Groundwater‐Derived Carbon Promotes Hypoxia and Acidification in a Large Tropical Estuary

Abstract Submarine groundwater discharge (SGD) derived nutrient inputs have been extensively documented. However, SGD‐derived carbon fluxes remain largely unconstrained, representing a critical gap in most coastal carbon budgets. Here, we resolve SGD and dissolved carbon budgets in the Pearl River Estuary (PRE), the largest estuary in Southern China surrounded by the world's largest urban conglomerate. Broadly‐defined SGD contributes 89%–96% of the dissolved inorganic carbon (DIC) pool (2–4 times riverine inputs) and 20%–70% of the dissolved organic carbon (DOC) fluxes of the PRE. SGD transports DIC exceeding total alkalinity (TAlk) by 2.7–7 times, potentially driving pH decline and acidification of nearshore waters. Groundwater p CO 2 values are 10–36 times higher than estuarine waters. SGD‐derived DOC mineralization can decrease estuary water pH by 0.04–0.16 units and increase CO 2 by 6.0–90.0 μmol L −1 , affecting local coral populations and benthic organisms. SGD also reduces seawater dissolved oxygen (DO) by 12–150 μmol L −1 and fuels the development of hypoxic zones. Overall, SGD regionally intensifies seawater hypoxia and acidification, creating challenging conditions for coral reef survival in an already stressed ecosystem. Our findings demonstrate that SGD should be integrated into carbon budgets and ecological assessments of the land‐ocean continuum.