Effects of the Kuroshio Extension jet and pinch-off ring on epipelagic mesozooplankton

The eastward flow of the Kuroshio Extension (KE) is accompanied by intense eddy activities at both the mesoscale and submesoscale levels, exerting a substantial effect on biogeochemical processes in the upper ocean. The present study focused on how the KE jet and pinch-off cyclonic eddy influenced mesozooplankton community structure and functional traits in the northwestern Pacific. The zooplankton communities in the investigated regions formed a four-segment pattern from north to south, with significant differences in the species and functional trait compositions of each group. The KE stream acted as a barrier that prevented water exchange between the Kuroshio-Oyashio mixed water (KOMW) and North Pacific Subtropical Gyre (NPSG) regions. A comparatively higher proportion of current–feeding omnivore–herbivore zooplankton was recorded in the NPSG region when compared to the KOMW and KE regions, which can be attributed to the influence of the pinch-off cyclonic eddy. Red Noctiluca scintillans was carried from the coast of Japan to open water by the KE’s eastward advection, and it bloomed (1.2 × 10⁴ ind·m⁻³) in the KOMW under favorable hydrological and nutritional environments, changing the community structure of zooplankton. The pinch-off cyclonic eddy could increase zooplankton abundance but had no significant impact on species composition and functional traits. In addition, the divergence effect of the eddy resulted in greater zooplankton abundance at its edges than in the center. This study enhanced our knowledge of the impacts of the KE on zooplankton communities and has significant implications for understanding pelagic plankton and nutrient responses to pinch-off mesoscale eddies.