Submarine groundwater discharge as a major driver of coastal carbon fluxes

Submarine groundwater discharge (SGD) is a major yet often overlooked driver of coastal carbon cycling. We examined two contrasting coastal basins: a bay dominated by karstic groundwater discharge and a fjord influenced by riverine inputs in the west coast of Ireland. Radioisotope-based (Ra and Rn) models and seasonal measurements were used to quantify water and carbon fluxes. We hypothesize that high-alkalinity groundwater from karstic aquifers delivers elevated dissolved carbon (DIC and DOC) and modulates air-sea CO₂ exchange and carbon outwelling to the ocean. We further assess whether groundwater inputs act to buffer or intensify coastal acidification along the land–ocean continuum, providing new insights into how groundwater chemistry regulates carbonate equilibria and CO₂ dynamics in contrasting coastal environments. Groundwater discharge was about 30% lower than river discharge yet contributed ~34 times more DIC (899 ± 453 vs 26 ± 22 mmol m² d^-1) and similar DOC fluxes (62 ± 31 vs 65 ± 51 mmol m² d^-1, respectively) to the coastal basins. Rivers (TA/DIC = 0.5 ± 0.2) showed a stronger acidifying effect than groundwater (TA/DIC = 0.9 ± 0.1) due to the strong buffering capacity of karst aquifers derived from carbonate dissolution. Bicarbonate outwelling from the coastal basins to the ocean and CO₂ emissions to the atmosphere from the SGD influenced bay (155 ± 63 and 155 ± 121 mmol m² d^-1, respectively) exceeded those from the river fed fjord (87 ± 109 and 67 ± 33 mmol m² d^-1), highlighting the disproportionate role of groundwater-derived alkalinity in regulating carbon fluxes across the land-ocean interface.