LAUSR

Abstract Coastal upwelling can enhance primary production, thus playing an important role in fisheries and global carbon cycle. The high concentration of Chlorophyll a induced by coastal upwelling generally occurs in a confined region. However, an unusual phytoplankton bloom, which lasted more than 16 days in June 2018 with a patch about 90 km wide and 400 km long in the upwelling region east of Hainan Island, was captured by satellite chlorophyll a data. The coastal upwelling provided nutrients, and the horizontal advection induced by the interaction between the western boundary current of the South China Sea (SCS) and warm eddy provided a dynamic background for long-distance transportation of phytoplankton. In addition, the relative abundance of phytoplankton in the coastal upwelling region east of Hainan Island, where salinity change was fairly large due to precipitation, confirmed that sea water with low salinity provided a suitable environment for phytoplankton to maintain their growth. We also find that continuous high-salinity water from enhanced vertical mixing due to strong wind and strong northward horizontal advection transport induced by the western boundary current of the SCS inhibited the growth of phytoplankton and blocked the horizontal transport of phytoplankton. This is the first time phytoplankton bloom triggered by eddy-wind interaction in the eastern Hainan coastal upwelling system is reported, providing a new insight into irregular horizontal transportation of nutrients and understanding spatiotemporal variation of phytoplankton in the upwelling region of the SCS.