Yearly Variations in Nutrient Supply in the East China Sea Due To the Zhejiang Coastal Upwelling and Kuroshio Intrusion

The East China Sea (ECS) is a marginal sea with high productivity, especially in summer. In the Zhejiang coastal region of ECS, the large 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 continental ECS. However, the yearly variations in these two processes are not consistent, which complicates coastal nutrient supplies. In this study, to understand the contributions of ZCU and KI to the nutrient supplies, nutrient concentrations, and phytoplankton biomass in ECS as well as their yearly variations, a physical-biochemical coupled model was applied. According to model results and observations, the years 2013 and 2018 were identified as strong and weak ZCU years during 2010-2018 period, respectively. In 2013, nutrient and chlorophyll-a concentrations were higher than 2018 in nearshore 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 ECS in summer.