Bull. Jpn. Soc. Fish. Oceanogr. 77(1), Page 1-9, 2013   Source of sedimentary organic matter in Obama Bay estimated from stable isotope and C/N ratios Takahisa Sato1 ‡, Ryo Sugimoto2 and Osamu Tominaga2 1 Graduate School of Biosciences and Biotechnology, Fukui Prefectural University, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan 2 Research Center for Marine Bioresources, Faculty of Marine Biosciences, Fukui Prefectural University, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan ‡ E-mail: s0994002@fpu.ac.jp The total organic carbon (TOC), total nitrogen (TN), C/N ratio and carbon and nitrogen stable isotope ratio (δ13C and δ15N) of sedimentary organic matter (SOM) were measured to estimate the amount and source of surface SOM in Obama Bay, the central coast of Wakasa Bay. The concentration of TOC and TN of surface SOM in Obama Bay were 1.5-32.7 mg C･g−1 and 0.2-2.4 mg N･g−1, respectively. Spatial variations of TOC and TN in sediments were low in the western area. In contrast, they were higher in eastern area and dramatically increased at the mouth of the Kita and Minami rivers. These maximum values of TOC and TN were similar to those of Osaka Bay which is known as a eutrophic sea area. The δ13C values and C/N ratio varied between -21.1‰ and -28.7‰, and 6.5 and 17.9 respectively. Judging from the δ13C and C/N ratio, the major origin of SOM in the western area was marine organic matter (mainly marine phytoplankton), whereas the major source of SOM in the eastern area was terrestrial organic matter (mainly terrestrial plants). These results are possibly related to the location of two main rivers, low flocculation in the estuaries and counterclock-wise current in Obama Bay. Key words: SOM, terrestrial plants, carbon stable isotope ratio, C/N ratio