Phytoplankton change in the Kuroshio region of the East China Sea associated with the Kuroshio frontal eddy

The lower-trophic level ecosystem in the Kuroshio region of the East China Sea (ECS) during summer generally shows typical subtropical features, i.e., small size pico-phytoplankton such as cyanobacteria dominates in the euphotic layer under the oligotrophic condition due to the stable stratification. It is also known that such stable physical and chemical conditions are often disturbed by the Kuroshio frontal eddy. In this study, we investigated the phytoplankton dynamics associated with the passing of Kuroshio frontal eddies. We conducted comprehensive observations around the Kuroshio region in the ECS in the mid-June, 2012 and 2014. We observed drastic changes in phytoplankton community in the subsurface water where the dominant groups of phytoplankton quickly changed from pico-phytoplankton to micro-phytoplankton. We found cyclonic frontal eddies with 25 km diameter at the 50 m and 75 m depth near the drastic change of phytoplankton community. These drastic changes from the oligotrophic ecosystem to the eutrophic ecosystem were caused by the upwelling of nutrient-rich subsurface water associated with the passing of the cyclonic frontal eddies. Therefore, the lower-trophic level ecosystem in the Kuroshio region of the ECS can quickly adapt to short-term disturbances by frontal eddies.