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11 páginas, 3 tablas, 6 figuras ; There has been growing interest in quantifying the capacity of seagrass ecosystems to act as carbon sinks as a natural way of offsetting anthropogenic carbon emissions to the atmosphere. However, most of the efforts have focused on the particulate organic carbon (POC) stocks and accumulation rates and ignored the particulate inorganic carbon (PIC) fraction, despite important carbonate pools associated with calcifying organisms inhabiting the meadows, such as epiphytes and benthic invertebrates, and despite the relevance that carbonate precipitation and dissolution processes have in the global carbon cycle. This study offers the first assessment of the global PIC stocks in seagrass sediments using a synthesis of published and unpublished data on sediment carbonate concentration from 403 vegetated and 34 adjacent un-vegetated sites. PIC stocks in the top 1m of sediment ranged between 3 and 1660MgPIC ha1, with an average of 654 24MgPIC ha1, exceeding those of POC reported in previous studies by about a factor of 5. Sedimentary carbonate stocks varied across seagrass communities, with meadows dominated by Halodule, Thalassia or Cymodocea supporting the highest PIC stocks, and tended to decrease polewards at a rate of 8 2MgPIC ha1 per degree of latitude (general linear model, GLM; p < 0:0003). Using PIC concentrations and estimates of sediment accretion in seagrass meadows, the mean PIC accumulation rate in seagrass sediments is found to be 126.3 31.05 g PICm2 yr1. Based on the global extent of seagrass meadows (177 000 to 600 000 km2), these ecosystems globally store between 11 and 39 Pg of PIC in the top metre of sediment and accumulate between 22 and 75 Tg PIC yr1, representing a significant contribution to the carbonate dynamics of coastal areas. Despite the fact that these high rates of carbonate accumulation imply CO2 emissions from precipitation, seagrass meadows are still strong CO2 sinks as demonstrated by the comparison of carbon (PIC and POC) stocks between vegetated and adjacent un-vegetated sediments. ; This study was funded by the EU FP7 project Opera (contract no. 308393), the project EstresX funded by the Spanish Ministry of Economy and Competitiveness (contract no. CTM2012-32603), the CSIRO Marine and Coastal Carbon Biogeochemistry Cluster and the Danish Environmental Protection Agency within the Danish Cooperation for Environment in the Arctic (DANCEA). I. Mazarrasa was supported by a PhD scholarship of the Government of the Balearic Islands (Spain) and The European Social Founding (ESF), and N. Marbà was partially supported by a Gledden visiting fellowship of The Institute of Advanced Studies UWA ; Peer reviewed

The term Blue Carbon (BC) was first coined a decade ago to describe the disproportionately large contribution of coastal vegetated ecosystems to global carbon sequestration. The role of BC in climate change mitigation and adaptation has now reached international prominence. To help prioritise future research, we assembled leading experts in the field to agree upon the top-ten pending questions in BC science. Understanding how climate change affects carbon accumulation in mature BC ecosystems and during their restoration was a high priority. Controversial questions included the role of carbonate and macroalgae in BC cycling, and the degree to which greenhouse gases are released following disturbance of BC ecosystems. Scientists seek improved precision of the extent of BC ecosystems; techniques to determine BC provenance; understanding of the factors that influence sequestration in BC ecosystems, with the corresponding value of BC; and the management actions that are effective in enhancing this value. Overall this overview provides a comprehensive road map for the coming decades on future research in BC science. ; P.I.M. and C.E.L. were supported by an Australian Research Council Linkage Project (LP160100242). C.M.D. was supported by baseline funding from King Abdullah University of Science and Technology. T.K. and K.W. were supported by JSPS KAKENHI (18H04156) and the Environment Research and Technology Development Fund (S-14) of the Ministry of the Environment, Japan. B.D.E. was supported by Australian Research Council grants DP160100248 and LP150100519. D.A.S. was supported by the UK Natural Environment Research Council (NE/K008439/1), and D.K.J. was supported by the CARMA project (8021-00222B), funded by the Independent Research Fund Denmark. Funding was provided to P.M. by the Generalitat de Catalunya (MERS, 2017SGR 1588) and an Australian Research Council LIEF Project (LE170100219). This work is contributing to the ICTA 'Unit of Excellence' (MinECo, MDM2015-0552). O.S. was supported by an ARC DECRA (DE170101524). N.M. was supported by the Spanish Ministry of Economy, Industry and Competitiveness (MedShift project). N.B. was supported by the UK Research Councils under Natural Environment Research Council award NE/N013573/1. J.W.F. was supported by the US National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Grant No. DEB-1237517. R.S. had the support of FCT, project FCT UID/MAR/00350/2018. I.E.H. was supported by Ramon y Cajal Fellowship RYC2014-14970, co-funded by the Conselleria d'Innovació, Recerca i Turisme of the Balearic Government and the Spanish Ministry of Economy, Industry and Competitiveness. The University of Dundee is a registered Scottish charity, no. 015096. J.P.M. was supported by the Smithsonian Institution and the National Science Foundation Long-Term Research in Environmental Biology Program (DEB-0950080, DEB-1457100, DEB-1557009).

An amendment to this paper has been published and can be accessed via a link at the top of the paper. ; P.I.M. and C.E.L. were supported by an Australian Research Council Linkage Project (LP160100242). C.M.D. was supported by baseline funding from King Abdullah University of Science and Technology. T.K. and K.W. were supported by JSPS KAKENHI (18H04156) and the Environment Research and Technology Development Fund (S-14) of the Ministry of the Environment, Japan. B.D.E. was supported by Australian Research Council grants DP160100248 and LP150100519. D.A.S. was supported by the UK Natural Environment Research Council (NE/K008439/1), and D.K.J. was supported by the CARMA project (8021-00222B), funded by the Independent Research Fund Denmark. Funding was provided to P.M. by the Generalitat de Catalunya (MERS, 2017SGR 1588) and an Australian Research Council LIEF Project (LE170100219). This work is contributing to the ICTA 'Unit of Excellence' (MinECo, MDM2015-0552). O.S. was supported by an ARC DECRA (DE170101524). N.M. was supported by the Spanish Ministry of Economy, Industry and Competitiveness (MedShift project). N.B. was supported by the UK Research Councils under Natural Environment Research Council award NE/N013573/1. J.W.F. was supported by the US National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Grant No. DEB-1237517. R.S. had the support of FCT, project FCT UID/MAR/00350/2018. I.E.H. was supported by Ramon y Cajal Fellowship RYC2014-14970, co-funded by the Conselleria d'Innovació, Recerca i Turisme of the Balearic Government and the Spanish Ministry of Economy, Industry and Competitiveness. The University of Dundee is a registered Scottish charity, no. 015096. J.P.M. was supported by the Smithsonian Institution and the National Science Foundation Long-Term Research in Environmental Biology Program (DEB-0950080, DEB-1457100, DEB-1557009).