Evaluation of a Re-meandering Project in the Shibetsu River From an Aspect of Primary Production Process

27 Mar Evaluation of a Re-meandering Project in the Shibetsu River From an Aspect of Primary Production Process

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Abstract

本研究では河川における平面形状の違いが物質・エネルギーの動態に及ぼす影響を把握することを目的として，標津川の再蛇行化を取り上げ，一次生産速度に及ぼす影響を直線区間と蛇行区間の比較から明らかにし，その要因を分析した．観測結果から，蛇行区間における一次生産速度は直線区間と比較して大きかった．河床付着物中のChl.a が蛇行区間で有意に大きかったことから，両区間における一次生産速度の差は蛇行区間における高い底生藻現存量に起因していると考えられた．また，河床表層を占める河床材粒の粒径は，蛇行区間では瀬・淵構造の発達に伴い相対的に大きく，砂の面積占有率は直線区間で大きかった．更に，蛇行区間では湾曲の影響により，砂は平常時においても掃流状態で流下していたのに対し，蛇行区間では掃流砂が横断方向に偏った分布を有し，横断面の一部でしか確認できなかった．底生藻の現存量が直線区間で相対的に小さく，一次生産速度が減少したのも，このような河床環境の違いを反映したものと考えられた．

English Abstract

A re-meandering project was implemented for the stream eco-system restoration of the Shibetsu River, Hokkaido Island, the northern part of Japan. It has been well known that the alteration of planview has an influence on stream structure such as pools and riffles, but there is only a little knowledge about changes in the energy flow of stream ecosystems. The present study evaluated the alteration of primary production rate brought by the re-meandering project comparing a re-meandering section with an upstream straight section. The single station diel O2 curve method and the light and dark bottle method were applied for the measurement of primary production rate, and both methods indicated that primary production rate was higher in the re-meandering section than the straight section. The a lysis of factors determining primary production rate showed that the standing crop of epilithic algae was lower in the straight section than the re-meandering section. The field observation gave the different view of lateral distribution of bedload transport between two sections: bedload transport in the straight section was considerably uniform across the channel, whereas it was heterogeneous and the transport was limited to the small area of the re-meandering section. Previous studies show that bedload distribution tends to be skewed in curved channels and the frequent collision of bedload particles with substrate leads to the standing crop reduction of epilithic algae. From these findings, we concluded that infrequent dislodgement of epilithic algae induced by the re-meandering treatment with small bedload area resulted to the high standing crop and it enhanced the primary production rate of the re-meandering section.