The characteristics of the nutrient budget in the broad East China Sea (ECS) shelf are jointly influenced by nutrient fluxes from the Changjiang River (CJR) discharge on its shoreside, Kuroshio Current intrusion on its seaside, and throughflow of Taiwan Warm Current (TWWC) as well as cycles among organic and inorganic substances. Understanding the spatiotemporal variable nutrient dynamics under the control of the external exchange and internal biogeochemical processes is crucial to quantifying the nutrient budget and biogeochemical response over the ECS. In this study, we used the coupled physical-chemical-biological model, CMOMS (China Sea Multi-scale Ocean Modelling System, https://odmp.hkust.edu.hk/cmoms/) to investigate the nutrient budget and underlying dynamics in the ECS shelf. We found that physical processes control the nutrient concentration and its 3D distribution over the shelf. Specifically, the upsloping nutrient flux from Kuroshio intrusion supplies the nutrient-rich waters from the depths below 200 m due to topographically induced geostrophic cross-isobath transport and bottom Ekman transport. Unlike the previous studies that regarded the intrusion as the primary nutrient source, only about 11% and 5.4% of the intruded NO₃ and PO₄ fluxes, respectively, reach the ECS with depths less than 100 m isobath. The northward TWWC plays a more critical role in nutrient redistribution than nutrient contribution in the ECS. Over the entire shelf, NO₃ regenerated through nitrification played a crucial role for phytoplankton growth. In addition, blooms under favorable temperature quickly absorbed nutrients discharged from the CJR in the nearshore, suggesting that the CJR has less effect on the offshore region. In winter, surface nutrient concentration peaked offshore due to the cold-water limitation and strong mixing inhibition on the phytoplankton growth. Overall, the nutrient budget in the ECS was regulated by the hydrodynamics-inducing extrinsic fluxes from adjacent seas and dynamics of intrinsic biogeochemical cycling.
 

nutrient budget, nutrient dynamics, East China Sea