The environmental parameters properly explained δ13CPOM variability, but not the difference among classes (III and IV) because it was significant in the GLM. Temperature, nitrate concentration, and C:N ratio were found to be significantly different across classes III and IV. Furthermore, the class IV samples were mostly from the surface layer during the summer, but the surface layer samples during summer were not classified as the class III. Nakatsuka et al. (1992) reported that δ13CPOM levels are high in the late phytoplankton bloom phase. Thus, the high δ13CPOM with low nitrate concentration and high C:N ratio in the class IV samples may be attributed to active carbon assimilation under nitrate depletion conditions. However, because the chlorophyll a concentration was low in the class IV samples, additional processes must be considered. According to the T-S diagram, class IV POM was mainly collected in the warm and saline waters in 2019. This may indicate that the isotope fraction in this water is different; for example, an increase in diatom abundance elevates δ13CPOM (Lowe et al., 2014), but the diatom contribution in the SOJ is low during the summer (Kodama et al., 2022b). The phytoplankton community structure was not assessed in this study and will need to be investigated more in the future. In contrast to class IV samples, low δ13CPOM samples in class III were mostly observed in nitrate-rich waters, indicating that primary production is not active, which might be due to light limitation and deep mixing; the previous study’s iron limitation for primary production was rejected (Fujita et al., 2010). However, comparable environmental conditions of classes III and IV samples were observed in the classes I and II samples, and hence the fundamental reason why the δ13CPOM was low and high in classes III and IV, respectively, is yet unclear.