The Kuroshio intrusion from the Luzon Strait significantly affects ecosystems in the South China Sea (SCS), especially during the Northeast Monsoon, a time when field observations are notably sparse and where vertical mixing induced by strong winds can obscure the effects of the Kuroshio intrusion. In this study, we address these gaps by reanalyzing data from 20 cruises (5338 samples) in the SCS between 2004 and 2015. We also carried out two dedicated field cruises during the Northeast and the Southwest Monsoon in 2018. Field observations from both cruises revealed a consistent unimodal relationship between total chlorophyll a (Chla) concentrations in the upper 50 m of the water column and the index of the Kuroshio intrusion. Specifically, we observed that a stronger Kuroshio intrusion during the Northeast Monsoon significantly enhanced Chla concentrations. This enhanced Chla concentration during the Northeast Monsoon was primarily driven by increases of Synechococcus and nanophytoplankton that contrasted with the dominance of Prochlorococcus during the Southeast Monsoon. Our analysis of long-term remote sensing data corroborated these findings and demonstrated a consistent pattern wherein intrusion by the Kuroshio led to elevated Chla concentrations, particularly during the Northeast Monsoon. Importantly, we identified a significant positive correlation between the intensity of the Kuroshio intrusion and the magnitude of the Chla increase. However, our findings also suggested a concerning possibility: weakening of the Kuroshio intrusion intensity over time might diminish future biogeochemical effects on SCS ecosystems. Continued monitoring and research will be crucial to understanding and responding to these changes.

Kuroshio intrusion,Chlorophll a,Phytoplankton community,Southern China Sea