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The increase in climate-related extreme events and ecosystem degradation demands consistent and sustainable climate mitigation efforts. Seagrass playing a key role in nature-based carbon sequestration mitigation strategy. Here, we investigated the role of coral reef connectivity in blue carbon dynamics with seagrass meadows with coral reef connectivity (SC areas) and without coral reef connectivity (SG areas) in Palk Bay, India. The high sediment organic carbon was recorded in SC areas (90.26 ± 25.68 Mg org.C/ha) and lower in SG areas (66.96 ± 12.6 Mg org.C/ha). The maximum above-ground biomass (AGB) was recorded in Syringodium isoetifolium (35.43 ± 8.50) in SC areas and the minimum in Halophila ovalis (7.59 ± 0.90) in SG areas, with a similar trend observed in below-ground biomass (BGB). Our findings highlight the importance of coral reefs in enhancing the blue carbon potential of seagrass ecosystems and underscore the need for integrated conservation and restoration strategies for coral reefs and seagrasses. • Study reveals the site-specific habitat for SC & SG of carbon storage in Palk Bay • The mean above-ground biomass of Syringodium isoetifolim was 35.43 ± 8.50 g dwt/m2. • The sediment organic carbon stock of Cymodocea rotundata was 84.79 ± 1.37 Mg org.C/ha • Coral reef–seagrass habitat identified as potential climate change mitigation hotspot [ABSTRACT FROM AUTHOR]

Mangrove forests play a crucial role in the coastal environment. A plethora of research is focusing on mangroves' role in climate mitigation through sequestering and storing CO 2. This study aims to assess the organic carbon (OC) stocks and carbon sequestration rate (CSR) of the largest stands of mangroves in the Red Sea Coast of Egypt, to evaluate their significance in climate mitigation strategies. Intensive sampling was conducted in mangrove stands of Safaga, Qulaan, and Hamata to investigate OC and sediment properties along depth intervals to 1 m deep. Additionally, OC stored in the above- and below-ground biomass was estimated using Landsat 9 OLI/TIRS satellite images. TOC content (∼0.50%) and stocks in the sediments (54.49 ± 31.71–86.00 ± 29.00 MgC/ha) were very low compared to the global average for mangrove stands, and comparable to the mangrove forest in the Middle East region. This can be attributed to the desert and semi-desert conditions, limited riverine input, and low precipitation rates. Moreover, the above-ground biomass, estimated from Normalized Difference Vegetation Index, exhibited extremely low OC storage. Therefore, the sediment acted as the main pool of organic carbon in these mangrove ecosystems. The research findings revealed that mangrove stands in Egypt exhibit low CSR, ranging from 11.94 to 18.02 g C m−2 year−1. Additionally, the study determined that the stable nature of mangrove stands in Egypt makes their annual CO 2 emissions negligible. Despite local studies suggesting otherwise, it is highly unlikely that mangroves in Egypt contribute to the climate mitigation related to CO 2 sequestration. Therefore, it is important to implement restoration plans to ensure the preservation of other ecological services provided by mangroves along the Red Sea Coast. • Sediments OC stock ranged from 54.49 ± 31.71 to 86 ± 29 Mg C/ha, in 1m-cores in mangroves of Egypt. • Above- and below-ground carbon stock ranged from 0.08 to 0.45 Mg C/ha as estimated by Landsat 9 OLI/TIRS satellite. • CSR ranged from 11.97 to 18.02 g OC/m2/year. • Mangroves of Egypt showed insignificant contribution to climate mitigation related to CO 2 sequestration. [ABSTRACT FROM AUTHOR]

