It has been well documented that since 1850 the amount of phosphorus as well as nitrogen input to coastal seas around the world has increased by a factor of ten, thus raising concern over eutrophication. Here, evidence is provided to show that despite a large riverine input of nutrients to the East China Sea (ECS), only a small fraction (7% for P and 33% for N) of the external nutrient supply supporting new production is provided by the total riverine input. The major nutrient supply in fact originates from the on-shore advection of the subsurface Kuroshio waters. Whether the ECS will become eutrophicated, therefore, actually depends on the amount of phosphorus supply supported by on-shore advection. This supply, though, will very likely be reduced after the completion of the Three Gorges Dam, and this will lead to a diminished productivity in the ECS. ¿ 2000 American Geophysical Union