Keywords:N and C isotope ratios, food chain, Oyashio region, isotopic fractionation
Carbon and nitrogen isotope ratios (δ15N and δ13C) of organisms are controlled by not only biological factors, such as catabolism and assimilation, but also physical environmental conditions that influence the isotope ratios of primary producers (phytoplankton). To examine how different water properties (i.e., nutrients and temperature) affect δ15N and δ13C in marine food webs, we measured δ15N and δ13C of zooplankton in three distinct water masses: coastal Oyashio (COY), Oyashio (OY) and warm-core ring (WCR) water along the A-line monitoring transect (38N-42.5N, 144.5-147.5E) in March 6- 26, 2015, cruise of the KH-15-1 of the R/V Hakuho-maru of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and in February 28 – March 16, 2015, cruise of the WK-15-03 of the R/V Wakataka-maru of the Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency (FRA). Zooplankton samples were collected from 150 m depth to the surface using vertical tow of a NORPAC twin net (45cm mouth diameter, 0.355 mm mesh size). After collection, all samples were classified using a stereomicroscope into species or genus level, and only adults were used for isotopic analysis. Water samples were collected from the surface to 250m depth profiles for the measurements of δ15N (NO3+ + NO2-). We compared the trophic fractionations of carbon and nitrogen isotopes (Δδ13C, Δδ15N) of zoopalankton among the three water masses. We found that δ15N of chaetognatha at COY tended to be higher than OY and WCR, but a simple relationship between Δδ15N and Δδ13C, regardless of species or water masses. Combined with this Δδ15N-Δδ13C relation, the isotopic ratios of zooplankton would allow us to predict C and N isotope ratios of higher trophic consumers, such as carnivorous fish and seabirds.