Small Island Developing States (SIDS) face complex socio-economic and environmental threats, making them particularly vulnerable to climate change. Blue Carbon (BC) ecosystems (mangrove forests, tidal marshes, and seagrass meadows) provide climate regulation services by sequestering and storing carbon, presenting an opportunity for SIDS to address climate change and implement Paris Agreement commitments in their Covid-19 recovery. BC habitat decline not only reduces carbon sequestration benefits provided, but can also result in sediment disturbance and the release of previously stored carbon back into the atmosphere. In this work, a scenario analysis informed by a stakeholder workshop and scientific and socio-economic expectations is used to assess the economic importance of Grenada's BC (mangroves and seagrasses) over the next 10, 25 and 50 years. Our findings indicate that sequestration benefits are severely diminished under Business-as-Usual habitat loss, but still marginally outweigh losses from carbon emissions, with overall welfare gains of US$0.5–1.9 million over 50 years. To stimulate economic recovery post-pandemic, stakeholders anticipated a realistic scenario of increased habitat decline resulting in overall losses of US$5.4–19.4 million in the next 50 years. However, if ecosystems are maintained, overall carbon benefits could reach US$10.7 million, while a 20% increase in mangroves over the next 25 years provides benefits reaching US$11.1 million between 2020 and 2070. These results demonstrate a significant increase in value when BC ecosystems are maintained and not disturbed, preventing the release of previously stored carbon and enhancing sequestration capacity. Restoration benefits are marginal, compared to conservation, supporting claims that conservation is more cost effective. • A scenario analysis is used to value Grenada's BC climate mitigation service. • Maintaining BC extents retains sequestration worth up to $10.7million over 50 years. • BAU habitat loss lowers sequestration and releases stored carbon, reducing benefits. • More severe habitat declines result in welfare losses up to US$19.4million by 2070. • Integrating BC into SIDS' Paris Agreement commitments can enrich climate mitigation. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 fluxes were measured in three subtropical mangroves for 16 site years. • VPD stress inhibits mangrove productivity during both exposed and inundated periods. • Meteorological factors and VPD have interactive effects on mangrove productivity. • Tides have independent effects on mangrove productivity through salinity changes. • The threshold effect range of VPD stress on mangrove productivity is 2.50–2.95 kPa. Subtropical mangroves have great potential to sequester atmospheric carbon dioxide (CO 2) and thus significantly contribute to climate change mitigation. Meanwhile, the carbon cycling processes in subtropical mangroves are vulnerable to the changing climatic conditions, such as the warming-induced vapor pressure deficit (VPD) increases. However, the impacts of VPD on net ecosystem production (NEP) in subtropical mangroves remain poorly understood due to a lack of detailed assessment of NEP responses to VPD among different mangrove forests over a long-term observational period. In this study, we deployed eddy-covariance systems to measure net ecosystem CO 2 exchange at three subtropical mangrove forests for 16 site-years, comprising two natural and one artificial forests. We employed a state-of-the-art data-driven modeling approach (i.e., Shapley additive explanations (SHAP) framework based on extreme gradient boosting model), which enabled us to explore the interactive effects of meteorological and tidal factors (e.g. salinity) with VPD on the mangrove NEP. We revealed that air temperature, global solar radiation (GR) and wind speed have significant interactions on the response of NEP to VPD stress in subtropical mangroves. For instance, when GR was high, the SHAP interaction values of VPD and GR on NEP decreased with increasing VPD, but when GR was low, the trends were the opposite. However, instead of identifying interactive effects between tidal salinity and VPD on mangrove NEP, we came across potential independent influence of salinity on the same. SHAP analysis was also able to disentangle the impact of VPD from other abiotic drivers. Thus, we evaluated the threshold effect of VPD stress on NEP loss in subtropical mangroves and observed a range of 2.50–2.95 kPa. Above this range, VPD stress leveled off. The subtropical mangrove responses to VPD should be therefore considered in the dynamic global vegetation models to increase the accuracy in carbon cycle simulations in the future. [ABSTRACT FROM AUTHOR]

Wetland area and condition are declining globally despite their importance to climate change mitigation and biodiversity. Introduced ungulate species are contributing to the global decline. Their impacts on wetlands are widespread and varied, however poorly understood. We summarise global impacts of introduced unmanaged and domesticated ungulates on wetlands highlighting potential outcomes of their removal. We place an emphasis on Australia due to the disproportionate impacts of ungulates on wetlands and potential for emerging carbon and biodiversity markets to incentivise private investment in wetland conservation and restoration. Our Systematic Literature Review assessed impacts of cattle, pigs, horses, deer, buffalo, sheep, camels, and other ungulates on wetlands. There were 372 relevant resources from 35 countries, with highest representation from Australia and the United States. The majority related to cattle (29 %) and pigs (19 %). More impacts were reported in freshwater wetlands (51 %) than marine (19 %). A quarter of studies related to riparian habitats. Ungulate impacts varied geographically and among climates. More studies reported soil damage, weed dispersal, decreased vegetation cover, and woody vegetation suppression than neutral or positive changes in these metrics. Decreases in richness and abundance of native flora and fauna were more frequently reported than increases. Of 33 studies reporting wetland carbon impacts, 24 reported increased CO 2 emissions due to loss of soil carbon or vegetation biomass. Ungulate exclusion from wetlands could enhance carbon stocks and biodiversity, however further studies comparing wetland typologies and carbon dynamics are needed to quantify levels of enhancement given differences in ungulate species and environments. • Wetlands are declining despite their importance for climate change and biodiversity. • Impacts of feral and domesticated ungulates on wetlands vary by ecology, region, climate, and wetland type. • Ungulates damage soils and reduce ground cover, which may increase carbon emissions. • Reported ungulate impacts on flora and fauna are most often negative, but effects depend on taxa and region. • Control of ungulates in wetlands may support wetland conservation and restoration. [ABSTRACT FROM AUTHOR]

Conservation of ecosystems is an important tool for climate change mitigation. Seagrasses, mangroves, saltmarshes and other marine ecosystems have particularly high capacities to sequester and store organic carbon (blue carbon), and are being impacted by human activities. Calls have been made to mainstream blue carbon into policies, including carbon markets. Building on the scientific literature and the French voluntary carbon standard, the ' Label Bas-Carbone ', we develop the first method for the conservation of Posidonia oceanica seagrasses using carbon finance. This methodology assesses the emission reduction potential of projects that reduce physical impacts from boating and anchoring. We show how this methodology was institutionalized thanks to a tiered approach on key parameters including carbon stocks, degradation rates, and decomposition rates. We discuss future needs regarding (i) how to strengthen the robustness of the method, and (ii) the expansion of the method to restoration of seagrasses and to other blue carbon ecosystems. [Display omitted] • Blue carbon methodologies are interesting tools to finance conservation • The first institutional blue carbon methodology in Europe is described • France has high potential to develop blue carbon projects • Expansion of methodologies should rest on precautionary principles [ABSTRACT FROM AUTHOR]

