We compared seasonal and spatial variations in phytoplankton biomass between a location upstream of a rivermouth barrage in the Tone River and a freshwater reservoir in Isahaya Bay in order to examine phytoplankton growth in areas affected by dam emplacement. Simple linear regression analyses were conducted between water renewal rates and phytoplankton biomass（chlorophyll a concentration）. Signifi cant negative correlations were found just upstream of a rivermouth barrage in the Tone River and in the riverine zone of a freshwater reservoir in Isahaya Bay from spring to summer. However, no signifi cant correlation existed between water renewal rate and phytoplankton biomass in the reservoir itself, which suggests that other environmental factors affect phytoplankton biomass at this location. Growth of the phytopkankton community increases the organic matter load to coastal areas, and an estimated 24％ of the organic carbon load into Isahaya Bay was derived from autochthonous production in the Isahaya Bay reservoir.