Carbon export from submarine groundwater discharge in a semi-enclosed bay: Impact for the buffering capacity against coastal ocean acidification

Submarine groundwater discharge (SGD) serves as an important pathway for the transport of dissolved carbon from land to ocean, significantly affecting the coastal biogeochemical cycles. However, the impact of SGD-derived dissolved carbon on the coastal carbon budget remains poorly understood. This study first quantified SGD and associated dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total alkalinity (TA) fluxes in Daya Bay using mass balance models based on radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra). We then constructed carbon mass balance models to evaluate the impact of SGD-derived carbon on the buffering capacity against coastal ocean acidification. The estimated SGD fluxes ranged from 0.80 × 10⁷ to 2.64 × 10⁷ m³ d⁻¹. The DIC, DOC and TA fluxes from SGD were 17.90–36.44 mmol m⁻² d⁻¹, 0.93–2.13 mmol m⁻² d⁻¹, and 21.19–28.47 mmol m⁻² d⁻¹, respectively. Based on carbon mass balances, the DIC flux from SGD was 19–39 times the riverine input, accounting for 27.16 % ∼ 37.64 % of the total carbon source. These results suggest that SGD is a major contributor to DIC, significantly affecting the coastal carbon budget. Furthermore, the average TA:DIC ratio of groundwater discharging into Daya Bay was approximately 1.13. High TA exports enhance the buffering capacity of the coastal ocean and contribute bicarbonate to the ocean, playing a significant role in the ocean carbon sequestration process. This study demonstrates the importance of SGD-derived dissolved carbon in the assessment of coastal carbon budgets.