Since the term "blue carbon" was coined by the report of Nellerman et al. (2009) the marine carbon cycle has firmly entered the realm of marine policy alongside its terrestrial neighbour, "green carbon" (Crooks et al., 2018). Many marine policy decisions rely on accurate information concerning the stocks of blue carbon in a region, the annual sequestration rates associated with those stocks and the threats posed to those stocks by human activities, and especially recently by bottom-trawling (e.g., Sala et al., 2021). Hence policy officials are reliant on accurate blue carbon scientific advice. However, at the present moment there is one topic that is contributing confusion to policy-science understanding, and that is the topic of organic vs. inorganic carbon. The aim of this short note is to clarify the differences between these two types of blue carbon and to recommend how they are treated in policy formulation and the provision of scientific advice. [ABSTRACT FROM AUTHOR]

Abstract 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 p CO 2 –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 p CO 2 –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 p CO 2 –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 p CO 2 –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 p CO 2 -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. Highlights • Seagrass features differed between control and low pH stations inconsistently in the two vents. • Carbon content and its surficial accumulation decreased at high p CO 2 –low pH conditions. • Carbon storage capacity of the seagrass may not increase at high p CO 2 -low pH conditions. [ABSTRACT FROM AUTHOR]

As interest in natural climate mitigation solutions continues to grow, there is an essential role for coastal and ocean ecosystems ("blue carbon") to play. To meet mitigation targets, however, it is crucial that human actions to protect or restore blue carbon sinks are based on solid science and actionable human management opportunities to increase sinks and reduce emissions from ecosystem loss. Here, we reaffirm the role of coastal wetlands in climate mitigation opportunities. We update the state of the science regarding existing blue carbon pathways and explore expanding the blue carbon opportunities to new systems. Specifically, we analyze new science for those blue carbon pathways we categorize as "emerging" (e.g., management interventions involving macroalgae - both cultivated and wild, tidal flats, and marine sediments) where human action may be able to increase these sinks, but we currently have insufficient information to ensure that their climate mitigation benefit is additional. We revisit those that are "non-actionable" (e.g., management interventions involving calcifying organisms and marine fauna) where the scientific evidence is clear that there is no mitigation benefit, or the science is too uncertain to claim that human action can definitively increase these carbon sinks. With limited funding for climate action, it is critical that efforts focus on projects with the most mitigation potential. Blue carbon management is only one piece in the climate mitigation challenge, but it is an important one. Continued scientific research and investment in developing policy pathways is warranted for these actionable and emerging systems. At the same time, the world must continue to work to decrease emissions rapidly, invest in other natural climate solutions, and scale renewable energy technologies. [ABSTRACT FROM AUTHOR]

The saltmarsh carbon storage potential is a key topic in blue carbon research and climate policy. Ecosystem service valuations provide valuable information to policymakers for habitat management and climate change mitigation policies. Yet, only few saltmarsh valuation studies have included the carbon storage service in the UK context. This paper investigates how the public values saltmarsh ecosystem services, focussing on the carbon storage service. We used a choice experiment to elicit the willingness to pay (WTP) of a representative sample of the Scottish public to support interventions that would maintain or improve the provision of these services. Furthermore, we tested the effect of information on individuals' preferences and WTP with a split sample approach where one group received a treatment in the form of additional information. We found that (i) all attributes had a significant influence on individuals' choices; (ii) both groups had, on average, a positive marginal WTP for all presented ecosystem services; (iii) the treated sample had, on average, no significantly different marginal WTP for carbon storage than the control group. This paper adds to the limited literature on the saltmarsh carbon storage ecosystem service and demonstrates a developed nation's public's openness to nature-based climate change mitigation solutions. [Display omitted] • The Scottish public on average prefers saltmarsh management over the status quo. • Providing additional information reduced heterogeneity in WTP for carbon storage. • Most respondents preferred an improvement of all saltmarsh ecosystem services. • Therefore management should focus on both climate change mitigation and adaptation. [ABSTRACT FROM AUTHOR]

This study quantified the organic carbon (C org) stocks in Thalassia hemprichii meadows that are (i) adjacent to mangroves (MG), and (ii) without mangroves (WMG), in tropical Andaman and Nicobar Islands (ANI) of India. In the top 10 cm of the sediment, C org content was 1.8-fold higher at the MG sites than the WMG sites. The total C org stocks (sediment + biomass) in the 144 ha of seagrass meadows at MG sites (988.74 ± 138.77 Mg C) was 1.9-fold higher than in 148 ha of WMG sites. Protection and management of T. hemprichii meadows of ANI can lead to emission avoidance of around 5447.33 (MG; 3595.12 + WMG: 1852.21) tons of CO 2. The social cost of the carbon stocks in these T. hemprichii meadows is around US$ 0.30 and 0.16 million at the MG and WMG sites, respectively, showcasing the importance of ANI's seagrass ecosystems as nature-based solutions for climate change mitigation. • Thalassia hemprichii meadows adjacent to mangroves had higher carbon stocks. • External nutrient input is higher in T. hemprichii meadows adjacent to mangroves. • T. hemprichii biomass increased 2-fold when present adjacent to mangroves. • T. hemprichii meadows adjacent to mangroves have 1.9-fold higher carbon stocks. • Andaman seagrass ecosystems have potential to mitigate climate change. [ABSTRACT FROM AUTHOR]

