Furthermore, δ13CPOM and δ15NPOM were divided into four classes (I–IV) according to the two-dimensional GMM (Fig. 5). The mean ± SDs of δ13CPOM and δ15NPOM were –24.6 ± 1.2‰ and –2.1 ± 0.8‰ in class I (n = 11), –23.7 ± 1.2‰ and 3.1 ± 1.2‰ in class II (n = 441), –27.1 ± 1.0‰ and 2.0 ± 1.3‰ in class III (n = 21), and –20.7 ± 0.8‰ and 1.7 ± 1.0‰ in class IV (n = 34), respectively (Fig. 5b–c). The δ13CPOM values were significantly different among the different classes (p < 0.001, pairwise test with Tukey–Kramer adjacent), except between classes I and II (p = 0.06) (Fig. 5b). The δ15NPOM values were significantly different among the different classes (p < 0.001, pairwise test with Tukey–Kramer adjacent), except between classes III and IV (p = 0.76) (Fig. 5c). However, the mean ± SD values of δ15NPOM in classes III and IV were within the range of class II values (Fig. 5c). Therefore, the characteristics of δ13CPOM and δ15NPOM in classes I, II, III, and IV were middle-δ13CPOM and low-δ15NPOM, middle-δ13CPOM and high-δ15NPOM, low-δ13CPOM and high-δ15NPOM, and high-δ13CPOM and high-δ15NPOM, respectively.