Of note, we did not include the interaction terms in the GLMs. For instance, longitude–latitude interactions can reflect the 2-D variations. Moreover, the temperature–salinity (T–S) diagram is the most fundamental method for obtaining the characteristics of water masses. Therefore, although the interactions must be considered, the interaction terms of temperature and salinity could not be used instead of the T–S diagram in the GLMs. Instead of interaction terms, the classes based on the GMM were plotted on the T–S and nitrate-density (σt) (N-σt) diagrams to elucidate the effect of non-linear interactions on δ13CPOM or δ15NPOM.