Carbon and nitrogen dynamics in the coastal Sea of Japan inferred from 15 years of measurements of stable isotope ratios of Calanus sinicus

Both nitrogen and carbon dynamics have changed in the Sea of Japan. We hypothesized that the carbon and nitrogen stable isotope ratios (delta C-13 and delta N-15) of the copepod Calanus sinicus could record changes in the coastal environment of the Sea of Japan. Consequently, these isotope ratios were monitored during the spring at four stations from 2006 to 2020 to identify the changes in carbon and nitrogen dynamics. The delta C-13 values ranged from -24.7 parts per thousand to -15.0 parts per thousand and decreased from the spring bloom (February-March) to the post-bloom (June-July) seasons. These variations were attributed to changes in the physiology of both C. sinicus and phytoplankton delta C-13 contents. The delta N-15 values range from 2.8 parts per thousand to 8.8 parts per thousand, indicating that C. sinicus is a secondary producer; the tendency of the delta N-15 values to increase from the bloom to the post-bloom seasons was attributable to an increase in the delta N-15 of phytoplankton. A generalized linear model (GLM) approach indicated that >70 % of the variations in delta C-13 can be explained by sea surface temperature (SST), sea surface chlorophyll a concentration (SSC), carbon:nitrogen ratio of C. sinicus (C/N ratio), and geographic differences. The residuals of delta C-13 in the GLM decreased yearly (-0.035 parts per thousand yr(-1)). The GLM for delta N-15 of C. sinicus indicated that delta N-15 varies with the stage or sex in addition to SST, SSC, C/N ratio, and geographic differences. The delta N-15 values of female C. sinicus and stage V copepodites were the lowest and highest, respectively. The residuals of delta N-15 in the GLM did not exhibit a significant interannual trend. These results suggest that the carbon isotope ratio in the secondary producer has linearly changed in the coastal Sea of Japan over the past 15 years. Moreover, the changes in carbon dynamics of this area have been recorded and observed to impact the marine ecosystem, while the nitrogen dynamics have not been recorded despite the increasing nitrogenous nutrient inputs in this sea.