The carbon and nitrogen isotope ratios of POM are thought to be more changeable in the SOJ than in the other western North Pacific marginal seas. In the case of carbon, Kosugi et al. (2016) reported that the partial pressure of CO2 (pCO2) on the surface of the central and eastern parts of the SOJ (312–329 µatm) differs from that in the northwestern region (360–380 µatm). Furthermore, the carbon:nitrogen ratio (C:N ratio) of POM in the surface water of the northern ECS is extremely high (>40:1) near Japan (Gao et al., 2014), and this organic carbon-rich water may influence the carbon dynamics of the SOJ. In the case of nitrogen, primary production in the northeastern ECS is supported by a variety of nitrogen sources, such as atmospheric deposition, Changjiang River discharge, and Kuroshio waters with varying δ15N (Umezawa et al., 2021; Umezawa et al., 2014). As the decrease in phosphate concentration in the SOJ is mainly observed with an increase in the nitrogen supply in the ECS (Kodama et al., 2016; Kim et al., 2013), advection of nitrate and POM produced in the northeastern ECS is likely to influence the δ15Ns in the SOJ. Previous nutrient dynamics studies have revealed that the supplied nitrate in the surface productive layer in the SOJ originates from the Kuroshio, regeneration at the bottom of the ECS, the Changjiang diluted waters, and the atmosphere in the summer (Kim et al., 2011; Kodama et al., 2017; Kodama et al., 2015). In addition, deep mixing occurs in the SOJ in winter (Ohishi et al., 2019); thus, deep-seawater-originated nitrate contributes to the primary production as well. Although their contributions to the plankton community are limited, diazotrophs are present during the summer (Hashimoto et al., 2012; Sato et al., 2021). Thus, the δ15N of POM helps in identifying which nitrogen source mainly supports primary production in this sea. Therefore, in this study, we investigated the carbon and nitrogen stable isotope ratios in the southern SOJ (approximately ≤ 40°N). The main objectives were to: 1) demonstrate the spatial distribution of carbon and nitrogen stable isotope ratios, 2) understand the effects of horizontal advection of particulate organic carbon from the ECS, and 3) identify which nitrogen source mainly supports the primary production.