The horizontal variations in δ13CPOM are shown by the 1° × 1° grid median values (Fig. 3a–e). During winter, the median (± interquartile range, IQR) of the δ13CPOM was –25.0 ± 2.34‰, and low δ13CPOM was observed in the offshore waters (median ± IQR: –25.6 ± 3.20‰). During spring, low δ13CPOM was identified offshore, and the surface δ13CPOM (–24.6 ± 1.55‰) was lower than that of the subsurface (–23.7 ± 1.68‰, Wilcoxon test, W = 271, p = 0.021). During summer, the surface δ13CPOM (–22.9 ± 1.07‰) was significantly higher than that of the subsurface (–24.4 ± 1.86‰, Wilcoxon test, W = 36002, p < 0.001). The δ13CPOM in the subsurface during the summer was higher in the western than in the eastern part, while those of the surface were higher in the eastern than in the western part.