Bull. Jpn. Soc. Fish. Oceanogr. 76(4), Page 197-204, 2012   River-water spread in Bisan Strait with reference to nutrient supply to nori (Porphyra) farms Shuzo Takagi1 †, Youhei Nanba2, Tokishige Fujisawa3, Yasunori Watanabe4 and Tateki Fujiwara5 1 Okayama Prefectural Technology Center for Agriculture, Forestry and Fisheries, Research Institute for Fisheries Science, Setouchi, Okayama 701-4303, Japan 2 Okayama Federation of Fisheries Cooperative Associations, Minami, Okayama, Okayama 702-8024, Japan 3 Kagawa Prefectural Fisheries Experimental Station, Takamatsu, Kagawa 761-0111, Japan 4 Hokkaido National Fisheries Research Institute, Kushiro, Hokkaido 085-0802, Japan 5 Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto, Kyoto 620-0861, Japan † shiyuuzou_takagi@pref.okayama.lg.jp Culture production of nori (Porphyra) in the central part of Bisan Strait in the Seto Inland Sea is decreasing recently. The decrease is mainly due to the discoloration of nori caused by deficiency of dissolved inorganic nitrogen (DIN). We conducted two field observations to investigate the transport process of river water and accompanying DIN supply from rivers to the central part of Bisan Strait. River-water was flowing out to the sea only during ebb tide; the discharge reached its maximum at low tide. The emitted riverine water mass was transported westward by the flood current in the sea, and the river water on the estuary zone was pushed back into the river during flood tide. As the river discharges increased, spread of the river plume became wider whereas movement of riverine water mass did not change. In winter, nori cultivation season, the lower-salinity water was containing higher DIN concentration. These results suggest that nori cultured in this area was intermittently supplied with DIN from rivers. When the river discharge increases, the DIN supply increases. Key words: Bisan Strait, river water, nori (Porphyra), nutrient supply