Submarine Groundwater Discharge on the Western Shelf of the Northern South China Sea Influenced by the Pearl River Plume and Upwelling

Submarine groundwater discharge (SGD) is a crucial material transport pathway across the continent–ocean margins. To evaluate the geochemical impacts of SGD on the western shelf of the northern South China Sea (NSCS), salinity, temperature, and radium were investigated in the summer of 2012. With Ra box models, the flux of SGD was estimated to be (1.0 ± 0.44) × 10 8 m 3 day −1 (3.0 ± 1.3 cm day −1 ) in the upwelling‐influenced area and (5.4 ± 2.3) × 10 8 m 3 day −1 (1.8 ± 0.8 cm day −1 ) in the no‐upwelling area. The flux of SGD in the no‐upwelling area estimated using a three end‐member mixing model was (2.1 ± 1.7) × 10 8 m 3 day −1 (0.7 ± 0.5 cm day −1 ), consistent with the flux estimates above. In the upwelling‐influenced area, the average net fluxes of dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP), dissolved silicate (DSi), dissolved carbon (including dissolved inorganic carbon, DIC, and organic carbon), and total alkalinity (TA) via SGD were 1–27 times greater than those from the upwelling. In the no‐upwelling area, the average SGD‐associated DIN, SRP, DSi, TA, and dissolved carbon fluxes were 22%–42% equivalent to the Pearl River estuarine exports. The net SRP flux from SGD could support 11%–13% of the new production on the shelf. The concentration of DIC on the shelf could increase by about 50 μmol L −1 due to the net DIC fluxes via SGD. These results indicate that SGD is a significant nutrient and carbon source more important than the upwelling and second to the Pearl River on the western shelf of the NSCS and needs to be considered in the shelf biogeochemistry.