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Seasonal stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were investigated around the Kuroshio from 2008 to 2019 (n = 474). Significant seasonality in POM δ13C and δ15N were observed in the coastal and offshore areas divided by the northern edge of the Kuroshio. Seasonal mean δ13C was highest during the summer (-23.2 ± 1.1‰ and -23.7 ± 1.1‰ in coastal and offshore areas, respectively) and lowest during the winter (-24.6 ± 0.8‰ and -25.0 ± 0.9‰ in coastal and offshore areas, respectively) in both areas. Seasonal mean δ15N exhibited different spatial variations. The mean δ15N value in the coastal area was the lowest during the winter (1.0 ± 1.9‰) and increased to a similar level (-3‰) during the other three seasons. In contrast, δ15N in the offshore area near the surface decreased from the spring (2.5 ± 1.5‰) to the summer (1.2 ± 1.7‰). These spatial and seasonal differences in δ15N are significant in generalized linear models and the generalized additive models, and suggest that the nitrogen sources used in biological production differ between coastal and offshore areas and between seasons. Nitrate originating in deeper water and rivers is the main sources used for new production throughout the year in the coastal area. Contributions from atmospheric deposition and nitrogen fixation are significant in the offshore area during the summer, while nitrate originating in deeper water is main during winter, spring, and fall in the offshore area. [ABSTRACT FROM AUTHOR]