Second, the nitrogenous (nitrate) source of the SOJ exhibited variability. Previous studies in the Northern ECS (Umezawa et al., 2014; Umezawa et al., 2021) found four nitrate sources with varying δ15NNO3. The nitrate with high-δ15NNO3 (8.3‰) originated from the Changjiang freshwater in July, the nitrate with low-δ15NNO3 (2.0‰) originated from the Changjiang estuary in July, the δ15NNO3 in the water originating from the Kuroshio is 5.5–6.0‰ in February, and that originating from atmospheric deposition is -4‒0‰ (Umezawa et al., 2014; Umezawa et al., 2021). Furthermore, active nitrogen fixation occurs in the northwestern part of the ECS (Shiozaki et al., 2010), and δ15NPOM originating from nitrogen fixation is -2.1‒0.8‰ (Minagawa and Wada, 1986). According to these results, the δ15NPOM formed by nitrate assimilation and nitrogen fixation exhibited a wider range. In fact, in the ECS, where the TWC originates, δ15NPOM near the surface varies widely by about -5–9‰ during summer (Gao et al., 2014) and 2–6‰ in autumn (Wu et al., 2003). Horizontal advective transport of nitrate from the ECS is the key factor for controlling primary production in the SOJ throughout the summer (Kodama et al., 2015; Kodama et al., 2017). POM originating from diverse nitrogenous sources will be mixed during the horizontal advection processes in the TWC and the ECS, and POM from the ECS is expected to flow into the SOJ with nitrate. The numerous nitrogen source contributions would obscure the δ15NPOM and nitrate concentration in the SOJ.