Here, a simulation of the relationship between the δ15NPOM and nitrate concentration was performed. The δ15NNO3 was set to 0–8.3‰ (Umezawa et al., 2014; Umezawa et al., 2021), the kinetic isotope effects to 3‰ (Sigman et al., 2009), and the supplied nitrate concentration to 0.05–5 µM. The δ15NPOM was then calculated by mixing nitrate-origin POM and nitrogen-fixation-origin POM. Based on observations in the southern ECS in summer, the contribution of nitrogen fixation to nitrate assimilation in the water column was reported as 10–82% (Liu et al., 2013).The contribution of nitrogen fixation to the primary production in Liu et al. (2013) (1.9–5.8%) was comparable to that of the SOJ in June (~3.8%) (Sato et al., 2021). Therefore, the contribution of nitrogen fixation to δ15NPOM was set at 10–82% (Liu et al., 2013). δ15NPOM produced with nitrogen fixation was set at -2.1–0.8‰ (Minagawa and Wada, 1986). Assuming that the parameters and fraction of remanent nitrate (0–100%), except kinetic isotope effects, were randomly varied in this simulation, and the sample size was set to 500, the insignificant relationship between the δ15NPOM and nitrate concentration was observed in ~30% of the simulations (replicated 1000 times). On the other hand, when the δ15NNO3 was adjusted to 5–6‰, the significant negative relationship between δ15NPOM and nitrate concentration was consistently observed. This result supports our hypothesis that the relationship between the δ15NPOM and nitrate concentration is disrupted by the numerous nitrogen sources.