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

Abstract

Abstract. Both nitrogen and carbon dynamics have changed in the Sea of Japan. We hypothesized that the carbon and nitrogen stable isotope ratios (δ13C and δ15N) 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 δ13C values ranged from −24.7 ‰ to −15.0 ‰ 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 δ13C contents. The δ15N values range from 2.8 ‰ to 8.8 ‰, indicating that C. sinicus is a secondary producer; the tendency of the δ15N values to increase from the bloom to the post-bloom seasons was attributable to an increase in the δ15N of phytoplankton. A generalized linear model (GLM) approach indicated that >70 % of the variations in δ13C 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 δ13C in the GLM decreased yearly (−0.035 ‰ yr−1). The GLM for δ15N of C. sinicus indicated that δ15N varies with the stage or sex in addition to SST, SSC, C/N ratio, and geographic differences. The δ15N values of female C. sinicus and stage V copepodites were the lowest and highest, respectively. The residuals of δ15N 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.