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• 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]

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]

• We assess the potential inclusion of blue carbon within Australia's Emissions Reduction Fund, emphasizing issues and approaches that have global relevance. • We identify twelve potential management actions then quantify and discuss the five most promising activities, encompassing the protection, restoration and creation of mangroves, tidal marshes and seagrasses. • On a per area basis, mean abatement intensity of organic carbon was highest for the management activity ' (re)introduction of tidal flow' which may result in mean annual abatement of 13 – 15 Mg C org ha−1 yr−1 for mangrove and 6 – 8 Mg C org ha−1 yr−1 for tidal marsh. • Our approach offers a template that uses best available information to identify options for carbon abatement through management of coastal landscapes. There is increasing interest in protecting, restoring and creating 'blue carbon' ecosystems (BCE; mangroves, tidal marshes and seagrasses) to sequester atmospheric CO 2 -C and thereby contribute to climate change mitigation. While a growing number of countries aspire to report greenhouse gas emission and carbon sequestration changes from these ecosystems under voluntary international reporting requirements, few countries have domestic policy frameworks that specifically support the quantification and financing of carbon emission abatement through BCE management. Australia, as home to approximately 5–11% of global blue carbon stocks, has a substantial interest in the development of blue carbon policy. Here we assess the potential inclusion of blue carbon within Australia's Emissions Reduction Fund, emphasizing issues and approaches that have global relevance. We used a participatory workshop of scientific experts and carbon industry stakeholders to identify blue carbon management actions that would meet the requirements of the Fund. In total, twelve actions were assessed for their greenhouse gas emission abatement potential and the ability to measure abatement reliably, using a combination of available data and qualitative and quantitative methods, including expert knowledge. We identify and discuss the five most relevant and promising activities, encompassing the protection, restoration and creation of mangroves, tidal marshes and seagrasses. On a per area basis, mean abatement intensity of organic carbon (C org) was highest for the (re)introduction of tidal flow resulting in establishment of mangrove (13–15 Mg C org ha−1 yr−1) and tidal marsh (6–8 Mg C org ha−1 yr−1), followed by land use planning for sea-level rise for the creation of new mangrove habitat (8 Mg C org ha−1 yr−1). The avoided disturbance of existing mangroves, tidal marshes and seagrasses has the twofold benefit of avoiding remineralisation of existing stocks, plus the future annual abatement associated with the net sequestration of atmospheric CO 2 -C as C org with the continued functioning of these BCE. Our approach offers a template that uses best available information to identify options for carbon abatement through management of coastal landscapes, and details current knowledge gaps and important technical aspects that need to be considered for implementation in carbon crediting schemes. [ABSTRACT FROM AUTHOR]

• Molecular analysis of 3200 yr Posidonia australis seagrass soil record (Australia). • Tracing diagenetic effects on sheaths and root/rhizome detritus. • Ageing-induced selective degradation of polysaccharides and enrichment of lignin. • Minor constituents cutin, suberin and tannin also preferentially depleted. • Overall C storage capacity associated with lignocellulose chemical recalcitrance. Seagrass ecosystems are recognised for their role in climate change mitigation, due to their capacity to form organic-rich sediments. The chemical recalcitrance of seagrass organs is one characteristic driving carbon storage, but the molecular background of this feature is poorly understood. We assessed molecular composition changes of Posidonia australis sheaths (SH) and roots plus rhizomes (RR) along a sediment core, encompassing 3200 cal. yr BP, by means of nuclear magnetic resonance spectroscopy (13C NMR), conventional analytical pyrolysis (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). Significant trends with depth (age) in the composition of both SH and RR remains of P. australis were observed from all methods. In general terms, polysaccharides become depleted (degraded) and lignin enriched (selectively preserved) as age increases, and the minor constituents cutin, suberin and condensed tannin are also preferentially depleted during ageing in both fractions. Molecular changes with ageing were smaller in SH, especially regarding polysaccharides, indicative of a superior stability compared to RR. The molecular changes observed are most pronounced within the first 75 cm of the record, which reflects the recalcitrance of P. australis detritus once it is buried below that depth (corresponding to approximately 700 cal. yr BP). The capacity of P. australis to act as a long-term carbon sink seems to be mainly related to the resistance of buried lignocellulose materials to decomposition. The results on diagenetic effects on the molecular fingerprint of seagrass detritus contribute to our understanding of carbon sequestration in Blue Carbon ecosystems. Furthermore, data comparison of the methods applied using principal component analysis (PCA) allowed us to identify consistencies, discrepancies and complementarities. [ABSTRACT FROM AUTHOR]

Blue carbon ecosystems (BCE) include mangrove forests, tidal marshes, and seagrass meadows, all of which are currently under threat, putting their contribution to mitigating climate change at risk. Although certain challenges and trade-offs exist, remote sensing offers a promising avenue for transparent, replicable, and cost-effective accounting of many BCE at unprecedented temporal and spatial scales. The United Nations Framework Convention on Climate Change (UNFCCC) has issued guidelines for developing blue carbon inventories to incorporate into Nationally Determined Contributions (NDCs). Yet, there is little guidance on remote sensing techniques for monitoring, reporting, and verifying blue carbon assets. This review constructs a unified roadmap for applying remote sensing technologies to develop cost-effective carbon inventories for BCE – from local to global scales. We summarise and discuss (1) current standard guidelines for blue carbon inventories; (2) traditional and cutting-edge remote sensing technologies for mapping blue carbon habitats; (3) methods for translating habitat maps into carbon estimates; and (4) a decision tree to assist users in determining the most suitable approach depending on their areas of interest, budget, and required accuracy of blue carbon assessment. We designed this work to support UNFCCC-approved IPCC guidelines with specific recommendations on remote sensing techniques for GHG inventories. Overall, remote sensing technologies are robust and cost-effective tools for monitoring, reporting, and verifying blue carbon assets and projects. Increased appreciation of these techniques can promote a technological shift towards greater policy and industry uptake, enhancing the scalability of blue carbon as a Natural Climate Solution worldwide. • Blue carbon ecosystems (BCE) play a key role in climate change mitigation. • Remote sensing techniques are cost-effective for monitoring BCE extent and carbon stocks. • A tradeoff exists between spatial resolution and historical coverage of remotely sensed data. • New technologies can facilitate the inclusion of BCE in national inventories and Nationally Determined Contributions (NDCs). • Our decision tree guides the choice of remote sensing methods for monitoring BCE. [ABSTRACT FROM AUTHOR]