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Effects of the Nagara Rivermouth Barrage on planktonic algal development are examined based on the data continuously recorded by the Japanese Ministry of Construction (MoC) and Water Resource Development Public Corporation (WRDPC) since 1994 when the barrage was completed. Many predictions concerning algal occurrence and water pollution are reviewed. Remarkable algal occurrences were observed many times under low discharge conditions mainly from late spring to fall after barrage construction. The dominant species were centric diatoms, i.e., Cyclotella atoms and C. meneghiniana in summer, and Stephanodiscus spp. in winter. Additionally, accumulations of blue-green algae were also observed. Longitudinal distribution of algal biomass resulted from a balance between production and sedimentation, which were dependent on the discharge. Judging from flora and population dynamics, the barrage enhanced potamoplanktonic algal production by its prolongation of retention time. In spite of high algal production, the target level of organic matter content as evaluated by Biochemical Oxygen Demand (BOD).has attained the Environmental Quality Standards for Water Pollution based on the Basic Law for Environmental Pollution. This is partly attributable to sedimentation and zooplankton grazing at a lower observatory. For water use, it should be noted that fluctuation in the dosage of activated carbon in a water purification plant was partially correlated to that in algal occurrences. For further assessment and improvement of river eutrophication after barrage construction, it is necessary to obtain a correct understanding of primary production in Japanese rivers and to review past pre-dictions made on misconceptions before construction.
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