短報　SHORT COMMUNICATION

小室 隆^１）・山室 真澄^１）＊

１）東京大学大学院新領域創成科学研究科　〒277-8563 千葉県柏市柏の葉5-1-5 環境棟562

Takashi KOMURO^１） and Masumi YAMAMURO^１）＊ : Estimation of the aquatic plant community area in Lake Shinji reconstructed with the aerial photographs taken by the US forces in 1940's. Ecol. Civil Eng. 16(1), 51-59, 2013.

1）Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563 Japan

Abstract: We reconstructed the distribution of aquatic plant community, including submerged plant, in Lake Shinji using aerial photographs taken by the US forces in October 1947. We identified the submerged plant community with the following features: shade and darker than sand and open water; profile, round in shape and existing below the water surface. We found only submerged plant community in the lake. Although Phragmites australis has been widely planted as the restoration project at Lake Shinji, emergent plant community was found only near the river mouths. Floating-leaved plant community or submerge plant community with extended leaves, that protrude from the water surface, were not found in the lake. The area of the submerged plant community was calculated as ca. 3km². Transparency had been formerly estimated to be 3m, but our results suggest that it was more than 4m in 1947. At present, the lake bottom at a depth of 4m consists mostly of mud. On the other hand, data from 1947 shows that the lake bottom was composed partly of sand, based on its reflection of white light and the existence of a ripple at the bottom. The deepest areas covered with submerged plant community were found at the center of the north shore of Lake Shinji, and its depth was estimated about 4m. Recently, submerged plants with extended leaves, that protrude from the water surface, have invaded Lake Shinji. Our results show that such type of submerged plants are not the same species or the same type of those found in the past which dominated the area.

Key words: Charophyte, emergent plant, submerged plant, transparency, nature restoration

摘要

島根県に位置する宍道湖を対象に，1947年10月に撮影された米軍空中写真を用いて，沈水植物を含む水草の分布範囲を復元した．沈水植物の判読は，「水域や砂地より暗く，輪郭部が丸みをおび，波の下に存在する」という基準に従って行った．写真判読の結果，沈水植物以外の水草（抽水植物・浮葉植物）は湖内では確認されなかった．現在の宍道湖では自然再生としてヨシが植栽されているが，抽水植物が確認されたのは流入河川の一部に限られた．浮葉植物，もしくは水面上まで葉を伸ばす沈水植物は，全写真で確認できなかった．沈水植物の面積は約3km²と計算された．また当時3mとされていた透明度は，本研究の結果では4m以上と判断された．現在は泥質である水深4mの湖底の一部は，光を強く反射し白くなっていることや，一部の写真で白い部分に砂漣が認められたことから，砂質であったと推定された．沈水植物は，宍道湖北岸中央部で最深となる水深4mまで繁茂していた．宍道湖では近年，葉を水面まで展開するタイプの沈水植物が侵入している．本研究の結果から，これらは過去に優占していた種・タイプではないことが確認された．

2012年9月25日受付, 2013年8月8日受理
＊e-mail: yamamuro@k.u-tokyo.ac.jp