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Assessing historical environmental conditions linked to habitat colonization is important for understanding long‐term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold‐temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20‐ and 24‐fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO2 removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services. Plain Language Summary: This study investigated the historical colonization of eelgrass (Zostera marina), an important marine vascular plant in cold‐temperate coastal regions. Sediment cores from eelgrass meadows on the Swedish west coast dating back as far as 14,000 years were examined to understand the time‐course of eelgrass colonization and the subsequent modification of the environment. We found that eelgrass colonization began approximately 2,000 years ago, coinciding with the development of shallow, sheltered conditions that favored eelgrass growth. As eelgrass became established, this led to substantial habitat and sediment changes, including a 20‐ and 24‐fold increase in carbon and nitrogen accumulation, respectively. This highlights the crucial role of eelgrass as a provider of important ecosystem services, such as regulation of climate, nutrient retention, and sediment protection. We also examined the potential effects of climate change on eelgrass growth and health, predicting that decreased water clarity and altered water flow pose the greatest risks. Overall, this study adds valuable insights into the relationship between eelgrass and its environment, aiding in conservation and restoration efforts to mitigate climate change and maintain essential ecosystem services. It emphasizes the importance of specific environmental conditions for successful eelgrass colonization and restoration. Key Points: Decreased hydrodynamics and water depth created a favorable environment for eelgrass establishment about 2,000 years agoCarbon and nitrogen burial increased in order of magnitudes following seagrass colonizationPaleoecological information on environmental conditions linked to seagrass colonization can aid conservation and restoration efforts [ABSTRACT FROM AUTHOR]

Seagrass meadows are effective carbon sinks due to high primary production and sequestration in sediments. However, methane (CH4) emissions can partially counteract their carbon sink capacity. Here, we measured diffusive sediment‐water and sea‐air CO2 and CH4 fluxes in a coastal embayment dominated by Posidonia oceanica in the Mediterranean Sea. High‐resolution timeseries observations revealed large spatial and temporal variability in CH4 concentrations (2–36 nM). Lower sea‐air CH4 emissions were observed in an area with dense seagrass meadows compared to patchy seagrass. A 6%−40% decrease of CH4 concentration in the surface water around noon indicates that photosynthesis likely limits CH4 fluxes. Sediments were the major CH4 source as implied from radon (a natural porewater tracer) observations and evidence for methanogenesis in deeper sediments. CH4 sediment‐water fluxes (0.1 ± 0.1–0.4 ± 0.1 μmol m−2 d−1) were higher than average sea‐air CH4 emissions (0.12 ± 0.10 μmol m−2 d−1), suggesting that dilution and CH4 oxidation in the water column could reduce net CH4 fluxes into the atmosphere. Overall, relatively low sea‐air CH4 fluxes likely represent the net emissions from subtidal seagrass habitat not influenced by allochthonous CH4 sources. The local CH4 emissions in P. oceanica can offset less than 1% of the carbon burial in sediments (142 ± 69 g CO2eq m−2 yr−1). Combining our results with earlier observations in other seagrass meadows worldwide reveals that global CH4 emissions only offset a small fraction (<2%) of carbon sequestration in sediments from seagrass meadows. Plain Language Summary: Seagrass meadows are hotspots for marine carbon storage in sediments. Part of the sediment carbon can be released as carbon dioxide and methane (CH4). Methane has 45–96 times more powerful global warming effect than carbon dioxide. If seagrass meadows release CH4, the emissions counteract their climate mitigation potential. We measured greenhouse gas concentrations and fluxes in a seagrass‐dominated Mediterranean embayment. Low CH4 coincided with oxygen produced from seagrass photosynthesis. Areas with dense seagrass meadows had lower CH4 emissions. Overall, seagrass‐dominated coasts were a small source of CH4 that offset only <2% of carbon buried in sediments on local and global scales. Hence, seagrass meadows remain an effective carbon sink. Key Points: High‐resolution CH4 observations reveal diel cycles linked to seagrass photosynthesisDense seagrass meadow had lower sea‐air CH4 flux than the area with patchy and dead seagrassCH4 emissions were a small offset to seagrass C sequestration on local and global scales [ABSTRACT FROM AUTHOR]

