Bull. Jpn. Soc. Fish. Oceanogr. 83(3), Page 171-180, 2019   Supplying path of nutrients and mechanism maintaining chlorophyll a at a high concentration in Nagatsura-ura Lagoon, Miyagi, Japan Kenji Kaneko1*†, Yutaka Okumura2 and Motoyuki Hara1 1 Graduate School of Agricultural Science Tohoku University, 468–1 Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi 980–8572, Japan 2 Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3–27–5 Shinhama-cho, Shiogama, Miyagi 985–0001, Japan * Department of Marine Bio-Science, Faculty of Life Science and Biotechnology, Fukuyama University, 452–10 Ohama-cho, Onomichi, Hiroshima 722–2101, Japan † E-mail: kkaneko@fukuyama-u.ac.jp Nagatsura-ura Lagoon is the main sea area producing oysters in Miyagi, Japan. However, there is little information regarding the nutrient supply process or the primary production process that maintains high productivity of oysters in the lagoon. Seasonal changes in nutrients and chlorophyll a concentration (as an indicator of primary production) were surveyed in the lagoon and the surrounding area from 2014 to 2015. Chlorophyll a concentration was found to be high from late March to September, and especially high from June to September. The amount of river water containing rich nutrients (NO3–N and SiO2–Si) flowing from the Shin-Kitakami River into the lagoon increased from March to September due to increases in both the river flow rate and the frequency of northeast winds blowing from the mouth of the river to the lagoon. Moreover, the release rate of certain nutrients (PO4–P and NH4–N) from the sediment of the seafloor increased due to rising water temperature and decreasing dissolved oxygen concentrations at the bottom layer of the lagoon after June. It was concluded that the high chlorophyll a concentration over the long period from March to September is due to the supply of nutrients from the river and from the sediment in the lagoon. Key words: chlorophyll a, nutrients, Nagatsura-ura Lagoon, oyster culture