Studies have suggested that some mangrove soils might contribute to wetland methane (CH 4 ) production and emissions, especially when the mangroves are disturbed. CH 4 emissions were measured seasonally from nine locations on two mangrove creeks on Naples Bay in southwest Florida, USA. One of the tidal creeks has been impacted in the past few decades with alteration of upstream watersheds and freshwater inflows; the other creek was an un-impacted reference tidal creek. Our study sites were established along a longitudinal gradient (distance to the creeḱs mouth) with different plant communities and freshwater influences, which were further evaluated by measurements during the dry and wet seasons. CH 4 emission measurements were also performed during the flood and ebb tides (n = 6) to incorporate the influence of water level fluctuations on CH 4 emissions. Additionally, hydroperiods and soil pore water electrical conductivity (EC) as a measure of salinity were measured along the study creeks. Our study showed very small, if not negligible, CH 4 emissions from mangrove soils in this southwest Florida location at all sampling locations and sampling times. Despite our collecting over 1900 methane samples from methane emission chambers, most analyses of rates of change in the chambers showed no methane emissions. Seasonal averages ranged from 0.24 to 1.68 mg CH 4 -C m −2 d −1 (annual average of 0.32 g CH 4 -C m −2 y −1 ). Ironically, the lowest methane emissions were at the end (December–January) of a typical wet season of daily rainfall and were highest in the dry season (March–April) when freshwater inputs from watersheds and precipitation were negligible. Water level fluctuations, freshwater inputs, and plant species composition did not play a significant role in CH 4 emissions. There appeared to be a slight pattern of methane emissions versus air temperature but the relationship was not linear. Combining our results with carbon sequestration rates in a companion study suggested that mangroves in southwest Florida are clearly net sinks of both carbon and radiative forcing and therefore beneficial for mitigating climate warming. [ABSTRACT FROM AUTHOR]

Seagrasses comprise a substantive North American and Caribbean Sea blue carbon sink. Yet fine-scale estimates of seagrass carbon stocks, fluxes from anthropogenic disturbances, and potential gains in sedimentary carbon from seagrass restoration are lacking for most of the Western Hemisphere. To begin to fill this knowledge gap in the subtropics and tropics, we quantified organic carbon (C org ) stocks, losses, and gains from restorations at 8 previously-disturbed seagrass sites around the Gulf of Mexico (GoM) (n = 128 cores). Mean natural seagrass C org stocks were 25.7 ± 6.7 Mg C org ha − 1 around the GoM, while mean C org stocks at adjacent barren sites that had previously hosted seagrass were 17.8 Mg C org ha − 1 . Restored seagrass beds contained a mean of 38.7 ± 13.1 Mg C org ha − 1 . Mean C org losses differed by anthropogenic impact type, but averaged 20.98 ± 7.14 Mg C org ha − 1 . C org gains from seagrass restoration averaged 20.96 ± 8.59 Mg ha − 1 . These results, when combined with the similarity between natural and restored C org content, highlight the potential of seagrass restoration for mitigating seagrass C org losses from prior impact events. Our GoM basin-wide estimates of natural C org totaled ~ 36.4 Tg for the 947,327 ha for the USA-GoM. Including Mexico, the total basin contained an estimated 37.2–37.5 Tg C org . Regional US-GoM losses totaled 21.69 Tg C org . C org losses differed significantly among anthropogenic impacts. Yet, seagrass restoration appears to be an important climate change mitigation strategy that could be implemented elsewhere throughout the tropics and subtropics. [ABSTRACT FROM AUTHOR]

Seagrass meadows deliver a range of ecosystem services, where one of the more important is the capacity to store carbon and serve as sinks for atmospheric carbon dioxide. The capacity of seagrass meadows for carbon storage might, however, be modified and complicated by several factors; one important factor is the possible effects of calcification within the meadows. In tropical areas, seagrass meadows can contain high proportions of calcareous organisms, which through their calcification may cause release of CO 2. To study this aspect of the CO 2 balance within tropical seagrass systems, we investigated the air-water CO 2 flux in seagrass mesocosms with different plant community compositions, i.e. mixtures of seagrass and calcifying macroalgae, having similar overall photosynthetic oxygen evolution rates. The measured CO 2 fluxes changed both in rate and direction over the day and were significantly related to plant community composition. Downward fluxes of CO 2 were found only over vegetation with high proportion of seagrass and in the afternoon, whereas occurrence of calcifying algae appeared to reverse the flow. A partial least squares (PLS) regression model indicated that pH, pCO 2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO 2 fluxes. Our findings show that algal calcification might partly counteract the carbon sequestration in seagrass meadows. • CO 2 fluxes over the surface were significantly related to plant community composition. • Downward fluxes of CO 2 were found only over vegetation with high proportion of seagrass. • pH, pCO 2 and dissolved inorganic carbon (DIC) were the primary environmental variables predicting the CO 2 fluxes. [ABSTRACT FROM AUTHOR]

