The surface δ13CPOM and δ15NPOM from July–September exhibited significant interannual variation (ANOVA, p < 0.05). The significant difference in interannual lsmean δ13CPOM (pairwise test with Tukey–Kramer adjacent, p = 0.036) was observed between 2019 (highest: –21.9 ± 0.35‰) and 2018 (lowest: –23.5 ± 0.35‰). The interannual lsmean δ15NPOM was the lowest in 2019 (1.5 ± 0.31‰), and significantly different (pairwise test with Tukey–Kramer adjacent, p = 0.016) from that in 2018 (3.0 ± 0.36‰). In the other combinations, there was no significant difference in the pairwise test. The temperature, salinity, and chlorophyll-a concentration in the surface layer during summer did not vary significantly (ANOVA, p > 0.1).