Intrusion of Kuroshio Enhances Phytoplankton Biomass and Diversity in the East China Sea

Abstract

The Kuroshio, the North Pacific Ocean's most robust western boundary current, brings a rich infusion of nutrients (especially phosphate) and warm-water species into the East China Sea (ECS), profoundly shaping its environmental conditions and ecological processes. However, the precise impact of Kuroshio intrusion on the composition and distribution of phytoplankton communities remains unclear. We hypothesized that the Kuroshio intrusion alleviates phosphorus limitations on phytoplankton growth and enhances phytoplankton diversity within the ECS. We collected phytoplankton samples and relevant physicochemical data from the ECS across four seasons in 2011. We observed that phytoplankton abundance and chlorophyll a concentration were significantly higher during summer and autumn compared to winter and spring. Notably, elevated phytoplankton biomass was detected in the Zhejiang coastal waters during spring, and along the boundary between Kuroshio Subsurface Water and Changjiang Diluted Water during summer and autumn. The dissolved inorganic phosphorus (DIP) carried by Kuroshio subsurface water mitigated the phosphorus limitation in summer and stimulated the growth of phytoplankton in spring and autumn. Redundancy analysis revealed a strong association between low-salinity species and nutrient levels, while warm-water species appeared to be influenced primarily by temperature. Generalized additive models further elucidated that phytoplankton biomass in coastal region was primarily influenced by nitrogen/phosphorus (N/P) ratio, and silicate/nitrogen (Si/N) ratio, whereas in Kuroshio region, DIP and N/P ratio played more significant roles. In mixed region, phytoplankton biomass was influenced by temperature, Si/N ratio, and stratification. The intrusion of the Kuroshio significantly enhanced phytoplankton diversity (species number, warm-water species number, and Shannon index) in the Kuroshio region compared to the coastal and mixed regions. These findings underscore the substantial influence of Kuroshio intrusion on the spatial and seasonal variations of phytoplankton biomass and community composition within the ECS.