Based on historical data and previous researches, significant concentration variations of nutrients (i.e., NO^-₃, SiO^2-₃, and PO^3-₄), in the western Yellow Sea and the East China Sea and during the recent 40～50 years, were studied, in which [NO^-₃] showed a rapid increase whereas [SiO^2-₃] and [PO^3-₄] displayed considerable decreases. The increase of [NO^-₃] and decrease of [SiO^2-₃] were consistent with their concentration changes in the Changjiang Estuary; however, the [PO^3-₄] decrease was obscure in the Changjiang Estuary and appeared more evidently in the enclosed areas, such as the Bohai Sea and the Yellow Sea. It was speculated that marine primary productivity was enhanced by the increasing NO^-₃ fluxes and as a result, PO^3-₄ uptakes by phytoplankton were greatly simulated, which led to the observed [PO^3-₄] decrease in the study area. According to the above assumption, a simple box model was established, through which the required increase rate of N^-₃ outputs was calculated to satisfy the [PO^3-₄] decrease. The calculation results showed that, if the increase of nitrogen flux was completely ascribed to NO^-₃ transports via rivers, the increase of average NO^-₃ concentrations in these rivers was at least 7.8 μmol/L during the recent several decades.