Two recent studies have indicated that the carbon and nitrogen stable isotope ratio of POM varies in the western North Pacific marginal seas (Ho et al., 2021; Kodama et al., 2021). The δ13C of POM increased as the phytoplankton abundance increased in the southwestern ECS, which is consistent with isotope fractionation that occurs during photosynthesis (Ho et al., 2021). The δ15N of this area decreased with freshwater intake and coastal upwelling in the summer, and was negatively correlated with nitrate concentration (Ho et al., 2021). Gao et al. (2014) reported the horizontal distribution of δ13C and δ15N, and that δ13C largely varies in this sea. Active nitrogen fixation decreases the δ15N of POM in offshore waters in the Kuroshio area during the summer, whereas the δ15N of POM remains high in the coastal waters (Kodama et al., 2021).