原著論文 ORIGINAL PAPER

Seasonal changes of submerged macrophyte community with different depth profile in a calm bay of Lake Biwa, Japan.

Akihisa HATTORI

Faculty of Liberal Arts and Education, Shiga University, 2-5-1 Hiratsu, Otsu, Shiga 520-0862, Japan

服部昭尚: 琵琶湖の穏やかな湾内における水深構造の違いと沈水植物群落の季節変化. Ecol. Civil. Eng. 7(1), 1-11, 2004.

滋賀大学教育学部　〒520-0862　大津市平津2-5-1

Abstract:The structure of a submerged macrophyte community （vegetation cover, species richness and species composition） and the seasonal changes were investigated at the littoral zone （＜6m deep） of a calm bay in Northern Basin of Lake Biwa, Japan, where the transparency is high and no waterfowl grazers exist. To evaluate the effect of a small difference in depth profile on the community structure, the vegetation map of 35　m-belt transect （4m wide） was made monthly at two sites （30m apart） between March and November 1997. In total, the same 11 species were observed in the two sites, including two alien species, Egeria densa and Elodea nuttallii, and an endemic species, Vallisneria biwaensis. There was no significant difference in species richness in a census between the two sites. The species zonation was unclear in both sites: various mixed species patches were found throughout the study period. There were differences in the growth patterns of five species between the two sites. In a calm and clear-water littoral zone, where there is no disturbance by wave action, a small difference in depth profile can affect the growth patterns of several macrophyte species and then affect the three-dimensional structure of the macrophyte community, which can be a crucial determinant of the habitat structure of small aquatic animals.

Key words: depth profile, growth pattern, Lake Biwa, seasonal change, submerged, macrophyte

摘要

穏やかで透明度が高く，水鳥がほとんど生息しない琵琶湖の奥出湾（西浅井町大浦）において， 沈水植物群落の構造とその季節変化をSCUBA潜水により調査した．水深構造の違いが沈水植物群落へ及ぼす影響を把握できるように， 岸から35mのベルトトランゼクト（幅4m）を水深構造が異なり30m離れた2地点に設置し，1997年5月から11月まで毎月1回，水中植生図を作成した． 両地点とも，外来種2種（オオカナダモとコカナダモ）と固有種1種（ネジレモ）を含む11種が観察され，毎月の出現種数に地点間で有意差はみられなかった． 両地点とも帯状分布が不明瞭で，混合群落がモザイク状に分布していた． 少なくとも5種類（ホザキノフサモ，オオササエビモ，エビモ，ヒロハノセンニンモ，ヒロハノエビモ）には，2地点間で成長パターンに違いが見られた． 本調査場所では，沈水植物の種組成を決める主要因の一つとされる波浪の影響（波浪による撹乱）が少なかったため， 明瞭な帯状分布が成立しなかったと考えられる．本研究は，水深構造のわずかな違いが，種組成や種数ではなく， いくつかの種の生長パターンに影響を及ぼすことを明らかにした． また，種毎の生長パターンの違いは，水生動物の生息地に重要な沈水植物群落の立体構造に変化をもたらす可能性がある．

Recieved 19 August 2003; Accepted 25 March
e-mail: hattori@sue.shiga-u.ac.jp