The negative relationship between δ13CPOM and salinity or nitrate concentration has been previously reported in Kuroshio and western North Pacific boundary currents (Kodama et al., 2021). The negative relationship between salinity in the Kuroshio area was considered to be a mixture of POM formed in the estuary (Kodama et al., 2021), where phytoplankton bloom occurs and then high δ13CPOM POM are formed (Savoye et al., 2003; Ogawa and Ogura, 1997). Additionally, the elevated δ13CPOM may be due to sediment resuspension in the Changjiang estuary (Gao et al., 2014). Although our samples were occasionally obtained near Japanese coastal regions, the less saline water of this sea is mostly due to the influence of the ECS (Kosugi et al., 2021). In the ECS, δ13CPOM is negatively associated with salinity (Ho et al., 2021), and high δ13CPOM (> –20‰) is detected in the Changjiang diluted waters (Gao et al., 2014). However, Gao et al. (2014) did not report the hydrographic characteristics in the high δ13CPOM area, and thus, the relationship between salinity and δ13CPOM was unknown. The direct influence of terrestrial organic matter from China may be ignored: the δ13CPOM in the Changjiang river is < –25‰ (Gao et al., 2014), while the contribution of terrestrial organic matter is <10% 500 km away from the Changjiang estuary (Wu et al., 2003). The effects of Japanese local rivers cannot be ignored, where river-originated δ13CPOM is ~ –24‰ and lower than the ocean-originated δ13CPOM (Antonio et al., 2012), and thus, the direct influence of terrestrial organic matter is deemed to be restricted in the SOJ based on the relationship between salinity and δ13CPOM. These results suggest that POM with high-δ13CPOM from the less saline waters of the ECS is transported into the SOJ and influences the spatiotemporal variation of δ13CPOM in the SOJ, particularly during the summer, when the Changjiang-origin freshwater inputs to the SOJ via the Tsushima Strati are the highest among the seasons (Morimoto et al., 2009). However, in the instance of the Delaware Estuary in the United States, high seasonality of δ13CPOM (–17‰ in spring while –32‰ in summer) was reported (Fogel et al., 1992), and it was a possibility that POM with low-δ13CPOM was formed in both the Changjiang estuary and Delaware Estuary. Carbon assimilation and decomposition processes in the less-saline waters, which were not observed in this study, must thus be investigated in the future. We could not come up with a plausible explanation for the negative relationship between latitude and nitrate concentration.