Bull. Jpn. Soc. Fish. Oceanogr. 79(3), Page 117-129, 2015   Water purification in the river mouth tidal flat with an abundant supply of the Manila clam Ruditapes philippinarum: A model study in June in Rokujo tidal flat, at the mouth of the Toyo River in Mikawa Bay, Japan Ryota Sone1†, Satoru Kamohara1*, Satoshi Yamada1, Teruaki Suzuki2 and Teruhiro Takabe3 1 Aichi Fisheries Research Institute, 97 Wakamiya, Miya, Gamagori, Aichi 443-0021, Japan 2 Graduate School of Environmental and Human Science, Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, Aichi 468-8502, Japan 3 Research Institute of Meijo University, 1-501 Shiogamaguchi, Tempaku, Nagoya, Aichi 468-8502, Japan * Present: Fisheries Administration Division, Department of Agriculture, Forestry and Fisheries, Aichi Prefectural Government, 3-1-2 Sannomaru, Naka-ku, Nagoya, Aichi 460-8501, Japan † E-mail: riyouta_sone@pref.aichi.lg.jp To evaluate the water purification function of the Manila clam Ruditapes philippinarum on the Rokujo tidal flat at the mouth of the Toyo River in Mikawa Bay, we calculated the nutrient budget using box model analysis in June. In addition, we investigated the biomass and growth of the juvenile Manila clam population and discuss the mechanism of nitrogen cycling in the tidal flat. The results of the box model analysis indicated that the removal rate of particulate organic nitrogen (PON) was 18.5 mgN·m−2·h−1 and the elution of dissolved total nitrogen (DTN) was 6.3 mgN·m−2·h−1. The total nitrogen (TN) removal rate was accordingly 12.2 mgN·m−2·h−1. In comparison to previous studies, the removal rates of PON and TN for the Rokujo tidal flat were larger than those of other tidal flats in Ise-Mikawa Bay. In terms of nitrogen cycling in the Rokujo tidal flat, it is suggested that the large juvenile Manila clam population enhances the removal of PON, and that their high gross-growth efficiency lowers the DTN returned to the water column of the tidal flat. Additionally, it is considered that DTN is rapidly absorbed by seaweeds or benthic microalgae. These mechanisms thus contribute to the high removal of both PON and TN in the Rokujo tidal flat. In order to facilitate ecosystem restoration of coastal bay estuaries, it is necessary to promote the restoration of tidal flats in river mouths, where primary production is high and red tides frequently occur. Key words: water purification, river mouth tidal flat, nitrogen budget, box model analysis, Manila clam, population growth