特集: 長良川河口堰の影響に関するモニタリング評価
FEATURE: Evaluation of Monitoring on Impacts of Estuary Barrage in Nagara River
総説 REVIEW

長良川河口堰建設後の浮遊藻類発生とその環境影響

村上哲生

名古屋女子大学家政学部生活環境学科　〒457-0003 名古屋市瑞穂区汐路町3-40

Tetuo MURAKAMI: Development of planktonic algae and its environmental impacts subsequent to construction of the Nagam Rivermouth Barrage. Ecol. Civil. Eng. 5(1), 41-51, 2002.

Nagoya Women's University 3-40, Shioji-cho, Mizuho-ku, Nagoya 457-0003, Japan

Abstract: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 predictions made on misconceptions before construction.

Key words: environmental assessment, Nagara River, potamoplankton, river eutrophication, rivermouth barrage

摘要

長良川河口堰が完成した1994年以来,建設省・水資源開発公団が収集してきた資料等に基づき,河口堰が浮遊藻類に及ぼす影響を調査し,さらに,既存の藻類発生や水質汚濁についての予測を再検討した.
河口堰の建設後,低流量の時期,特に晩春から秋にかけて,頻繁な浮遊藻類の発生が認められた.優占的な種類は,中心珪藻類であり,夏にはCyclotella atomusとC. meneghinianaが,冬には,Stephanodiscus spp.が目立った.また,藍藻類の集積現象が見られることもあった.河川軸に沿った藻類の分布は,流量によって変化する増殖と沈降の速度により決定された.浮遊藻類相とその密度の変動から,河口堰の運用により,堰湛水の滞留日数が長期化し,真の河川棲浮遊藻類の増殖が促進されたものと判断された.藻類生産の増加にも関わらず,法に基づく環境基準は概ね達成されているが,これは堰直上流での沈降と動物プランクトンによる摂食の効果と考えられる.水利用に関しては,河口堰湛水が給水されている浄水場での活性炭の使用量と藻類量の変動が無関係ではないことが注目される.
河口堰運用後の河川富栄養化の現状評価と改善には,日本の河川における一次生産の概念の見直しと,誤った理解に基づく過去の予測の見直しが必要である.

2001年9月3日受付，2001年12月4日受理
e-mail: murakami@nagoya-wu.ac.jp