Bull. Jpn. Soc. Fish. Oceanogr. 74(3), Page 167-175, 2010   Disappearance of the bloom of the harmful dinoflagellate Karenia mikimotoi induced by the occurrence of bottom intrusion in Shitaba Bay, Uwa Sea, in 2007 Atsushi Kaneda1†, Yoshitsugu Koizumi2, Daisuke Takahashi3, Kayoko Fukumori4, Xinyu Guo5 and Hidetaka Takeoka5 1 Fukui Prefectural University, Department of Marine Bioscience, 1-1 Gakuen-cho, Obama, Fukui 917-0003, Japan 2 Ehime Research Institute of Agriculture, Forestry and Fisheries, Fisheries Research Center, 5516 Shitaba, Uwajima, Ehime 798-0104, Japan 3 Hydrosphere Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan 4 Center for Ecological Research, Kyoto University, 2-509-3 Hirano, Otsu, Shiga 520-2113, Japan 5 Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan †e-mail: kaneda@fpu.ac.jp As a blooming (or akashio) of a harmful dinoflagellate Karenia mikimotoi occurred in coastal areas of the Uwa Sea, Japan, in the summer of 2007, intensive observations were conducted in Shitaba Bay on the Uwa Sea. These observations were to examine how spatio-temporal distributions of K. mikimotoi’s akashio are related to physical phenomena, such as kyucho (intrusion of warm oceanic water into the surface layer) and bottom intrusion (intrusion of cold oceanic water into the bottom layer from the shelf-slope region). The population density of K. mikimotoi in the surface layer began to decrease on around 17 July before disappearing entirely on around 21 July. Mooring observation and repeated surveys showed that vertical circulation due to a bottom intrusion occurred on around 17 July. With the occurrence of the phenomenon, chlorophyll a (chl-a) concentration, which is an index of K. mikimotoi abundance, varied. In addition, the area of high chl-a concentration was likely moved passively by ocean currents along with the bottom intrusion. Time-series sampling data from an adjacent bay exhibited temporal changes that were similar to those of the K. mikimotoi blooming in Shitaba Bay, suggesting that physical oceanic perturbations significantly affect the disappearance of K. mikimotoi bloom in the Uwa Sea. Key words: Karenia mikimotoi, red tide, Uwa Sea, bottom intrusion, water exchange