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In order to improve the DO condition of a lake or sea, authors are developing the liquid film gas dissolution technique. This paper presents a summary of this technique, experimental results of DO improvement, and performance evaluation method for the DO improvement considering the DO concentration of the water to be treated. This DO improvement method transforms the water into bubble clusters which essentially makes water elements of the thin film of bubbles. This method yields a large value of the dissolved gas concentration gradient in the water and therefore attains an efficient gas dissolution. Also, this method treats deep water utilizing the airlift effect by aeration in the shallow part of the pipe, thus saving energy. The authors have shown experimentally that the DO increment depends on the DO concentration of the water to be treated, and proposed the equivalent DO increment as an index of the DO increment which can be used irrespective of the DO concentration to be treated and which indicates the potential of the DO increment for the same working condition from the theory of the aeration device. The validity of this index was checked using the result of the present experiments.

Ecological traits of five Zostera marina populations in and around Hiroshima Bay were investigated in a luxuriant season of 2004. Study sites were located at the most inner (Ajina; St.1), center (O-kurokami-jima; St. 2) and near the mouth area (northern coast of Yahiro-jima; St. 3) of Hiroshima Bay, and at the inner area (Ihono-sho; St. 4) and the outside (Heigun-jima; St.5) of Yanai Bay which is the strait adjacent to Hiroshima Bay. The distribution depth (C.D.L.) was different among the populations. The lower depth of Zostera distribution was limited to 1-2 m at St.1 and 2 by algal-mat of Ulva spp. and other macroalgae, though it was 4-6 m at St. 3-5. Mean biomass was ranged in 120-180g DWm-2, but shoot size and density was different among the populations. Mean density ranged between 88-278 shoots m-2, and the populations at St.3 and 5 exhibited higher densities with smaller shoots especially in shallow stands of the two population. These stands also exhibited higher proportion of below-ground biomass to total biomass. On the contrary, populations in inner areas (St. 1 and 4) exhibited lower density with larger shoots. Physical conditions such as water motion induced by waves were different among the habitats which were shown in grain size compositions of the sediments, and this could affect the ecological traits of each Z. marina population. 広島湾および周辺海域の5カ所の生育地のアマモZostera marinaの生態的特性を2004年の繁茂期に調査した。調査場所は広島湾奥部の阿品（St.1），同湾央部の大黒神島（St.2），同湾口部の屋代島（St.3），柳井湾奥部の伊保庄（St.4），および同湾外部に位置する平郡島（St.5）である。アマモの分布水深は生育地により異なり，St.1，St.2では海底でのアオサ類等の堆積により，分布下限水深は1-2m（C.D.L. 基準）までである一方，他の生育地では4-6mまで分布した。アマモの平均現存量は120-180g DWm-2の範囲にあったが，アマモの株のサイズと密度は生育地により異なっていた。平均株密度は生育地により88-278 shoots m-2の範囲であり，St.3およびSt.5では特に浅所で小型の株が密生した。これらの群落では，総現存量に占める地下部現存量の割合が相対的に大きかった。一方，湾奥部に位置するSt.1とSt.4では，アマモの株は大型である一方生育密度は低かった。群落内の底質の粒度組成の相違から，波浪流動等の物理的環境が生育地間で異なることが示唆され，それぞれの場所のアマモの生態的特性はその影響を受けていると考えられた。 本調査は，水産庁委託「生物多様性に配慮したアマモ場造成技術開発調査事業」（平成16～18年度）の一環として実施されたものである。