Payments for carbon services (C-PFES) is a mechanism used to meet national emission reduction targets via either forest protection or reforestation. However, while carbon is a unique environmental service traded in national and international markets, there are challenges in implementing C-PFES schemes at the local level. Simply put, the views of local participants who will undertake the abatement activities have not been considered. By analysing at the local level the views of potential sellers, potential buyers and intermediaries, this paper investigated the feasibility of applying C-PFES for mangroves in Ca Mau province, Vietnam. While local stakeholders had a strong understanding of the likely impacts of climate change on their production and were aware of the environmental roles of mangroves, the carbon sequestration capacity of mangroves was rarely mentioned, and this will be a challenge in implementing a C-PFES scheme. While potential sellers and intermediaries showed strong support for C-PFES, potential buyers were less likely to join the scheme. As the carbon sequestration capacity of mangroves can also be purchased by international entities, it is recommended that the scope of the current Vietnamese C-PFES scheme be broadened to allow participation of entities from outside Ca Mau province, including private companies, government agencies and international investors. We also identified the need for education training programs to enhance stakeholders' perspectives prior to the implementation of C-PFES at the local level. This study provides insights for countries with mangrove ecosystems seeking to implement a C-PFES. [ABSTRACT FROM AUTHOR]

Indonesia has the largest extent of mangroves globally, supporting communities at the local scale while contributing to climate change mitigation at the global scale. Yet, they are highly threatened by multiple anthropogenic and natural stressors. Thus, in this study, we aim to qualitatively assess Indonesia's mangrove ecosystems by conducting a literature review applying the Driver-Pressure-State-Impact-Response (DPSIR) framework. By using the framework, we identified multiple stressors and existing management efforts in 27 provinces of Indonesia. For instance, we identified aquaculture expansion as the most frequented driver of mangrove loss leading to the conversion of mangroves to shrimp farms in 20 provinces. In terms of responses, mangrove rehabilitation and community-based management have a long history in Indonesia while economic valuation and payment of ecosystem services are yet to be established across the country. The identification of multiple stressors and/or threats is instrumental to addressing the causes of degradation of globally-important mangroves in Indonesia if effectively translated into policies, which remains a future challenge. [ABSTRACT FROM AUTHOR]

• Climate change was identified as the main threat for seagrass soil CO 2 emissions. • Direct threats have the highest potential risk for CO 2 emissions at a local scale. • Empirical data on seagrass CO 2 emissions following disturbance is scarce. • Ranking of threats for seagrass CO 2 emissions can aid management and policy. Seagrass meadows are natural carbon storage hotspots at risk from global change threats, and their loss can result in the remineralization of soil carbon stocks and CO 2 emissions fueling climate change. Here we used expert elicitation and empirical evidence to assess the risk of CO 2 emissions from seagrass soils caused by multiple human-induced, biological and climate change threats. Judgments from 41 experts were synthesized into a seagrass CO 2 emission risk score based on vulnerability factors (i.e., spatial scale, frequency, magnitude, resistance and recovery) to seagrass soil organic carbon stocks. Experts perceived that climate change threats (e.g., gradual ocean warming and increased storminess) have the highest risk for CO 2 emissions at global spatial scales, while direct threats (i.e., dredging and building of a marina or jetty) have the largest CO 2 emission risks at local spatial scales. A review of existing peer-reviewed literature showed a scarcity of studies assessing CO 2 emissions following seagrass disturbance, but the limited empirical evidence partly confirmed the opinion of experts. The literature review indicated that direct and long-term disturbances have the greatest negative impact on soil carbon stocks per unit area, highlighting that immediate management actions after disturbances to recover the seagrass canopy can significantly reduce soil CO 2 emissions. We conclude that further empirical evidence assessing global change threats on the seagrass carbon sink capacity is required to aid broader uptake of seagrass into blue carbon policy frameworks. The preliminary findings from this study can be used to estimate the potential risk of CO 2 emissions from seagrass habitats under threat and guide nature-based solutions for climate change mitigation. [ABSTRACT FROM AUTHOR]

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. • C, N and P content was high for sediments and seagrass tissues. • C stocks were up to 2.5 times more in vegetated sediments. • C, N and P accumulation are dependent on seagrass species. [ABSTRACT FROM AUTHOR]

To align with international climate efforts to remain within 1.5 degrees of the earth temperature, Indonesia requires concerted measures from actors to preserve and restore carbon rich ecosystems, especially blue carbon ecosystem. Although studies have suggested the importance of blue carbon ecosystems in contributing to Indonesian climate action, translating science to policy remains a challenge. Mapping actors and the pattern of information exchange related to blue carbon can help identify potential barriers in the blue carbon governance process and policy development. This study uses Social Network Analysis and integrates it with results from in-depth qualitative evaluation of institutional respondents. Data is obtained through questionnaires and semi-structured interviews with representatives from a broad range of organizations. It was found that the actor who oversees the fulfilment of the climate commitment, which is the most common objective of the network, is not a central actor. Second, the actors with the highest degree of centrality received little trust from other actors. Third, overall, the network has low quality ties. Each of these hinders knowledge providers' ability to make an impact on policy development. By critically examining the interactions between actors, this research casts new light on the overlooked problem of the significance of the network in blue carbon governance process. [ABSTRACT FROM AUTHOR]

