The East China Sea (ECS) is a marginal sea with high productivity, especially in the summer. In the Zhejiang coastal region of the ECS, the large local phytoplankton biomass in summer is attributed to the nutrients pumped up by Zhejiang coastal upwelling (ZCU), and oceanic nutrients supplied by the Kuroshio intrusion (KI) onto the continental ECS. However, the interannual variations in these two processes are not consistent, which complicates coastal nutrient supplies. In this study, to distinguish the contributions of ZCU and KI to the interannual variations of nutrient supplies, nutrient concentrations, and phytoplankton biomass, a physical-biochemical coupled model was applied. According to the model results and observations, the years 2013 and 2018 were identified as strong and weak ZCU years within the 2010–2018 period, respectively. In 2013, nutrient and chlorophyll-a concentrations were higher than 2018 in coastal areas, but nutrient concentrations were lower than 2018 at the bottom of wide offshore areas. Strong wind stress in 2013 induced strong ZCU and shoreward net nutrient flux along the 60-m isobath, which was seaward in 2018. The fewer nutrient supplies from KI in the spring of 2013 decreased the nutrient concentrations at the bottom of offshore areas, but its influence on nearshore nutrient concentrations was not as obvious as that caused by ZCU. Phytoplankton biomass in the nearshore and offshore areas reflected different mechanisms of nutrient supplies. In conclusion, the intensity of ZCU, rather than KI, is the principal factor influencing the coastal primary productivity of the ECS in summer.