A bloom of a new picoplankter species Aureococcus anophagefferens occurred in several estuaries in the northeast coast of the U.S.A. in 1985. The phenomenon is believed to be the result of a complex chain of physical-chemical-biological factors. Based on

The features of 228Ra and 226Ra distribution covering the salinity range from less than 0•1% to 35% in the Ota River estuary and extending to the Seto Inland Sea have been presented. Using this data and a mass balance of radium in this region, the source of radium from the river-estuarine sediment is shown to be more important than that from the sediments themselves of the Seto Inland Sea. The flux of Ra from the sediments is 0•84 dpm 228Ra cm-2 year-1 and 0•08 dpm 226Ra cm-2 year-1 about 17 times higher than those from river for 228Ra and 8 times for 226 Ra. By applying the One-Dimensional Advection-Diffusion model and assuming a steady state of radium in the Seto Inland Sea, the appropriate value of the horizontal eddy diffusion coefficient is in the range 1•1 × 106 - 1•4 × 106 cm2 s-1.