• We estimated economic value of sequestered carbon in California's coastal habitat. • Lack of site-specific carbon sequestration data hampers current valuation efforts. • Sub-national blue carbon policy can help countries keep Paris Agreement commitments. • Valuation of sequestration helps governments meet commitments to greenhouse gas goals. The emissions reduction pledges made by individual countries through the 2015 Paris Agreement represent the current global commitment to mitigate greenhouse gas emissions in the face of the enduring climate crisis. Natural lands carbon sequestration and storage are critical for successful pathways to global decarbonization (i.e., as a negative emissions technology). Coastal vegetated habitats maintain carbon sequestration rates exceeding forest sequestration rates on a per unit area basis by nearly two orders of magnitude. These blue carbon habitats and their associated carbon sequestration benefits are vulnerable to losses from land-use change and sea-level rise. Incorporation of blue carbon habitats in climate change policy is one strategy for both maintaining these habitats and conserving significant carbon sequestration capabilities. Previous policy assessments have found the potential for incorporation of coastal carbon sequestration in national-level policies, yet there has – to date – been little inclusion of blue carbon in the national-scale implementation of Paris commitments. Recently, sub-national jurisdictions have gained attention as models for pathways to decarbonization. However, few previous studies have examined sub-national level policy opportunities for operationalizing blue carbon into climate decision-making. California is uniquely poised to integrate benefits from blue carbon into its coastal planning and management and its suite of climate mitigation policies. Here, we evaluated legal authorities and policy contexts addressing sequestration specifically from blue carbon habitats. We synthesized the progressive action in California's approaches to mitigate carbon emissions including statutory, regulatory, and non-regulatory opportunities to incorporate blue carbon ecosystem service information into state- and local-level management decisions. To illustrate how actionable blue carbon information can be produced for use in decision-making, we conducted a spatial analysis of blue carbon sequestration in several locations in California across multiple agencies and management contexts. We found that the average market values of carbon sequestration services in 2100 ranged from $7,730 to $44,000 per hectare and that the social cost of carbon sequestration value was 1.3 to 2.7 times the market value. We also demonstrated that restoration of small areas with high sequestration rates can be comparable to the sequestration of existing marshes. Our results illustrate how accessible information about carbon sequestration in coastal habitats can be directly incorporated into existing policy frameworks at the sub-national scale. The incorporation of blue carbon sequestration benefits into sub-national climate policies can serve as a model for the development of future policy approaches for negative emissions technologies, with consequences for the success of the Paris Agreement and science-based decarbonization by mid-century. [ABSTRACT FROM AUTHOR]

Seagrass restoration has been suggested as a Blue Carbon (BC) strategy for climate change mitigation. For Nationally Determined Contributions (NDC) and carbon crediting schemes, BC projects need to demonstrate 'additionality', that is enhanced CO 2 sequestration and/or avoided greenhouse gas emissions following management actions. This typically requires determining soil carbon accumulation rates (CAR), which is often done using radionuclides or surface elevation tables to estimate sedimentation rates. Here we undertook a case study, using 210Pb and 14C dating, to detect possible changes in C org stocks and CAR following the loss and partial recovery of Posidonia seagrass meadows in South Australia since 1980–90s. The 210Pb data revealed a lack of accumulation of excess 210Pb in most sites, suggesting negligible accumulation of sediments, intense mixing of the upper layers, or accumulation of reworked sediments, precluding the estimation of reliable CAR at decadal time scales. This limitation was also encountered with 14C. The inability to compare sites over analogous periods of time prevented quantifying differences in soil C org sequestration, thereby to demonstrate additionality. The lack of significant differences in soil C org stocks among sites which never suffered seagrass loss, those showing recovery and those with no recovery (5.7 ± 1.2, 4.5 ± 0.7 and 3.3 ± 0.3 kg C org m-2 within the top meter, respectively) also precluded estimates of soil C org gains or losses. Our findings demonstrate that, while 210Pb and 14C provide important information on sediment deposition dynamics, it is not straightforward to demonstrate additionality using radionuclides in low depositional seagrass habitats exposed to hydrodynamic energy, features which may be encountered in seagrass sites. We provide insights for the selection of suitable habitats for seagrass BC projects, suggest possible alternative methods for estimating additionality, and discuss the implications of the findings for the implementation of seagrass BC strategies to mitigate greenhouse gas emissions. Image 1 • Demonstrating C org gains and losses in BC projects constrained by methods and hydrodynamics. • Determining CARs using radionuclides may be impossible in some seagrass sites. • Carefully site selection should precede the implementation of seagrass BC projects. • Depositional environments protected from hydrodynamic energy are more suitable for BC projects. [ABSTRACT FROM AUTHOR]

