Drivers of variability in Blue Carbon stocks and burial rates across European estuarine habitats

The implementation of climate change mitigation strategies based on the conservation and restoration of Blue Carbon ecosystems requires a deep understanding of themagnitude and variability in organic carbon (Corg) storage across and within these ecosystems. This study explored the variability in soil Corg stocks and burial rates across and within intertidal estuarine habitats of the Atlantic European coast and its relation to biotic and abiotic drivers. A total of 136 soil cores were collected across saltmarshes located at different tidal zones (high marsh, N = 45; low marsh, N = 30), seagrass meadows (N=17) and tidal flats (N=44), and from the inner to the outer sections of five estuaries characterized by different basin land uses. Soil Corg stocks were higher in high-marsh communities (65±3 Mg ha-1) than in low-marsh communities (38 ±3 Mg ha-1), seagrass meadows (40 ± 5 Mg ha-1) and unvegetated tidal flats (46± 3Mg ha-1) whereas Corg burial rates also tended to be higher in highmarshes (62±13 g m-2 y-1) compared to low marshes (43 ± 15 g m-2 y-1) and tidal flats (35 ± 9 g m-2 y-1). Soil Corg stocks and burial rates decreased from inner to outer estuarine sections in most estuaries reflecting the decrease in the river influence towards the estuary mouth. Higher soil Corg stocks were related to higher content of silt and clay and higher proportion of forest and natural land within the river basin, pointing at new opportunities for protecting coastal natural carbon sinks based on the conservation and restoration of upland ecosystems. Our study contributes to the global inventory of Blue Carbon by adding data from unexplored regions and habitats in Europe, and by identifying drivers of variability across and within estuaries. The authors acknowledge the financial support fromthe Government of Cantabria through the “Fénix Programme”. IM was supported by a Juan de la Cierva Incorporación postdoctoral fellowship of the Spanish Ministry of Science, Innovation and Universities (JC2020-045917-I).OS was supported by I+D+i projects RYC2019-027073-I and PIE HOLOCENO 20213AT014 funded by the Spanish Ministry of Science, Innovation and Universities (MCIN/AEI/10.13039/501100011033) and FEDER. The IAEA is grateful for the support provided to its Marine Environment Laboratories by the Government of the Principality of Monaco. PM was supported by the Australian Research Council. J.G.O. acknowledges the financial support of the Spanish Ministry of Science, Innovation and Universities, through the “Maria de Maeztu” program for Units of Excellence (CEX2019-000940-M) and the Generalitat de Catalunya (MERS; 2017 SGR–1588). This research was carried out with the contribution of the LIFE Programme of the European Union to the Project ADAPTA BLUES (ref. LIFE18 CCA/ES/001160). This document reflects only the authors' view and the Agency/Commission is not responsible for any use that may be made of the information it contains.