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応用生態工学 7(2), 165-172, 2005

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特集:標津川再生事業の概要と再蛇行化実験の評価 FEATURE:Shibetsu River Restoration Project
事例研究 CASE STUDY

ECEロゴマーク

標津川河跡湖の水質環境

野崎健太郎1)*・ 紀平征希2)・ 山田浩之3)・ 岸 大弼4)・布川雅典3)・ 河口洋一5)

1)椙山女学園大学人間関係学部 〒471-0131 愛知県日進市岩崎町竹ノ山37-234
2) 滋賀県立大学湖沼環境実験施設 〒522-0057 滋賀県彦根市八坂町3165
3) 北海道大学大学院農学研究科 〒060-8589 北海道札幌市北区北9条西9
4) 北海道大学北方生物圏フィールド科学センター 〒060-0809 北海道札幌市北区北9条西9
5) 独立行政法人土木研究所自然共生研究センター 〒501-6021 岐阜県羽島郡川島町笠田官有地無番地

Kentaro NOZAKI1)*, Masaki KIHIRA2), Hiroyuki YAMADA3), Daisuke KISHI4), Masanori NUNOKAWA3) and Yoichi KAWAGUCHI5): Limnological characteristics of an oxbow lake in a lower reach of the Shibetsu River in Hokkaido Island, Japan.. Ecol. Civil. Eng. 7(2), 165-172, 2005.

1) School of Human Sciences, Sugiyama Jogakuen University, Takenoyama 37-234, Iwasaki-cho, Nisshin, Aichi 471-0131, Japan
2) Limnological Laboratory, The University of Shiga Prefecture, Hassaka 3165, Hikone, Shiga 522-0057, Japan
3) Graduate School of Agriculture, Hokkaido University, Kita 9, Nishi 9, Kita-ku, Sapporo, Hokkaido 060-8589, Japan
4)Field Science Centre for Northern Biosphere, Hokkaido University, Kita 9, Nishi 9, Kita-ku, Sapporo, Hokkaido 060-0809, Japan5)Aqua Restoration Research Centre, Incorporated Administrative Agency, Public Works Research Institute, Kasada, Kawashima, Gifu 501-6021, Japan

Abstract:Limnological characteristics of a shallow oxbow lake (maximum depth about 2 meters) were investigated in a lower reach of the Shibetsu River in Hokkaido Island, Japan. Water temperatures, under-water light attenuation coefficients, and concentrations of dissolved oxygen, nitrogen, phosphorus and chlorophyll-a were measured on July 21 and November 17 in 2001 and July 30 in 2002. Water temperature varied among sampling stations and depths, ranging from 10℃ to 24℃ in July, while all measurements were constant at about 5℃ in November. Dissolved oxygen concentrations at all stations on all sampling dates were 10 mg L-1 or more. Its maximum concentration reached 25 mg L-1 with 250% saturation at the lake bottom on July 21 in 2001. The depths of sampling stations that obtained high concentrations of dissolved oxygen were between 50 cm and 60 cm. Water temperatures at these depths ranged from 10℃ to 15℃, which were lower than surface water by 5℃ to 10℃. A large filamentous green alga, Spirogyra sp., propagated in these depths. Concentrations of dissolved nitrogen in lake water varied from 11 μg L-1 to 250 μg L-1, which declined in July at all stations and depths. Concentrations of PO43--P showed a range from 7 μg L-1 to 14 μg L-1, not so different among stations, depths and sampling dates. Particulate phosphorus and chlorophyll-a concentrations were approximately constant between 33 μg L-1 and 35 μg L-1 and 10 μg L-1 and 13 μg L-1, respectively. The concentrations of total phosphorus and chlorophyll-a on 21 July 2001 indicate that trophic status of the oxbow lake is in meso-eutrophic conditions. Under-water light attenuation coefficients (k) ranged from 1 m-1 to 2 m-1 at all stations and sampling dates. These values are comparable to the maximum value for eutrophic lakes with water-bloom of phytoplankton. However, because chlorophyll-a concentrations in this oxbow lake were lower than those in the eutrophic lakes, under-water light seemed to be attenuated by non-living particles and dissolved organic matter. Therefore, there is a possibility that the oxbow lake is in dystrophic conditions. These results suggested that the limnological characteristics of the oxbow lake were greatly different from those of the main channel of the Shibetsu River. It is believed that the oxbow lake maintains the characteristics of a low-land marsh, which was former landscape of the lower region of the Shibetsu River watershed.

Key words: dissolved oxygen, dystrophic condition, oxbow lake, trophic status, under-water light attenuation

摘要

標津川下流域(北海道標津町)に位置する浅い河跡湖(最大水深2m)の水質環境(水温,水中光の消散係数,溶存酸素,窒素,リン,クロロフィルa)を2001年7月21日,11月17日,2002年7月30日に調査した.水温は7月には地点間,水深間で10〜24℃の違いが観察された.11月にはほぼ5℃で均一であった.溶存酸素濃度は常に10mg L-1以上を示し,最大値は,25mg L-1,飽和度で250%に達し,2001年7月21日に湖底付近で観察された.高い溶存酸素濃度が得られた地点は,水深が60〜100cmで,表層より水温が5〜10℃低く(10〜15℃),大型糸状緑藻Spirogyra sp.が繁茂していた.湖水中の溶存態窒素濃度は,4〜250mg L-1の幅で変動し,7月に大きく低下した.リン酸態リン濃度は,7〜14mg L-1の幅で変動したが,溶存態窒素に比べて変動の幅は小さかった.懸濁態のリン量は33〜35mg L-1,クロロフィルa量は10〜13mg L-1であり,おおよそ一定であった.夏期の湖水中の全リン濃度とクロロフィルa量は,この河跡湖が中栄養と富栄養の中間の水質を持つことを示した.水中光の消散係数は,1〜2m-1であり,富栄養湖の最大値に匹敵した.湖水中のクロロフィルa量は富栄養湖ほど多くはないので,水中光を大きく減らしているのは,植物プランクトン以外の懸濁物質や溶存有機物であると考えられる.河跡湖周辺の原風景が低湿地であったことを考えると,この河跡湖は湿地に多く見られる腐植栄養的な性質を持つ水環境である可能性が高い.これらの研究結果から,河跡湖の水質環境は,現在の標津川本川とは大きく異なっており,むしろ,かつての低湿地環境が残存している場であることが推定される.

line 2004年1月27日受付,2004年5月10日受理
1) e-mail: ken@hs.sugiyama-u.ac.jp line
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