The horizontal variations in δ15NPOM are shown in the same manner as δ13CPOM (Fig. 3f–i). Evidently, in winter, lower values (~ –2.0‰) were observed in the coastal area. δ15NPOM was higher in spring than in winter (3.12 ± 2.02‰ and 3.12 ± 2.02‰ in the surface and subsurface, respectively). Moreover, during spring, the δ15NPOM value in the offshore area was lower than that of the coastal area. In summer, the surface δ15NPOM (2.76 ± 1.68‰) was significantly lower than that of the subsurface (3.14 ± 1.39‰, Wilcoxon test, W = 19187, p = 0.02874). Furthermore, lower δ15NPOM values were identified in the offshore water, and high δ15NPOM values were observed in the coastal area of the northern part.