Bull. Jpn. Soc. Fish. Oceanogr. 81(3), Page 245-258, 2017   Study on the generation mechanism of turbidity that obstructs seaweed aquaculture: A case study of an aquaculture field for Monostroma nitidum in Izonoura, inner side of Matoya Bay Tomoaki Tasaki1†, Yuzou Akashi2, Toshirou Katou3, Tomoki Miyamukai4, Kazumasa Imao4, Teruaki Suzuki5 and Teruhiro Takabe5 1 Resource Recycling Division Nagoya Works Nippon Steel & Sumitomo Metal Corporation, 5-3 Tokai-machi, Tokai, Aichi 476-8686, Japan 2 Slag & Cement division Head office Nippon Steel & Sumitomo Metal Corporation, 2-6-1 Marunouchi, Chiyoda-ku, Tokyo 100-8071, Japan 3 Advanced Technology Research Laboratories, Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan 4 Chubu branch, Marine Biological Research Institute of Japan Co., Ltd., 3-9-2 Koubai-cho, Showa-ku, Nagoya, Aichi 466-0031, Japan 5 Graduate School of Environmental and Human Sciences, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, Aichi 468-8502, Japan 6 Graduate School of Environmental and Human Sciences Faculty of Science & Technology, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, Aichi 468-8502, Japan † E-mail: tasaki.pr5.tomoaki@jp.nssmc.com Izonoura-located in the inner area of Matoya Bay in Shima City, Mie Prefecture-prospered in the aquaculture of Monostroma nitidum, whose amount of production was over 300 tons per year in the 1970s. However, the production decreased rapidly after the 1980s, and the fishermen have demanded an investigation into its cause. We conducted observations of several environmental elements, including turbidity and weather, throughout the spring and summer of 2014 and analyzed the generation mechanism of high turbidity. It was found that the turbidity was higher than 7 FTU-17-66% and 26-73% of the time in spring and summer, respectively-which is considered to affect the growth of Monostroma nitidum. The following factors were considered to cause high turbidity: i) Decrease in river flow accompanied by dam construction reduced the mean flow from the inner to the central area, which would induce a clockwise circulation flow, ii) The surface residual outflow current through the canal weakened. Then, SS (Suspended Solid) from the river and that produced and re-suspended in Izonoura, does not readily outflow from Izonoura, iii) SS was re-suspended not only by the wind but also by the usual tidal current, and iv) Excess sedimentation of SS has caused the deterioration of bottom environments, benthic communities, and the self-purification function of the bottom. Key words: Monostroma nitidum, Sea bottom sediment, turbidity, re-suspension, continuous observation