短報 SHORT COMMUNICATION

河口域に見られる二つのプランクトン群集−起源, 挙動, 及び河口堰がプランクトン群集に及ぼした影響

村上哲生¹⁾・ 西條八束²⁾

1) 名古屋市環境科学研究所 〒457-0841 名古屋市南区豊田5-16-8
2) 〒464-0063 名古屋市名東区新宿2-247

Tetuo MURAKAMI¹⁾, Yatsuka SAIJO²⁾: Two planktonic communities in rivermouths - their origin, behavior and effects of a rivermouth barrage on planktonic communities. Ecol. Civil. Eng. 2(2), 199-204, 1999.

1) Environmental Pollution Research Institute of Nagoya City, 16-8 Toyota, Minami-ku, Nagoya 457-0841, Japan
2) 2-247 Shinjuku, Meito-ku, Nagoya 465-0063, Japan

Abstract:Data continuously recorded for one hour on chlorophyll a at lower reaches of the Nagara and Kiso Rivers made it clear that two planktonic communities with different origin have developed at their rivermouths. One was a community dominated by Cryptophyceae or Dinophyceae originating in a brackish or marine environment. Changes in their biomass synchronized with that in salinity. The other was a potamo planktonic diatom community drifting down from upstream, and the maximal algal biomass was recorded at low tide.
A rivermouth barrage in the Nagara enhanced potamoplanktonic algal production in the newly created freshwater region by prolongation of the river water retention time. Moreover a large algal biomass downstream of a construction site in the Kiso suggested that brackish or marine planktonic algae accumulation resulted from obstruction of dilution with freshwater and inhibition of exchange of the rivermouth water. The mechanism by which the algal biomass of each community was controlled after construction was different. For assessment and improvement of river eutrophica tion after barrage construction, a clear distinction between the two planktonic commu nities should be considered.

Key words: Kiso River, Nagara River, planktonic community, river eutrophication, rivermouth barrage

摘要

長良川・木曾川下流部におけるクロロフィルαの連続観測記録の解析により,淡水域と汽水・海域に起源を持つ二つの浮遊藻類群集が両川の河口部で見られる事が確認された.前者は,河川穣浮遊珪藻群集が河口域に流下したものであウ,その現存量は塩化物イオンの変化と逆位相の変動を示した.一方,後者は,汽水域から遡上した褐色鞭毛藻類,渦櫛鞭毛藻類を優占種とする群集で満潮時に高い現存量が記録された.
長良川河口堰は,その上流の湛水域における滞留日数を長期化することにより河川儀浮遊珪藻群集の増殖を促進することが示され,また,木曾川下流域においては, 堰による取水が,淡水による着駅や水の交換を阻害し, 汽水由来の浮遊藻類が集積する可能性が示唆された.河口構築物が由来の異なる二つの浮遊藻類群集の現存量の変化に及ぼす機構はそれぞれ異なっており,水質の現状評価や改善の施策の検討に当たっては,両群集を区別して考える必要がある.

1999年5月27日受付, 1999年7月29日受理
1) e-mail: kankaken@cjn.or.jp