Blue carbon (BC) ecosystems provide important and multiple ecosystem services, including climate change mitigation through carbon storage. However, these ecosystems are under the pressure of numerous anthropogenic stressors, such as population growth and industrialization, which jeopardize the benefits they provide. Although the common methodology is based on the use of a linear depth scale, this work highlights the need of sediment dating to evaluate temporal variation of organic carbon (C org) stocks and burial rates, since sedimentation patterns can be variable even between sites within a single ecosystem. Temporal records of C org stocks and burial rates during the past 100 years were determined in mangrove, seagrass and salt marsh ecosystems, from five representative areas of the Mexican marine coastal systems, by using 210Pb-dated sediment cores. Carbon stocks during the past 100 years (C org stock (100 years)) ranged from 67 to 269 Mg C org ha−1 in mangroves, 8–53 Mg C org ha−1 in seagrasses and 32–56 Mg C org ha−1 in salt marshes, and burial rates were 8–426 g m−2 yr−1, 2–252 g m−2 yr−1 and 1–60 g m−2 yr−1, respectively. Most of the stored C org (60–86%) accumulated during the Anthropocene (since 1950), and in mangrove cores recent C org stocks were up to six times higher than before the Anthropocene onset (1900–1950), most likely as a result of land use changes. Increased C org burial rates in seagrass ecosystems were mostly driven by mass accumulation rate changes (promoted by catchment erosion and sea level rise), and in mangroves and salt marshes by changes in C org content, likely promoted by an increase in productivity associated with nutrient enrichment by fertilizer runoff and sewage disposal. Despite the multiple anthropogenic stressors that positively affected C org stocks and burial rates in BC ecosystems, worldwide destruction of these valuable habitats will cause losses of long-term buried C org through significant CO 2 releases, thus conservation and/or restoration are effective actions for climate change mitigation. • C org burial and stock temporal change in Mexican blue carbon habitats are reviewed. • Notable differences in C org accumulation were observed within and among study sites. • Highest accumulation in seagrasses, mangroves and salt marshes occurred after 1950s. • Land use changes have enhanced C org burial rates and stock during the Anthropocene. • 210Pb dating is relevant for blue carbon stock evaluations in a global context [ABSTRACT FROM AUTHOR]

The study of a Posidonia sinuosa sedimentary archive has delivered a millenary record of environmental change in Cockburn Sound (Western Australia). Ecosystem change is a major environmental problem challenging sustainable coastal development worldwide, and this study shows baseline trends and shifts in ecological processes in coastal ecosystems under environmental stress. The concentrations and fluxes of biogeochemical elements over the last 3,500 years indicate that important changes in ecosystem dynamics occurred over the last 1,000 years, in particular after ~1900's, probably related to establishment of seagrass meadows in the area and to local and regional human activities (industry and coastal development), respectively. The establishment of seagrasses ~1,000 years ago in the area of study is supported by the appearance of Posidonia fibres from ~40 cm soil depth until the core top, higher δ13C values indicating a larger contribution of seagrass-matter to the soil organic carbon pool, and increased concentration of fine sediments driven by the effect of seagrass canopy in enhancing sedimentation. The comparison of organic carbon, nutrients and metal concentrations and fluxes between pre- and post-establishment of seagrasses shows that seagrass establishment resulted in up to 9-fold increase in the soil biogeochemical sink. In ~1900's, shifts in the concentrations of metals, carbonates, organic carbon, sediment grain size, and δ13C and δ15N values of the organic matter were detected, demonstrating an alteration in seagrass ecosystem functioning following the onset of European settlement. Anthropogenic activities, and in particular the construction of a causeway in 1970's, enhanced seagrass soil organic carbon and metal accumulation rates by 36- and 39-fold, respectively, showing that human-made structures can enhance the biogeochemical sink capacity of seagrasses. Here we reconstruct the impact of human activities on seagrass ecosystem dynamics and blue carbon, which can inform local management of Cockburn Sound and seagrass conservation for climate change mitigation and adaptation. • Seagrass archives help to understand and better manage ecological change. • Seagrass establishment ~1,000 years ago increased biogeochemical sink capacity. • Ecosystem shifts followed European settlement in an Australian embayment. • Human-made structures can enhance seagrass carbon and metal sequestration. [ABSTRACT FROM AUTHOR]

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

Coastal communities, knowingly or otherwise, rely on seagrasses for their livelihood, recreation and food source, among other services. However, despite its importance, seagrasses are not receiving similar attentions with its adjacent ecosystems, the mangroves, and coral reefs. Because of their role in climate change mitigation, seagrasses along with mangroves and salt marshes (the blue carbon ecosystems) are gaining attention recently. This preliminary study investigates the perception of coastal communities in Eastern Samar, Philippines on seagrasses. The results of the survey show that there is a level of high awareness among respondents to seagrass ecosystem services. Despite that, however, utilization remains low. The level of awareness varies on the type of ecosystem service; for instance, locals have a high awareness of provisioning services while low awareness of cultural services. The survey also included the perceived threats to seagrasses, where damages caused by natural disturbances are identified as the most concerning threat. This may be due to the geographic location of the sites that are frequented by typhoons. The perception potentially indicates depleted resources brought about by mismanagement or overlooking this natural resource. The results provide a more contextualized understanding on how local communities are aware of and interacting with, or the lack thereof, seagrass ecosystems. This could aid local government units (LGUs) and conservation groups in the area to craft practicable and sustainable management plans and threat-specific solutions. Location map of the study sites and the current condition of seagrass ecosystems as perceived by the local communities (95 respondents in Balangiga, 91 in Salcedo, 94 in Lawaan, and 92 in Balangkayan). Image 1 • Level of awareness of the locals varies on the type of ecosystem service. • About 28.6%–42.4% of the locals are not aware of the cultural services of seagrasses. • Frequency of fishing in seagrass beds is high for household consumption with 33.7%–58.5% of the locals. • Natural disturbances are the most concerning threats to seagrasses. • Availability of seagrass data is very limited in the study areas. [ABSTRACT FROM AUTHOR]

