Assessment of submarine groundwater discharge (SGD) and associated nutrient subsidies to Xiangshan Bay (China), an aquaculture area

Submarine groundwater discharge (SGD) is a predominant component of chemical fluxes in the solute budgets in coastal ecosystems because of its large flux and the corresponding concentrated constituents. A quantitative study regarding the mass balance model of ²²⁶Ra and ²²⁸Ra was performed to assess SGD and associated nutrient inputs in Xiangshan Bay (XSB), a typical Chinese aquaculture bay. Based on these mass balance models, the SGD rates in the XSB Channel were 8.9–13.2 cm d⁻¹. The SGD rates in the three corresponding embayments were 3.5–3.7 cm d⁻¹ in Embayment I, 5.6–5.8 cm d⁻¹ in Embayment II, and 4.5–5.8 cm d⁻¹ in Embayment III. Significant spatial variabilities in SGD rates were observed in the different XSB districts, reflecting that ocean dynamics and anthropogenic activities play significant roles at different scales. SGD-derived nutrient fluxes in XSB were the dominant sources of nutrient loading, and their magnitudes were approximately 1–2 orders larger than those of riverine input. SGD rates in the XSB Channel were approximately three times higher than those in the three embayments, but the SGD-derived nutrient rates in the XSB Channel were comparable with those in the three embayments, even significantly lower than that for NO₃-N in the inner regions of the embayment. These results imply that aquaculture activities and urbanization jointly influence SGD. When combined with the results of other studies, our observations demonstrate the significant comprehensive effects of multiple factors on SGD, a critical but overlooked nutrient source.