The monthly variations in δ13CPOM and δ15NPOM were evaluated using the samples collected along the SI-line (Fig. 4a–d). When the interannual variations were ignored, δ13CPOM was the lowest in April (lsmean ± SE: –26.1 ± 0.62‰) and highest in September (–22.4 ± 0.32‰) (Fig. 4a); no significant difference was observed during February, April, and June (Pairwise test with Tukey–Kramer adjacent, p > 0.05). The δ15NPOM in the surface layer was the lowest in February (–0.5 ± 0.40‰) and highest in June (2.7 ± 0.52‰), while a significant difference was not observed among June, July, and September (Fig. 4b). In the subsurface layer, the monthly variation of δ13CPOM was not significant (ANOVA, p = 0.09) (Fig. 4c), and that of δ15NPOM was significant (p = 0.026) but was not significant as per the pairwise test with Tukey–Kramer adjacent (p > 0.05) (Fig. 4d).