Bull. Jpn. Soc. Fish. Oceanogr. 69(1), Page 1-9, 2005   Seasonal and Inter-annual Variation in chlorophyll a Concentration in Kitanada Bay, Uwa Sea Yuichi HAYAMI†1, Shu OUGIYAMA2*, Masanori YAMADA3, Masahiko YAMADA3, Kimihiro TAKEMURA3 and Hidetaka TAKEOKA4 1 Faculty of Engineering, Ehime University, 3 Bunkyo-Cho, Matsuyama, Ehime 790-8577, Japan 3 Tsushima Town, Section of Fishery, 471 Iwamatsu-Kou, Tsushima Town, Ehime 798-3392, Japan 4 Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama, Ehime 790-8577, Japan†e-mail: hayami@dpc.ehime-u.ac.jp The seasonal and inter-annual variation in chlorophyll a concentration in the Kitanada Bay located at the eastern coast of the Bungo Channel was investigated using the data of oceanographic monitoring conducted by Tsushima town, Ehime Prefecture. The relationship between the variations in physical conditions and chlorophyll a concentration was also investigated. From April to October, the density structure of Kitanada Bay was estuarine type. Low density water spread from the bay head to bay mouth at the surface. On the other hand, it was inverse estuarine type from December to February, cold and high density water plunges along the bottom from bay head to the bay mouth. The chlorophyll a concentration in the bay was low in the inverse estuarine period and high during the estuarine type period especially in the surface layer. The net ecosystem production (NEP) of chlorophyll a was estimated with a box model analysis. The NEP of chlorophyll a amounted around 400 kg chl-a · day-1 in the estuarine type period, but it was around zero in the inverse estuarine period. The integrated NEP of chlorophyll a throughout the year indicates that the Kitanada Bay is a source of chlorophyll a for the offshore water. There was a good relationship between the mean summer chlorophyll a concentration in upper 5 m in and outside of the Kitanada Bay and the bottom water temperature in the offshore region that is an indicator of the activity of the bottom intrusion from the shelf slope. It indicates that the nutrient supply by the bottom intrusion in the Bungo Channel enhances the phytoplankton production even in the shallow tributary bay. It was supposed that the low chlorophyll a production in winter Kitanada Bay was caused by the decline of riverine input of nutrient and stop of nutrient transport from outside of the bay along the bottom due to the inverse estuarine circulation and the absence of bottom intrusion. Key words: chlorophyll a, seasonal variation, inter-annual variation, Kitanada Bay, Bungo Channel, bottom intrusion