Journal of Oceanography, Vol. 64 (No. 4), pp. 525-539, 2008
Sachiko Oguma1,2*, Tsuneo Ono2, Akira Kusaka2, Hiromi Kasai2, Yasuhiro Kawasaki2 and Tomonori Azumaya2
1Japan Society for the Promotion of Science, 6 Ichiban-cho, Chiyoda-ku, Tokyo 102-8471, Japan
2Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Katsurakoi, Kushiro 085-0802, Japan
(Received 17 August 2007; in revised form 10 December 2007; accepted 25 February 2008)
Abstract: In this study we used two stable isotopes, δ13C and δ18O, for water mass classification in the coastal region off eastern Hokkaido. δ13C* values, which were corrected for the biological effect, and δ18O values up to 300 m depth suggested that the isotopic character of the onshore and offshore water in the southern Okhotsk Sea, the Nemuro Strait and the western North Pacific could be explained by the mixing of three source waters: the Oyashio water (OYW), Soya Warm Current water (SWCW) and East Sakhalin Current water (ESCW). In summer, δ13C*-δ18O plots indicated mixing between SWCW from the southern Okhotsk Sea and OYW in the Pacific coast of southeastern Hokkaido, while temperature-salinity plots of the onshore water showed minimal difference from the offshore OYW. In winter, on the other hand, the mixed water of ESCW and OYW (or SWCW) appeared in the Pacific coastal region, distributed as cold, low salinity onshore water. Finally, we estimated mixing ratios of OYW, SWCW and ESCW in the coastal region of western North Pacific using their mean values of δ13C* and δ18O as endmembers. These results suggest seasonal and yearly changes of water mass combination en route from the southern Okhotsk Sea to the western North Pacific.