Unlike δ13CPOM, δ15NPOM was not adequately explained by environmental parameters based on the detection coefficient values. Temperature and salinity remained as explanatory variables in the variation of δ15NPOM in the ocean, but were not regarded as essential determinants (Sigman et al., 2009). Temperature had the opposite impact reported in the Kuroshio (Kodama et al., 2021). Temperature had no significant effect in the western south ECS (Ho et al., 2021), thereby suggesting that the negative impact of temperature on the δ15NPOM was specific to the SOJ; however, its mechanisms remain unclear.