open reading frames (ORFs). ORF1 encodes a putative replication factory matrix-like protein, while ORF2 encodes a RNA-dependent RNA polymerase (RdRp) domain. The fusion protein (ORF1+2) of ORF1 and ORF2, which mediates RNA replication, was produced using the +1 programmed ribosomal frameshifting (PRF) mechanism. The +1 PRF motif sequence, UUU_CGN, which is highly conserved among known amalgaviruses, was also found in ZmAV1 and ZmAV2. Multiple sequence alignment of the ORF1+2 fusion proteins from 24 amalgaviruses revealed that +1 PRF occurred only at three different positions within the 13-amino acid-long segment, which was surrounded by highly conserved regions on both sides. This suggested that the +1 PRF may be constrained by the structure of fusion proteins. Genome sequences of ZmAV1 and ZmAV2, which are the first viruses to be identified in common eelgrass, will serve as useful resources for studying evolution and diversity of amalgaviruses.

Dynamics of sediment carbon stocks across intertidal wetland habitats of Moreton Bay, Australia

Coastal wetlands are known for high carbon storage within their sediments, but our understanding of the variation in carbon storage among intertidal habitats, particularly over geomorphological settings and along elevation gradients, is limited. Here, we collected 352 cores from 18 sites across Moreton Bay, Australia. We assessed variation in sediment organic carbon (OC) stocks among different geomorphological settings (wetlands within riverine settings along with those with reduced riverine influence located on tide-dominated sand islands), across elevation gradients, with distance from shore and among habitat and vegetation types. We used mid-infrared (MIR) spectroscopy combined with analytical data and partial least squares regression to quantify the carbon content of ~2500 sediment samples and provide fine-scale spatial coverage of sediment OC stocks to 150 cm depth. We found sites in river deltas had larger OC stocks (175-504 Mg/ha) than those in nonriverine settings (44-271 Mg/ha). Variation in OC stocks among nonriverine sites was high in comparison with riverine and mixed geomorphic settings, with sites closer to riverine outflow from the east and south of Moreton Bay having higher stocks than those located on the sand islands in the northwest of the bay. Sediment OC stocks increased with elevation within nonriverine settings, but not in riverine geomorphic settings. Sediment OC stocks did not differ between mangrove and saltmarsh habitats. OC stocks did, however, differ between dominant species across the research area and within geomorphic settings. At the landscape scale, the coastal wetlands of the South East Queensland catchments (17,792 ha) are comprised of approximately 4,100,000-5,200,000 Mg of sediment OC. Comparatively high variation in OC storage between riverine and nonriverine geomorphic settings indicates that the availability of mineral sediments and terrestrial derived OC may exert a strong influence over OC storage potential across intertidal wetland systems.