Groundwater carbon exports dominate dissolved carbon budgets in a seagrass meadow karstic bay (Adriatic Sea, Croatia)

Coastal wetlands, encompassing saltmarshes, mangroves and seagrass meadows in carbon-enriched bays serve as crucial hotspots for carbon cycling. While submarine groundwater discharge (SGD) is recognized as important pathway for land-to-ocean dissolved carbon transport, its functional role in seagrass meadow ecosystems remains poorly constrained, representing a key knowledge gap in coastal carbon budgeting. This study employed a radium-quartet mass balance model to quantify SGD flux in Morinje Bay, a representative Posidonia oceanica meadow embayment along the Adriatic Sea in Croatia. The derived SGD flux ranged from 1.3 to 3.7 cm d^-1, consistent with the Mediterranean median value of 2.5 cm d^-1. Our first-hand evaluation revealed that SGD carbon exports from the seagrass meadow bay explained the majority of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), with contributions of 62 % and 42 % of total sources, respectively. Notably, the SGD-driven DIC flux (132 mmol m^-2 d^-1) exceeded DOC (22 mmol m^-2 d^-1) by 5.9-fold, surpassing integrated carbon burial and outwelling fluxes. These findings underscored dual role of SGD as both carbon source and effective retention mechanism within the seagrass meadow bay ecosystems. We therefore propose that robust assessments of coastal blue carbon sequestration capacity must integrate SGD-mediated carbon fluxes.