Radium-traced nutrient outwelling from the Subei Shoal to the Yellow Sea: Fluxes and environmental implication

The Subei Shoal is the largest sandy ridge in the southern Yellow Sea and is important source for nutrient loading to the sea. Here, the nutrient fluxes in the Subei Shoal associated with eddy diffusion and submarine groundwater discharge (SGD) were assessed to understand their impacts on the nutrient budget in the Yellow Sea. Based on the analysis of ²²³Ra and ²²⁴Ra in the field observation, the offshore eddy diffusivity mixing coefficient and SGD were estimated to be 2.3×10⁸ cm²/s and 2.6×10⁹ m³/d (16 cm/d), respectively, in the Subei Shoal. Combined the significant offshore decreasing gradients of nutrient in seawater of the Subei Shoal, the spatially integrated nutrient outwelling fluxes to the Yellow Sea were 262–1 465 µmol/(m²·d) for DIN, 5.2–21 µmol/(m²·d) for DIP and 711–913 µmol/(m²·d) for DSi. Compared to the riverine input, atmospheric deposition and mariculture, nutrient outwelling from the Subei Shoal might play an important role in nutrient budget of the Yellow Sea. These nutrient fluxes could provide 4.1%–23% N and 1.3%–5.3% P requirements for the primary productivity, and the deviated DIN/DIP ratios have the potential to affect the growth of phytoplankton in the marine ecosystem of the Yellow Sea.