Nutrient fluxes via radium isotopes from the coast to offshore and from the seafloor to upper waters after the 2009 spring bloom in the Yellow Sea

The horizontal and vertical transport of nutrient-rich water both from the coast and from the seafloor to the overlying water column should play an important role in supplying nutrients required for the periods of vegetative or reproductive growth of phytoplankton. In the present work, radium isotopes (²²³Ra, ²²⁴Ra and ²²⁶Ra) in the southern Yellow Sea were measured after a spring bloom in June 2009. The exponential-like decrease of ²²³Ra away from the coast to offshore waters yielded horizontal eddy diffusivities (K_h) of (2.93±1.47)×10⁷cm²s^-1 by neglecting the advection. This estimate was smaller than that with advection indicator by as much as 21% when using an analytic model for ²²³Ra and ²²⁶Ra. The corresponding horizontal nutrient fluxes were 1525μmolm^-2d^-1 (DIN), 15.9μmolm^-2d^-1 (DIP) and 826μmolm^-2d^-1 (DSi), which would supply around 16% of N and 3% of P requirements based on the primary productivity. The decrease of ²²⁴Ra and ²²³Ra activities from sediments to the upper water column suggests the vertical eddy diffusion coefficient (K_z) of 6.23±5.58cm²s^-1 below the thermocline, which was within the Yellow Sea Cold Water Mass (YSCWM). The calculated vertical fluxes of nutrient were 4945μmolm^-2d^-1 (DIN), 236μmolm^-2d^-1 (DIP) and 5315μmolm^-2d^-1 (DSi), accounting for up to 52% of N and 40% of P requirements for the phytoplankton growth. These results demonstrate the role of YSCWM as a relative nutrient-rich pool for the supply of nutrient to the southern Yellow Sea via the vertical diffusion process relative to the horizontal process during the summer season. Such processes will be strengthened during the weak density stratification in spring when algal blooms occur.