Submarine groundwater discharge and associated nutrient fluxes into the Southern Yellow Sea: A case study for semi-enclosed and oligotrophic seas-implication for green tide bloom

The biogenic elements concentrations in a coastal bay/estuary are strongly influenced not only by riverine input but also by submarine groundwater discharge (SGD) which has been identified as a typical process of land/ocean interactions in coastal zones. To assess the role of SGD in nutrient fluxes in the Southern Yellow Sea (SYS), ²²⁸Ra activities were measured in seawater collected in May 2014. Analyzing the sources and sinks of ²²⁸Ra, the flux of excess ²²⁸Ra through SGD was estimated to be (2.2 ± 1.0) ×10¹⁵ dpm yr⁻¹. Based on the ²²⁸Ra mass balance model, we estimated the average SGD flux to be approximately (1.3 ± 0.87) ×10¹² m³ yr⁻¹ over the entire SYS, which is at least 3.3 times the estimated annual delivery from the Changjiang River into the SYS (∼1.3 × 10¹¹ m³ yr⁻¹). The SGD-derived biogenic elements loads (dissolved inorganic nitrogen (DIN), phosphorus (DIP) and silicon (DSi)) were estimated as (487 ± 384) × 10⁹ mol yr⁻¹, (2.8 ± 2.2) × 10⁹ mol yr⁻¹, and (313 ± 259) × 10⁹ mol yr⁻¹, respectively, which are approximately 18 times, 7 times and 13 times the riverine input from both mainland China and Korea. The accumulation nutrient fluxes derived by SGD may play one of the most important roles in the green tide bloom that originated from the Subei Shoal zone in the SYS. Additionally, DIN and DIP via SGD can provide the necessary amounts of nutrient for recovering nutrient concentrations to normal levels after the green tide bloom is terminated.