汽水湖 (久美浜湾) における貧酸素水塊の解消過程 --冬季の鉛直混合を阻害する淡水流入--

Abstract

汽水湖である久美浜湾では, 冬季に貧酸素水塊が二枚貝養殖に被害を及ぼすが, その消長の詳細は未解明である。これを明らかにするため, 水温, 塩分, 溶存酸素濃度の分布の季節変動を調べた。貧酸素水塊の解消過程では, 亜熱帯性の湖沼や閉鎖性内湾でみられるような冬季の鉛直混合はみられず, 12月から3月にかけて, 湾口部からの外海水の進入および河川水の連行 (entrainment) によって貧酸素水塊が徐々に縮小した。久美浜湾のような本州中部日本海側に位置し, 流入河川の流域の標高が高くない汽水湖では, 冬季に降水量が多く, 降雨や数日で融ける降雪がただちに淡水として海域に流入するため, 上層に強い密度成層が形成され, 鉛直混合が阻害されていた。その結果, 夏季に底層で発生した貧酸素水塊は, 冬季まで残存し, 湾口から流入した外海水によって持ち上げられ, 中層貧酸素水塊となり, 養殖漁場へと到達することが明らかになった。

In Kumihama Bay, Kyoto, which is a brackish lake, hypoxic water is known to damage aquacultural bivalve in winter; however, the dynamics of hypoxic water mass has not been elucidated. To clarify this dynamics, we observed the monthly spatial distributions of water temperature, salinity, and dissolved oxygen concentration. In the breakdown process of hypoxic water mass, a decrease in hypoxic water mass induced by the tidal intrusion of seawater from the bay mouth from December to March and river flow entrainment was observed. Although cooling vertical mixing is generally observed in winter in lakes and bays in a temperate zone, that was not observed in Kumihama Bay. In Kumihama Bay, a brackish lake located in the central Honshu Island (mainland Japan) along the Sea of Japan, watershed precipitation is high in winter, and rainfall and snowfall that melts in a few days immediately flow into the bay. This flow of fresh water causes strong salinity and density stratifications in the upper layer of the bay, obstructing the cooling vertical mixing. As a result, the hypoxic water mass formed in summer remained until winter. In winter, the seawater that intruded from the bay mouth lifted the bottom hypoxic water mass to the middle layer, and the hypoxic water reached the bivalve aquaculture fields.