Coastal hypoxia is a widespread environmental concern, and its increasing occurrence during summer in the Pearl River Estuary (PRE) has attracted considerable attention. However, the potential contribution of submarine groundwater discharge (SGD), an important pathway for land-sea material transport, remains poorly quantified. During a cruise conducted in July 2021, we quantified SGD fluxes in the PRE using a radium mass-balance model and evaluated two pathways through which SGD may reduce bottom-water dissolved oxygen (DO): direct dilution by oxygen-depleted groundwater and oxygen consumption associated with the remineralization of SGD-derived dissolved organic carbon (DOC). Low-oxygen bottom waters occurred primarily downstream of the Modaomen and Huangmaohai subestuaries, with DO concentrations ranging from 1.24 to 2.92 mg L^-1. The inverse relationship between DO concentrations and radium activities, together with the spatial correspondence between low-oxygen waters and elevated SGD fluxes, suggests that SGD may contribute to the development and intensification of summer hypoxia in the PRE. On average, both SGD fluxes and dilution-derived apparent bottom-water oxygen-deficit fluxes were approximately one order of magnitude higher in the hypoxic zone than in the non-hypoxic zone, reaching 0.45 ± 0.16 versus 0.054 ± 0.13 m³ m^-2 d^-1 and 3150 ± 1120 versus 378 ± 910 mg m^-2 d^-1, respectively. The oxygen-deficit fluxes were estimated assuming an anoxic groundwater endmember. In the hypoxic zone, the dilution-derived oxygen-deficit flux was approximately one order of magnitude greater than the potential oxygen demand associated with the remineralization of SGD-derived DOC, whereas the two pathways were comparable in magnitude in the non-hypoxic zone. When hypoxic and non-hypoxic waters were considered jointly, the mean rate of oxygen depletion attributable to SGD-driven physical dilution across the PRE was higher than the corresponding summer estimates for the Bohai Sea and the Changjiang Estuary, both of which experience seasonal hypoxia. It was also comparable in magnitude to reported water-column oxygen consumption rates in several Chinese coastal hypoxic systems. Because the oxygen demand associated with SGD-borne ammonium and other reduced solutes was not quantified, the overall influence of SGD on bottom-water oxygen depletion may have been underestimated. These results highlight the potential importance of SGD in regulating the bottom-water oxygen budget of the PRE and indicate that both the direct input of oxygen-depleted groundwater and the remineralization of SGD-derived DOC may contribute to summer hypoxia in Chinese coastal waters.
 

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