Evaluation of surface water mixing and associated nutrient fluxes in the East China Sea using ²²⁶Ra and ²²⁸Ra

Advection and diffusion are recognized as two important processes in the mixing and exchange of coastal waters and associated nutrients. In this study, Ra isotopes (²²⁶Ra and ²²⁸Ra) are surveyed in the East China Sea (ECS) to investigate the advection and diffusion processes. Both one-dimensional (1D) and two-dimensional (2D) advection-diffusion models are applied to estimate the cross-shore and along-shore eddy diffusivities and advection velocities. The advection velocity is basically small in magnitude, suggesting its secondary role in transport. The cross-shore 1D model gives promising results on the diffusivity by 4.93×10⁵cm²s^-1. Sensitivity analysis shows that the cross-shore diffusivity is less sensitive whereas the along-shore diffusivity is quite sensitive to advection velocity. Introducing benthic Ra flux in the model decreases the eddy diffusivity. A quisi-2D method generates an along-shore diffusivity by 2.50×10⁷cm²s^-1, which is within the sensitivity range thus reliable. Based on the estimated diffusivity and advection velocities, we calculate the offshore nutrient fluxes (molm^-2d^-1) by 0.44 for dissolved inorganic nitrogen (DIN), 0.012 for dissolved inorganic phosphorous (DIP) and 0.26 for dissolved inorganic silicates (DSi). The along-shore fluxes (molm^-2d^-1) are 6.44 for DIN, 0.10 for DIP and 2.92 for DSi. Compared with nutrient inputs from other sources, e.g. river, sediments, and ocean, the horizontal mixing-derived nutrient fluxes contribute only <4% N, <1% P and <2% Si to the nutrient requirements for primary productivity in the study area. This study stresses the role of advection and diffusion in the material transport, i.e. nutrients and stoichiometry in the ECS.