Bull. Jpn. Soc. Fish. Oceanogr. 80(4), Page 269-282, 2016   Temporal and spatial variation in primary production in Obama Bay Hisami Honda1†*, Ryo Sugimoto2, Shiho Kobayashi3, Daisuke Tahara2 and Osamu Tominaga2 1 Graduate School of Biosciences and Biotechnology, Fukui Prefectural University, 1-1 Gakuen-cho, Obama, Fukui 917-0116, Japan 2 Research Center for Marine Bioresources, 49-8-2 Katsumi, Obama, Fukui 917-0116, Japan 3 Field Science Education and Research Center, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan * Present: Research Institute for Humanity and Nature, 457-4 Motoyama Kamigamo, Kita-ku, Kyoto 603-8047, Japan † E-mail: h_honda@chikyu.ac.jp Submarine groundwater discharge (SGD) is an important nutrient source for the coastal ecosystems of Obama Bay as well as river water. However, the response of phytoplankton to nutrients supplied from freshwater is unknown. In this study, we evaluated the spatial and temporal variability of primary production from phytoplankton in Obama Bay. From May 2011 to March 2012, we made monthly field observations of Chlorophyll-a (Chl-a) concentration and calculated potential primary productivity (PP) using a simple mathematical model. Chl-a concentrations and PP were each higher in the upper layer from November to February, when the river discharge was high. During the months from March to October, when the nutrients in the upper layer were almost depleted, Chl-a concentrations and PP were higher in the lower layer. In May 2011, we observed significantly higher concentrations of Chl-a (51.8 μg l-1) in the bottom layer at an area 2 km offshore from the river mouth. We also observed low-salinity water in the bottom layer at around the same area in June 2011, one week after a notable flood. Together these observations strongly suggest that SGD led to the phytoplankton bloom observed in the bottom layer during the spring. Key words: Obama Bay, chlorophyll-a maximum, primary production, submarine groundwater discharge