The T–S diagram demonstrated that the samples of class I were mostly (10 of 11 samples) in the water with the temperature and salinity within the 9.4–11.4°C and 33.877–34.038 ranges, respectively (Fig. 7a). Only a single sample of class I was identified at a temperature and salinity of 4.3°C and 34.037, respectively, in April (Fig. 7a). The one sample classified as class II was identified in April in the surface layer, with the temperature and salinity ranging from 9.4–11.4°C and < 34.038, respectively. The nitrate concentration, δ13CPOM, and δ15NPOM of this sample were 0.28 µM, –25.3‰, and 3.5‰, respectively. However, in the N-σt diagram, the samples were not characterized according to the classes (Fig. 7b).