Figure 4. Temporal variations of δ¹³C_POM (left) and δ¹⁵N_POM (right) along the monitoring line (SI-line); (a) Monthly variation of δ¹³C_POM in the surface layer, (b) monthly variation of δ¹³C_POM in the subsurface layer, (c) monthly variation of δ¹⁵N_POM in the surface layer, and (d) monthly variation of δ¹⁵N_POM 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 δ¹³C_POM in the subsurface layer (c). The numbers just below the horizontal axis lables indicate the sample numbers.