Bull. Jpn. Soc. Fish. Oceanogr. 83(3), Page 164-170, 2019   Seasonal variations in size-fractionated Chl-a and cell density of phytoplankton communities at sea surface in the inner part and the mouth of the coast of Tokyo Bay Shinji Hashimoto*† and and Chihiro Hashimoto* * Teikyo University of Science, Faculty of Life and Environmental Sciences, 2–2–1 Senju Sakuragi, Adachi, Tokyo 120–0045, Japan † E-mail: hashimoto@ntu.ac.jp To clarify the seasonal variations in the phytoplankton community and the mechanism in Tokyo Bay, the concentrations of size-fractionated (>10 µm, 2–10 µm, <2 µm) chlorophyll a (Chl-a) and nutrients, and the abundance of large-sized phytoplankton species (>10 µm) were investigated at the surface of the inner part and mouth of the bay from April to December 2012. In spring, fall and winter, the seasonal variations in the concentrations of size-fractionated Chl-a and the abundance of large-sized phytoplankton species were similar between the inner part and mouth of the bay. In April, the total Chl-a concentration was high during the observation periods at both stations, and the >10 µm fraction accounted for more than 50% of the total Chl-a concentration. Further, the large-sized diatom accounted for a high proportion of the phytoplankton community. However, in May, the <2 µm fraction and raphidophyte (Heterosigma akashiwo) were dominant at both stations. Consequently, it is probable that the succession of phytoplankton occurs synchronously in the inner part and mouth of the bay from April to May. Nitrate+nitrite concentrations rapidly decreased from April to May at both stations. Therefore, it is likely that the size-fractionated Chl-a concentration and the phytoplankton community in the bay were strongly affected by the nitrate+nitrite concentration. On the other hand, a difference was observed with the seasonal variations in the size-fractionated Chl-a concentrations and species composition of the phytoplankton community at both stations during summer. It is possible that the difference was caused by the nutrient concentrations. Key words: inner part of bay, mouth of bay, nutrient, phytoplankton community, size-fractionated chlorophyll a, Tokyo Bay