It has been documented that the global proliferation of dam construction on the major river has reduced nutrient and sediment loading to coastal environments. As a consequence, dams can impact marine ecological systems by changing nutrient concentrations and ratios in the coastal zone. From 1998--2004, we conducted a high resolution oceanographic investigation of the East China Sea (ECS) before and after the first filling phase (June 2003) of the Three-Gorges Dam (TGD). We found that the Si:N ratio in the River affected region changed from 1.5 in 1998 to 0.4 in 2004 with sediment loading significantly reducing (about 55%) at the Datong station after June 2003. Most importantly, we found that the PP had declined by 86% between 1998 and 2003, both measured during the high flood season. The results suggest that the ECS ecosystem may respond sensitively to changes in the nutrient supply arising from the TGD project.