An estimation of nutrient fluxes via submarine groundwater discharge into the Sanggou Bay-A typical multi-species culture ecosystem in China

Submarine groundwater discharge (SGD) is now recognized as an important process of land/ocean interactions in coastal zones (LOICZ). In this study, the initial results of the SGD-derived nutrient fluxes into Sanggou Bay, Shandong, China, in June 2012 are reported. This bay is a typical multi-species culture ecosystem with aquaculture species, such as kelp, scallops, shrimp, and oysters. Using a ²²⁴Ra/²²⁸Ra activity ratio (AR) apparent age model, the average residence time of water in the bay was estimated to be 5.12days, which was comparable with a flushing time of 6.08days. Based on the non-conservative inventory of ²²⁶Ra and ²²⁸Ra in the water column, the average SGD fluxes into the bay were evaluated to be between 2.59×10⁷ and 3.07×10⁷m³d^-1, and the SGD-derived nutrient fluxes (mol/month) had DIN (dissolved inorganic nitrogen) value between 1.16×10⁸ and 1.38×10⁸, DIP (dissolved inorganic phosphorus) value between 4.17×10⁵ and 4.92×10⁵, and DSi (dissolved silicon) value between 6.33×10⁶ and 7.50×10⁶. Additionally, the riverine nutrient inputs to the bay were an order of magnitude lower than the SGD-derived inputs for the DIN and DIP. To balance the nutrient budget, a fertilizer input of N (approximately 4.76×10⁷mol/month) and P (approximately 5.58×10⁶mol/month) is required for aquaculture activities. The fertilizer is a major fraction of the DIP input compared to the SGD. SGD-driven nutrient fluxes may play an important role for nutrient recycling and are required to support the high density of aquaculture activities in the Sanggou Bay.