Figure 3. Horizontal distributions of δ13CPOM (upper, a–e) and δ15NPOM (lower, f–j) in the Sea of Japan (1° × 1° grid median values for five years). The (a and f) winter, (b and g) in the surface layer of spring, (c and h) in the subsurface layer of spring, (d and i) in the surface layer of summer, and (e and j) in the subsurface layer of summer. Circle sizes reflect the sample numbers used for calculating the median values. The maps were made with Natural Earth, and Geospatial Information Authority of Japan.