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Human activities have caused sometimes dramatic changes to the marine environment globally and locally during the last half century. We hypothesized that the carbon and nitrogen stable isotope ratios (d13C and d15N) of the copepod Calanus sinicus, one of the dominant secondary producers of North Pacific coastal waters, would record anthropogenic impacts on the coastal environment of the Japan Sea. We monitored these isotope ratios during the spring at four stations in the Japan Sea from 2006 to 2020. The d13C values ranged from -24.7‰ to -15.0‰ and decreased from the spring bloom (February/March) to the post-bloom (June/July). This monthly variation was attributed to changes in both the physiology of C. sinicus and phytoplankton d13C. The negative correlation between the d13C values of C. sinicus and their carbon:nitrogen ratios reflected lipid accumulation by the copepods; high d13C values were associated with high sea surface chlorophyll a concentrations. The d15N values ranged from 2.8‰ to 8.8‰. The tendency of the d15N values to increase from the bloom to post-bloom was attributable to an increase of the d15N of the phytoplankton associated with nitrate depletion and Rayleigh fractionation. These monthly changes were synchronized among the four stations, but d13C and d15N differed significantly between stations. Interannual variations were statistically significant, but there were no significant monotonic trends. Interannual variations differed between d13C and d15N as well as among stations. These results suggest that local conditions rather than global-scale trends were the primary determinants of elemental cycles in this coastal ecosystem. [ABSTRACT FROM AUTHOR]