Upwelling Regulates Nutrient Supply and Phytoplankton Chlorophyll-a Regime on the East China Sea Shelf During Late Summer

The East China Sea (ECS) is among the most important and highly dynamic shelf seas, where the coupling of physical and biogeochemical processes is distinctive and may play a crucial role in regulating nutrient conditions and the pattern of primary production. Based on field observations during the late summer of 2019, the physical controls of nutrient supply and the subsequent influences on phytoplankton chlorophyll-a (Chl-a) regime were investigated in the ECS. The coastal upwelling originating from the Kuroshio Subsurface Water (KSSW) intrusion serves as a nutrient source on the inner East China Sea (ECS) shelf, and this inshore upwelling combined with the coastal water largely governs the surface Chl-a patterns. The halocline, thermocline and nutricline exhibit a vertically synchronous fluctuating phenomenon, and the coexistence of several subsurface cold rings/patches was significantly observed on the middle-outer ECS shelf, indicating intense vertical advection and the occurrence of upwelling. The subsurface cold rings greatly regulate the vertical hydrodynamics, constituting unique upward pathways of the onshore intruded KSSW on the middle-outer shelf. Nutrients can be upwelled within these cold rings to support primary production, thus partially leading to the spatial variability of subsurface Chl-a maximum in terms of intensity and depth. Our findings would considerably contribute to our understanding of the physical-biogeochemical-ecological processes on the ECS shelf. The East China Sea (ECS) is one of the most important continental shelves in the world. Exploring the physical-biogeochemical processes is of great significance for the in-depth understanding of ecosystem in this shelf sea. Here, specific attention was paid to the physical controls on the nutrient supply and the associated influences on phytoplankton chlorophyll-a (Chl-a) regime in the ECS. We show that the coastal/inshore upwelling originating from the nearshore branch of the Kuroshio Subsurface Water (KSSW) intrusion serves as a nutrient source on the inner ECS shelf. The co-occurrence of several subsurface cold rings clearly indicates the upward transport of the onshore intruded KSSW on the middle-outer ECS shelf. The subsurface cold rings may play a role in modulating the vertical hydrodynamics and related biogeochemical-ecological processes, leading to the upward delivery of nutrients and enhanced Chl-a levels. Our results would provide a sound scientific basis for the studies of biogeochemical and ecological dynamics in the ECS. The coexistence of several subsurface cold rings indicating upwelling on the middle-outer East China Sea shelf was observed Subsurface cold rings and coastal upwelling constituted unique upward pathways of the onshore intruded Kuroshio Subsurface Water Nutrients could be upwelled within the cold rings and potentially modulated the associated biogeochemical-ecological processes