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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]

The effect of 70% ethanol preservation (for three days) on bulk stable isotope measurements of carbon and nitrogen in marine predators (squid and fish) were examined. A total of 17 different species collected from a wide range of latitudes (15?S-65?N) in the Pacific Ocean were used in the analysis. In agreement with previous studies, ethanol preservation significantly increased δ15N by 0.8-1.7? relative to frozen control samples, regardless of species. Ethanol enriched δ13C of squid mantle tissue as well, but showed variable alterations in fish muscle compared with the control (lipids extracted by chloroform and methanol). An additional extraction treatment with 99.5% ethanol increased δ13C by 1.3-2.0? for squid, and by 0.3-1.3? for fish relative to the control, without a further shift of δ15N, and reduced the variability of δ13C alterations of fish muscle. After this treatment, both δ15N and δ13C of the controls could be estimated from the samples preserved in ethanol by fitting to regression lines with high correlation coefficients (r²>0.90). These results suggest that, with additional treatment, marine predator samples preserved in 70% ethanol are able to be used for stable isotopic analysis. [ABSTRACT FROM AUTHOR]