Bull. Jpn. Soc. Fish. Oceanogr. 75(3), Page 143-153, 2011   Short-term dynamics of a Chattonella antiqua bloom in the Yatsushiro Sea, Japan, in summer 2010: characteristics of its appearance in the southern area Goh Onitsuka1†, Kazuhiro Aoki1, Manabu Shimizu1, Yukihiko Matsuyama2, Katsunori Kimoto2, Hitoshi Matsuo3, Yuuki Kitadai3, Hiromi Nishi4, Yoshio Tahara4 and Kiyonari Sakurada4 1 National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan 2 Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira-machi, Nagasaki, Nagasaki 851-2213, Japan 3 Azuma-cho Fishery Cooperative Association, Takanosu, Nagashima-cho, Izumi-gun, Kagoshima 899-1403, Japan 4 Kagoshima Prefectural Fisheries Technology and Development Center, 160-10 Takadaue, Iwamotoaza, Ibusuki, Kagoshima 891-0315, Japan 5 Kumamoto Prefectural Fisheries Research Center, 2450-2 Naka, Oyano-machi, Kami-amakusa, Kumamoto 869-3603, Japan † onizuka@affrc.go.jp In summer 2010, a harmful bloom of the raphidophycean flagellate Chattonella antiqua occurred in the Yatsushiro Sea. The short-term dynamics of the C. antiqua bloom and oceanographic conditions were investigated by intensive field observations. The bloom first appeared in the west-central area of the Yatsushiro Sea at the end of June. It then expanded rapidly to include the entire area in early July, and high cell density with short-term fluctuations continued to exist till the end of July. In the southern area, increase in the cell density of C. antiqua was synchronized with the timing of low salinity water covering the surface layer, which originated in the fresh water discharge from the Kuma River in the northern area of the Yatsushiro Sea. This synchronization implies that the dense water containing C. antiqua was transported by a density-driven current from the north-central area, and/or cell density was enhanced by the stratification associated with the low salinity riverine water. A rapid increase in cell density in the surface layers and the subsequent deepening of dense water below 20 m occurred in the southern area in late July, resulting in enormous damage to cultured finfish. Key words: Chattonella antiqua, harmful bloom, Yatsushiro Sea, Kuma River, low salinity water, density-driven current