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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.

Phytoplankton assemblages are important for understanding the quality of primary production in marine ecosystems. Here, we describe development of a methodology for monitoring marine phytoplankton assemblages using an in situ multi-wavelength excitation fluorometer (MEX) and its application for seasonal observations in coastal and offshore areas around Japan. The MEX recorded the fluorescence excited with nine light-emitting diodes, temperature, and sensor depth. We prepared reference datasets comprising temperature, MEX fluorescence, and plant-pigment-based chemotaxonomy phytoplankton assemblages. Target MEX fluorescence was decomposed by reference MEX fluorescence using a linear inverse model for calculating coefficients after the reference data were limited by temperature, followed by reconstruction of plant-pigment-based chemotaxonomy of the target MEX fluorescence using the coefficients and the chemotaxonomy assemblages of the reference data. Sensitivity analysis indicated poor estimation of the proportion and/or chlorophyll a-based abundance of chlorophytes, haptophytes, prasinophytes, and prochlorophytes; however, limiting the estimations to five chemotaxonomic groups [diatoms, dinoflagellates, cryptophytes, cyanobacteria (cyanophytes and prochlorophytes), and other eukaryotes (chlorophytes, haptophytes, and prasinophytes)] resulted in positive correlations of both the proportion and abundances, suggesting that the five taxonomic abundances were well-estimated using the MEX. Additionally, MEX observations denoted spatial and seasonal variations of phytoplankton assemblages, with high contributions from other eukaryotes in every area and season, cyanobacteria highly during the summer in surface Kuroshio and Japan Sea waters, and diatoms in the Oyashio and Oyashio–Kuroshio transition areas and the Okhotsk Sea. Furthermore, ratios of water-column-integrated chlorophyll-based abundances to those on the surface at the chemotaxonomy group level were differed among the areas and groups. These findings suggested that phytoplankton-assemblage monitoring in the vertical direction is essential for evaluation of their current biomass, and that the MEX promotes the acquisition of these observations.

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 (δ13C and δ15N) 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 δ13C 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 δ13C. The negative correlation between the δ13C values of C. sinicus and their carbon:nitrogen ratios reflected lipid accumulation by the copepods; high δ13C values were associated with high sea surface chlorophyll a concentrations. The δ15N values ranged from 2.8 ‰ to 8.8 ‰. The tendency of the δ15N values to increase from the bloom to post-bloom was attributable to an increase of the δ15N of the phytoplankton associated with nitrate depletion and Rayleigh fractionation. These monthly changes were synchronized among the four stations, but δ13C and δ15N differed significantly between stations. Interannual variations were statistically significant, but there were no significant monotonic trends. Interannual variations differed between δ13C and δ15N 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.