Interannual variations in nutrient supply in the East China Sea
influenced by the Zhejiang coastal upwelling and Kuroshio intrusion
Abstract
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.