Figure 4. Temporal variations of δ13CPOM (left) and δ15NPOM (right) along the monitoring line (SI-line); (a) Monthly variation of δ13CPOM in the surface layer, (b) monthly variation of δ13CPOM in the subsurface layer, (c) monthly variation of δ15NPOM in the surface layer, and (d) monthly variation of δ15NPOM in the subsurface layer. Box plots show the mean (thick horizontal lines within boxes), standard deviation (boxes), maximum or minimum values (vertical bars), and outliers (closed circles). The lower-case italic characters near the boxes (x, and y) reflect the results of the pairwise test with Tukey–Kramer adjacent; the same character pairs showed an insignificant difference in the pair (p > 0.05). The pairwise test with Tukey–Kramer adjacent was not significant among any pairs of δ13CPOM in the subsurface layer (c). The numbers just below the horizontal axis lables indicate the sample numbers.