Carbon sequestration can also occur as alkalinity, dissolved inorganic and organic carbon (DIC and DOC) as well as detritus export beyond the habitat where carbon is fixed. Aquatic carbon sequestration refers to the long-term (century-scale) storage of atmospheric carbon in sediments, biomass and/or water. GLO:4D9/02nov22:lno12268-fig-0001.jpg PHOTO (COLOR): 1 Blue carbon ecosystems (mangroves, saltmarshes, seagrass meadows, and kelp forests) are carbon sequestration hotspots. A faster turnover of contemporary autochthonous organic carbon ("fast carbon") than old allochthonous organic carbon ("slow carbon") revealed a decoupling between carbon burial and anthropogenic CO SB 2 sb emissions. [Extracted from the article]

Tidal marshes rank among the ecosystems with the highest capacity to sequester and store organic carbon (Corg) on earth. To inform conservation of coastal vegetated ecosystems for climate change mitigation, this study investigated the factors driving variability in carbon storage. We estimated soil Corg stocks in tidal marshes across temperate Western Australia and assessed differences among geomorphic settings (marine and fluvial deltas, and mid-estuary) and vegetation type (Sarcocornia quinqueflora and Juncus kraussii) linked to soil biogeochemistry. Soil Corg stocks within fluvial and mid-estuary settings were significantly higher (209 ± 14 and 211 ± 20 Mg Corg ha−1, respectively; 1-m-thick soils) than in marine counterparts (156 ± 12 Mg Corg ha−1), which can be partially explained by higher preservation of soil Corg in fluvial and mid-estuary settings rich in fine-grained (< 0.063 mm) sediments (49 ± 3% and 47 ± 4%, respectively) compared to marine settings (23 ± 4%). Soil Corg stocks were not significantly different between S. quinqueflora and J. kraussii marshes (185 ± 13 and 202 ± 13 Mg Corg ha−1, respectively). The higher contribution of tidal marsh plus supratidal vegetation in fluvial (80%) and intermediate (76%) compared to marine (57%) settings further explains differences in soil Corg stocks. The estimated soil Corg stocks in temperate Western Australia's tidal marshes (57 Tg Corg within ~ 3000 km2 extent) correspond to about 2% of worldwide tidal marsh soil Corg stocks. The results obtained identify global drivers of soil Corg storage in tidal marshes and can be used to target hot spots for climate change mitigation based on tidal marsh conservation. [ABSTRACT FROM AUTHOR]

Coastal vegetated "blue carbon" ecosystems can store large quantities of organic carbon (OC) within their soils; however, the importance of these sinks for climate change mitigation depends on the OC accumulation rate (CAR) and residence time. Here we evaluate how two modeling approaches, a Bayesian age‐depth model alone or in combination with a two‐pool OC model, aid in our understanding of the time lines of OC within seagrass soils. Fitting these models to data from Posidonia oceanica soil cores, we show that age‐depth models provided reasonable CAR estimates but resulted in a 22% higher estimation of OC burial rates when ephemeral rhizosphere OC was not subtracted. This illustrates the need to standardize CAR estimation to match the research target and time frames under consideration. Using a two‐pool model in tandem with an age‐depth model also yielded reasonable, albeit lower, CAR estimates with lower estimate uncertainty, which increased our ability to detect among‐site differences and seascape‐level trends. Moreover, the two‐pool model provided several other useful soil OC diagnostics, including OC inputs, decay rates, and transit times. At our sites, soil OC decayed quite slowly both within fast cycling (0.028 ± 0.014 yr−1) and slow cycling (0.0007 ± 0.0003 yr−1) soil pools, resulting in OC taking between 146 and 825 yr to transit the soil system. Further, an estimated 85% to 93% of OC inputs enter slow‐cycling soil pools, with transit times ranging from 891 to 3,115 yr, substantiating the importance of P. oceanica soils as natural, long‐term OC sinks. Key Points: Conservative long‐term carbon burial rates can be estimated from soils below the rhizosphereEstimate uncertainty with pool models increases the ability to detect seascape‐level trendsPool model estimates of soil carbon diagnostics help to unveil dynamics governing carbon storage [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]

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]

Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO2 emission benefits of VCE conservation and restoration. Australia contributes 5–11% of the C stored in VCE globally (70–185 Tg C in aboveground biomass, and 1,055–1,540 Tg C in the upper 1 m of soils). Potential CO2 emissions from current VCE losses are estimated at 2.1–3.1 Tg CO2-e yr-1, increasing annual CO2 emissions from land use change in Australia by 12–21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions. Policies aiming to preserve vegetated coastal ecosystems (VCE) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here the authors assessed organic carbon storage in VCE across Australian and the potential annual CO2 emission benefits of VCE conservation and find that Australia contributes substantially the carbon stored in VCE globally. [ABSTRACT FROM AUTHOR]

Seagrass meadows are 'blue carbon' ecosystems widely recognised for their potential role in climate change mitigation. Previous studies have focused mainly on carbon storage within meadows and sediments. However, little is known about contribution of outwelling (i.e., lateral transport) to seagrass carbon budgets. Here, radium isotopes (223Ra and 224Ra) were used to assess dissolved inorganic carbon (DIC) and total alkalinity (TA) outwelling from a Mediterranean Posidonia oceanica meadow during early autumn. DIC outwelling was 114 ± 61 mmol m−2 day−1 and exceeded above-meadow CO 2 outgassing (3 ± 1 mmol m−2 day−1). Production of DIC was uncoupled from TA and fuelled by net heterotrophy and aerobic processes within the meadow. The small export of TA (5 ± 6 mmol m−2 day−1) implied that ∼90% of outwelled DIC may return to the atmosphere as CO 2 in offshore waters. Combining these fluxes with above-meadow outgassing suggested a total carbon loss that exceeded long term burial in sediments. Overall, the meadow acted as a carbon source to the atmosphere during the early autumn season. Further studies quantifying outwelling at multiple spatial and temporal scales are required to better resolve seagrass carbon budgets and their contribution to carbon sequestration. • Radium isotopes enabled calculation of mixing rates and outwelling fluxes. • Dissolved inorganic carbon outwelling exceeded above-meadow CO 2 outgassing. • Production of dissolved inorganic carbon and alkalinity was uncoupled. • Most of the outwelled dissolved inorganic carbon may return to the atmosphere offshore. • Dissolved inorganic carbon may be a significant component of seagrass carbon budgets. [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]

The term 'Blue Carbon' was coined about a decade ago to highlight the important carbon sequestration capacity of coastal vegetated ecosystems. The term has paved the way for the development of programs and policies that preserve and restore these threatened coastal ecosystems for climate change mitigation. Blue carbon research has focused on quantifying carbon stocks and burial rates in sediments or accumulating as biomass. This focus on habitat-bound carbon led us to losing sight of the mobile blue carbon fraction. Oceans, the largest active reservoir of carbon, have become somewhat of a blind spot. Multiple recent investigations have revealed high outwelling (i.e., lateral fluxes or horizontal exports) of dissolved inorganic (DIC) and organic (DOC) carbon, as well as particulate organic carbon (POC) from blue carbon habitats. In this paper, we conceptualize outwelling in mangrove, saltmarsh, seagrass and macroalgae ecosystems, diagnose key challenges preventing robust quantification, and pave the way for future work integrating mobile carbon in the blue carbon framework. Outwelling in mangroves and saltmarshes is usually dominated by DIC (mostly as bicarbonate), while POC seems to be the major carbon species exported from seagrass meadows and macroalgae forests. Carbon outwelling science is still in its infancy, and estimates remain limited spatially and temporally. Nevertheless, the existing datasets imply that carbon outwelling followed by ocean storage is relevant and may exceed local sediment burial as a long-term (>centuries) blue carbon sequestration mechanism. If this proves correct as more data emerge, ignoring carbon outwelling may underestimate the perceived sequestration capacity of blue carbon ecosystems. [Display omitted] • Odum's outwelling hypothesis is framed in a marine carbon sequestration context. • Mangrove, saltmarsh, seagrass and macroalgae ecosystems effectively sequester carbon. • Outwelling may exceed sediment burial as a long-term carbon sequestration mechanism. • Carbon outwelling estimates remain spatially and temporally limited. • It is time to focus not only on local carbon burial, but also exports to the sea. [ABSTRACT FROM AUTHOR]