Carbon and nitrogen dynamics in the Sea of Japan (SOJ) are rapidly changing. In this study, we investigated the carbon and nitrogen isotope ratios of particulate organic matter (δ13CPOM and δ15NPOM, respectively) at depths of ≤ 100 m in the southern part of the SOJ from 2016 to 2021. δ13CPOM and δ15NPOM exhibited multimodal distributions and were classified into four classes (I–IV) according to the Gaussian mixed model. A majority of the samples were classified as class II (n = 441), with mean ± standard deviation of δ13CPOM and δ15NPOM of –23.7 ± 1.2‰ and 3.1 ± 1.2‰, respectively. Compared to class II, class I had significant low δ15NPOM (-2.1 ± 0.8‰, n = 11), class III had low δ13CPOM (-27.1 ± 1.0‰, n = 21), and class IV had high δ13CPOM (-20.7 ± 0.8‰, n = 34). All the class I samples, whose δ15NPOM showed an outlier of total data sets, were collected in winter and had comparable temperature and salinity originating in Japanese local rivers. The generalized linear model demonstrated that the temperature and chlorophyll-a concentration had positive effects on δ13CPOM, supporting the active photosynthesis and phytoplankton growth increased δ13CPOM. However, the fluctuation in δ15NPOM was attributed to the temperature and salinity rather than nitrate concentration, which suggested that the δ15N of source nitrogen for primary production is different among the water masses.  These findings suggest that multiple nitrogen sources, including nitrates from the East China Sea, Kuroshio, and Japanese local rivers, contribute to the primary production in the SOJ.

Stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were seasonally investigated in the vicinity of the Kuroshio from 2008 to 2019 (n = 490). Generalized linear models (GLMs) showed significant seasonality of δ13C and δ15N of POM in the coastal (between Japan Main Island and the northern edge of the Kuroshio) and offshore (Kuroshio and more southern parts) areas. Seasonal climatological δ13C estimated based on the GLMs and seasonal median values of the environmental parameters was the highest in summer (-22.4 ± 0.2‰ and -22.9 ± 0.2‰ in the coastal and offshore areas, respectively) and lowest in winter in both areas (-23.9 ± 0.2‰ and -24.3 ± 0.2‰ in the coastal and offshore areas, respectively). Seasonal climatological δ15N showed different spatial variations from spring to summer. The δ15N value was the lowest during winter (0.8 ± 0.4‰), and increased to a similar level during the other three seasons in the coastal area (~3‰), suggesting that nitrate originating in the deep-sea water was the main source of new production from spring to autumn. In contrast, δ15N in the offshore areas decreased from spring (2.6 ± 0.4‰) to summer (0.9 ± 0.4‰), suggesting that the significant contributions of atmospheric deposition and nitrogen fixation in the summer were similar to those around Hawaii. Therefore, the nitrogen sources for biological production were different between the areas and seasons in the vicinity of the Kuroshio.