Submarine Groundwater Discharge in the Northern Bohai Sea, China: Implications for Coastal Carbon Budgets and Buffering Capacity.

Submarine groundwater discharge (SGD) has been widely recognized as an important source of dissolved nutrients in coastal waters and affects nutrient biogeochemistry. In contrast, little information is available on SGD impacts on coastal carbon budgets. Here, we assessed the SGD and associated carbon (dissolved inorganic carbon [DIC] and total alkalinity [TA]) fluxes in Liaodong Bay (the largest bay of the Bohai Sea, China) and discussed their border implications for coastal DIC budget and buffering capacity. Based on 223Ra and 228Ra mass balance models, the SGD flux was estimated to be (0.92–1.43) × 109 m3 d−1. SGD was the largest contributor of DIC, accounting for 55%–77% of the total DIC sources. The low ratio (<1) of SGD‐derived TA to DIC fluxes and negative correlation between radium isotopes and pH in seawater implied that SGD would potentially reduce seawater pH in Liaodong Bay. Combining the groundwater carbon data in Liaodong Bay with literature data, we found that the SGD‐derived DIC flux off China was 4–9 times greater than those from rivers. By analyzing the TA/DIC ratios in groundwater along the Chinese coast and related carbon fluxes, SGD was thought to partially reduce the CO2 buffer capacity in receiving seawater. These results obtained at the bay scale and national scale suggest that SGD is a significant component of carbon budget and may play a critical role in modulating coastal buffering capacity and atmospheric CO2 sequestration. Plain Language Summary: Submarine groundwater discharge (SGD) is an important process of land‐sea interaction, but its significance on coastal carbon budgets is still poorly understood. In this study, the SGD and associated carbon flux were quantified based on high‐resolution radium and carbon data in an urbanized bay (Liaodong Bay, China). By reviewing the coastal groundwater carbon data in the existing literature, this study provided a first approximation of SGD‐derived carbon fluxes at the shelf‐scale along the Chinese coastlines. The results suggested that SGD is an important component of dissolved inorganic carbon (DIC) budgets at the bay scale and shelf‐scale. SGD with low total alkalinity/DIC ratio would potentially reduce seawater pH and CO2 buffering capacity in receiving coastal waters. This study highlights the important but easily overlooked implications of SGD for coastal carbon budget and carbonate system. Key Points: Submarine groundwater discharge and associated carbon fluxes were estimated in the largest bay of the Bohai Sea, ChinaSubmarine groundwater discharge was an important component of coastal carbon budgets at the bay scale and shelf‐scaleGroundwater carbon inputs to ocean would modify local seawater pH and partially influence CO2 buffering capacity in receiving coastal waters [ABSTRACT FROM AUTHOR]