Estuarine, Coastal and Shelf Science
School of Science / Centre for Marine Ecosystems Research
Swedish Research Council (2020–00457) / I + D + i projects RYC2019-027073-I / PIE HOLOCENO 20213AT014 - MCIN/AEI/10.13039/501100011033 and FEDER / Programme for Requalification of the Spanish University System 2021–2023 (Ministerio de Universidades) / CESAM (UIDB/50017/2020, UIDP/50017/2020, and LA/P/0094/2020; FCT/MCTES)
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 CO2 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 CO2 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.
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