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

• Advection and diffusion processes were investigated using 226Ra and 228Ra in the East China Sea.
• A one- and two-dimensional advection–diffusion surface model was applied.
• Kx was not sensitive to advection, while Ky is sensitive to advection.
• Benthic Ra flux decreases the eddy diffusivity.
• Nutrient fluxes were calculated using both diffusion and advection.

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 × 105 cm2 s− 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 × 107 cm2 s− 1, which is within the sensitivity range thus reliable. Based on the estimated diffusivity and advection velocities, we calculate the offshore nutrient fluxes (mol m− 2 d− 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 (mol m− 2 d− 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.