• Organic carbon burial rates and sources were assessed across Papuan mangroves. • Organic carbon burial rates ranged between 0.21 and 1.19 Mg C ha−1 yr−1. • Soil organic carbon stocks in the top 50 cm varied between 62 and 179 Mg C ha−1. • Soil organic carbon stocks are sourced from autoch- and alloch-thonous sources. Mangrove organic carbon is primarily stored in soils, which contain more than two-thirds of total mangrove ecosystem carbon stocks. Despite increasing recognition of the critical role of mangrove ecosystems for climate change mitigation, there is limited understanding of soil organic carbon sequestration mechanisms in undisturbed low-latitude mangroves, specifically on organic carbon burial rates and sources. This study assessed soil organic carbon burial rates, sources and stocks across an undisturbed coastal mudflat and mangrove hydrogeomorphological catena (fringe mangrove and interior mangrove) in Bintuni Bay, West Papua Province, Indonesia. 210Pb radionuclide sediment dating, and mixing model of natural stable isotope signatures (δ 13C and δ15N) and C/N ratio were used to estimate organic carbon burial rates and to quantify proportions of allochthonous (i.e., upland terrestrial forest) and autochthonous (i.e., on-site mangrove forest) organic carbon in the top 50 cm of the soil. Burial rates were in the range of 0.21–1.19 Mg C ha−1 yr−1. Compared to the fringe mangroves, organic carbon burial rates in interior mangroves were almost twice as high. Primary productivity of C 3 upland forest vegetation and mangroves induced soil organic carbon burial in interior mangroves and this was consistent with the formation of the largest organic carbon stocks (179 ± 82 Mg C ha−1). By contrast, organic carbon stored in the fringe mangrove (68 ± 11 Mg C ha−1) and mudflat (62 ± 10 Mg C ha−1) soils mainly originated from upland forests (allochthonous origin). These findings clearly indicate that carbon sequestered and cycling in mangrove and terrestrial forest ecosystems are closely linked, and at least a part of carbon losses (e.g., erosion) from terrestrial forests is buried in mangrove ecosystems. [ABSTRACT FROM AUTHOR]

Our results concluded, that deforestation and degradation results in significant losses of TEC stocks from mangroves. While the prevention of deforestation and degradation is the most effective strategy for climate change mitigation and adaptation, it appears that restoration results in mangroves that can continue to combat climate change after 25–30 years. • Deforestation reduced total ecosystem carbon (TEC) stocks of intact mangroves by 60%. • TEC stocks from degraded mangroves forests did not differ from intact forests. • TEC stocks from 25-year-old restored mangroves were similar to intact mangroves. • A gridded sampling approach effectively captured TEC variability across an entire country. Mangrove forest conservation can help reduce global C emissions. Despite this benefit to climate change mitigation and adaptation, mangrove forests are being deforested or degraded at an alarming rate, though restoration efforts may offset these losses. The impacts of deforestation to C stocks are relatively intuitive and result in significant decreases in C stocks. It remains unclear how degradation from selective harvesting of trees affects C stocks or how effective restoration efforts are at restoring C stocks. Furthermore, total ecosystem C (TEC) stocks of pristine mangroves can significantly vary spatially. To address these issues, we conducted an intensive, national assessment of mangrove forests across Cambodia using a grid approach to: 1) examine how land use land cover (i.e., pristine, deforested, degraded, and restored forests) impacts TEC stocks, and 2) how TEC stocks vary spatially across the country. TEC stocks from deforested mangroves were always lower than pristine forests, resulting in an overall loss of 60% C (480 Mg C ha−1). However, TEC stocks from degraded and 25-year-old restored mangroves forests did not differ from pristine forests. Mean TEC in mangroves was 784.7 ± 30.1 Mg C ha−1, decreasing from 957.2 ± 32.8 Mg C ha−1 in the northern region to 628.9 ± 33.1 Mg C ha−1 in the central region to 386.2 ± 19.1 Mg C ha−1 in the southern region of Cambodia. Intensive sampling in mangroves across Cambodia verified impacts of deforestation reported elsewhere, revealed the lack of degradation impacts on TEC stocks, and demonstrated the effectiveness of restoration on TEC stocks after only 25 years. Our gridded sampling approach was able to capture spatial variability across Cambodia and provide a more realistic TEC stock information that can be used for national reporting or participation in C markets. [ABSTRACT FROM AUTHOR]