Field observations and satellite data consistently showed elevating Chlorophyll a (Chl a) concentration around the continental slope of the East China Sea, associated with increasing dissolved inorganic nitrogen (DIN) input in the subsurface waters. The accumulation of DIN in the upper mesopelagic waters was observed, but the relationship between the simultaneously increasing of both Chl a and DIN remains unclear. A possible mechanism came out that the upper mesopelagic waters upwell to the bottom of the continental shelf, and then contribute to the subsurface Chl a by strong vertical mixing in winter. By analyzing the strengths of upwelling and the mixing layer depths at the PN-line transect from 1997 to 2019, we confirmed that the DIN from the 200 to 500 m was probably the main driver on the elevating Chl a in the subsurface waters around the continental slope. Other processes such as atmospheric deposition, biological N₂ fixation, continental shelf pump and upstream Kuroshio transport contributed little to the primary production. These findings help us better understand the intricate dynamics of upwelling systems along continental margins and shed light on their pivotal role in sustaining and enhancing productivity levels.
 

upwelling,enhancing Chl a,nutrients,vertical mixing,East China Sea