Bull. Jpn. Soc. Fish. Oceanogr. 69(1), Page 10-17, 2005   Spatial and Temporal Distribution of Copepod Nauplii in Ariake Bay Yuji Okazaki†1, Yuko Hosoe2, Yuko Nonaka3 and Hideaki Nakata4 1 Tohoku National Fisheries Research Institute, 3-27-5 Shinhama, Shiogama, Miyagi 985-0001, Japan2 Graduate School of Science and Technology, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki 852-8521, Japan 3 Takeei Co. Ltd, 7-20-10 Nishikasai, Edogawa, Tokyo 134-8611, Japan4 Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki 852-8521, Japan†e-mail: okazakiy@affrc.go.jp We examined the spatial and temporal distribution of copepod nauplii in Ariake Bay. Surface density of copepod nauplii was high in the inner and center part of the bay, and lower toward the outer bay. Nauplii density increased in summer, which attained to over 9000 individuals l -1, and declined in winter. Their density was high in the upper layer (0-5 m depth) during summer and autumn, and the high density was correspondent with high chlorophyll a concentration. Most of the copepodites (including adult stage) were Oithona spp.; they were the most abundant in summer. Chlorophyll a concentration and copepod nauplii density were higher in the neap tide comparing to those in the spring tide, suggesting that differences in oceanographic conditions (eg. transparency and vertical stratification) due to the change in intensity of tidal mixing may strongly control plankton distribution in Ariake Bay. Key words: Ariake Bay, copepod nauplii, copepodites, chlorophyll a, tidal mixing