Phytoplankton Community Patterns in the Northeastern South China Sea: Implications of Intensified Kuroshio Intrusion During the 2015/16 El Nino

Kuroshio intrusion into the South China Sea (SCS) varies seasonally and interannually under different climatic conditions. To characterize the intensity of the climate-driven Kuroshio intrusion and its impact on the phytoplankton community in the northeastern South China Sea (NESCS), two cruises were conducted during the 2015/16 El Nino event. In situ observations and remote sensing data showed that the Kuroshio strongly intruded into the SCS in both summer and winter. The summer phytoplankton community was dominated by cyanobacteria (Trichodesmium erythraeum), associated with high surface temperature, shallower mixed layer depth, and low nutrient concentrations. The winter phytoplankton community was mainly dominated by cosmopolitan diatoms (Thalassiosira diporocyclus, Thalassionema nitzschioides, and Chaetoceros socialis), associated with low surface temperature, deeper mixed layers, and relatively higher nutrient concentrations. Meanwhile, the elevated abundance of T. erythraeum coupled well with the Kuroshio intrusion index during the El Nino summer. The phytoplankton network was unstable in summer but not in winter, induced by the Kuroshio intrusion. Our findings show that intensified Kuroshio intrusion during the El Nino event was a vital driving force for reshaping phytoplankton community dynamics of the NESCS through lateral advection and affecting temperature, mixed layer depth, and nutrient supply. Plain Language Summary In this study, we report the impacts of intensified Kuroshio intrusion on the phytoplankton community of the northeastern South China Sea during the 2015/16 El Nino event. We investigate the intensity of the climate-driven Kuroshio intrusion and its impact on the phytoplankton community. The phytoplankton community exhibited apparent seasonal variations, dominated by cyanobacteria and diatoms in summer and winter, respectively. The intensified Kuroshio intrusion contributed to higher abundance of cyanobacteria Trichodesmium erythraeum during the El Nino summer. In addition, Kuroshio intrusion significantly reduced phytoplankton network stability in summer but not in winter.