Another distinctive feature of the SOJ was the low-δ15NPOM designated class I, which was found in the winter and spring, and is characterized in the T-S diagrams. The low-δ15NPOM was mainly observed in temperature and salinity ranges of 9.4–11.4°C and 33.877–34.038, respectively. Wagawa et al. (2020) classified this water as "upper low salinity water" (ULSW). Despite the fact that the origin of USLW was unclear, Wagawa et al. (2020) proposed that it originated from Toyama Bay, with a less saline condition caused by the mixing with local Japanese rivers. It is reasonable to assume that the USLW is not mixed with the saline TWC water (its salinity is ~ 34.5). Because this saline TWC water originates from the Kuroshio Current, the δ15NNO3 of the saline TWC water is estimated to be 5.5–6.0‰ in accordance with Umezawa et al. (2014). At the same time, the δ15NNO3 of local Japanese rivers has been estimated to be 0–2‰ (Sugimoto et al., 2019), suggesting that the POM in the ULSW may have originated from lower δ15NNO3-nitrate than the Kuroshio-origin nitrate, and hence the δ15NPOM is lower than the other water masses. The horizontal distribution of ULSW is not reported, but we assumed that it was not large and confined to winter and spring based on Wagawa et al. (2020), hence the low-δ15NPOM area would be limited to season and area. Phytoplankton bloom is another possibility for class I. δ15NPOM rapidly declined at the start of the phytoplankton bloom phase but quickly rose with nitrate depletion (Nakatsuka et al., 1992). However, we believe that such scenario is uncommon and did not have a significant impact on our observations because the majority of class I samples were collected in February, and the phytoplankton bloom occurred at the end of March (Kodama et al., 2018; Maúre et al., 2017).