Ra Tracer-Based Study of Submarine Groundwater Discharge and Associated Nutrient Fluxes into the Bohai Sea, China: A Highly Human-Affected Marginal Sea

Nutrient concentrations in coastal bays and estuaries are strongly influenced by not only riverine input but also submarine groundwater discharge (SGD). Here we estimate the SGD and the fluxes of the associated dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicon (DSi) into the Bohai Sea based on a ²²⁶Ra and ²²⁸Ra mass balance model. This procedure shows that in the Bohai Sea the average radium activities (dpm 100 L⁻¹) are 42.8 ± 6.3 (²²⁶Ra) and 212 ± 41.7 (²²⁸Ra) for the surface water and 43.0 ± 6.1 (²²⁶Ra) and 216 ± 38.4 (²²⁸Ra) for the near-bottom water. According to the ²²⁸Ra/²²⁶Ra age model, the residence time in the Bohai Sea is calculated to be 1.7 ± 0.8 yrs. The mass balance of ²²⁶Ra and ²²⁸Ra suggests that the yearly SGD flux into the whole Bohai Sea is (2.0 ± 1.3) × 10¹¹ m³ yr⁻¹, of which the percentage of submarine fresh groundwater discharge (SFGD) to the total SGD is approximately (5.1 ± 4.1)%. However, the DIN and DSi fluxes from SFGD constitute 29% and 10%, respectively, of the total fluxes from the SGD. Moreover, nutrient loads, which exhibit high DIN/DIP from SGD, especially the SFGD, may substantially contribute to the nutrient supplies, resulting in the occurrence of red tide in the Bohai Sea.