Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

Seasonal stable isotope ratios of carbon and nitrogen (delta C-13 and delta N-15) 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 delta C-13 and delta N-15 were observed in the coastal and offshore areas divided by the northern edge of the Kuroshio. Seasonal mean delta C-13 was highest during the summer (-23.2 +/- 1.1 parts per thousand and -23.7 +/- 1.1 parts per thousand in coastal and offshore areas, respectively) and lowest during the winter (-24.6 +/- 0.8 parts per thousand and -25.0 +/- 0.9 parts per thousand in coastal and offshore areas, respectively) in both areas. Seasonal mean delta N-15 exhibited different spatial variations. The mean delta N-15 value in the coastal area was the lowest during the winter (1.0 +/- 1.9 parts per thousand) and increased to a similar level (similar to 3 parts per thousand) during the other three seasons. In contrast, delta N-15 in the offshore area near the surface decreased from the spring (2.5 +/- 1.5 parts per thousand) to the summer (1.2 +/- 1.7 parts per thousand). These spatial and seasonal differences in delta N-15 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.