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.

Variations in the Ratio of Carbon and Nitrogen Stable Isotopes of Particulate Organic Matter in the Southern Sea of Japan

“…The present study is the first to report δ 13 C POM and δ 15 N POM in a wide area of the southern SOJ. Previous studies have reported that the δ 13 C POM and δ 15 N POM in the southern SOJ [Antonio et al , 2012;Nakamura et al , 2022] were limited to coastal areas. The δ 13 C and δ 15 N values of sinking organic particles in the deep layer (> 500 m depth) have been reported in the SOJ [Khim et al , 2018;Nakanishi and Minagawa , 2003].…”

“…The explanatory variables used in the full GLMs were chosen based on the retraction of multicollinearity. We conducted other GLM approaches that included a quadratic expression in the model like Nakamura et al [2022], and a generalized additive model approach like Kodama et al [2021]. However, the deviance explained of the models did not improve; thus, the simple GLM approach was chosen.…”

“…In the SOJ, studies using a stable isotope ratio have been conducted to identify changes in carbon and nitrogen dynamics [Nakamura et al , 2022;Ohshimo et al , 2021]. The variation in the stable isotope ratio of particulate organic matter (POM) in the surface layer has rarely been reported and is limited to coastal areas [Antonio et al , 2012;Nakamura et al , 2022].…”

“…In the SOJ, studies using a stable isotope ratio have been conducted to identify changes in carbon and nitrogen dynamics [Nakamura et al , 2022;Ohshimo et al , 2021]. The variation in the stable isotope ratio of particulate organic matter (POM) in the surface layer has rarely been reported and is limited to coastal areas [Antonio et al , 2012;Nakamura et al , 2022]. The variation in the stable isotope ratio of POM is necessary because the stable isotope ratios of higher trophic organisms in this sea are not always reflected in the stable isotope ratio values of POM [Nakamura et al , 2022].…”

“…The variation in the stable isotope ratio of particulate organic matter (POM) in the surface layer has rarely been reported and is limited to coastal areas [Antonio et al , 2012;Nakamura et al , 2022]. The variation in the stable isotope ratio of POM is necessary because the stable isotope ratios of higher trophic organisms in this sea are not always reflected in the stable isotope ratio values of POM [Nakamura et al , 2022]. In addition, repeated observations indicate that the carbon and nitrogen stable isotope ratio of POM is unstable in the marginal seas of the western North Pacific [Ho et al , 2021;Kodama et al , 2021].…”