琵琶湖泥質粒子の堆積メカニズム  [in Japanese] Sedimentation mechanism of fine materials in Lake Biwa  [in Japanese]

湖水域における富栄養化の原因となる栄養塩類の移動メカニズムを明らかにするため, 湖底堆積物の分布状態を調査し, それをもとに堆積モデルを設定し, 堆積モデルの検証調査を実施した。この根拠として, 栄養塩類の水中での挙動が泥質粒子の挙動と似ていることがあげられる。堆積物の水平分布様式を説明できるモデルとして, 「cloud」モデルを考えた。これによって, 堆積速度と水深の相関が説明できる。多量の雨水が流入した直後の湖水の水中懸濁物濃度を調べると, 全層で懸濁物濃度が上昇していた。雪解け時以外では, さらに水温躍層以浅および湖底付近に他の部分の2倍程度の懸濁物濃度が観測されたが, これらは堆積作用にあまり関与していない。この結果, 「cloud」モデルが証明された。なお, 重金属濃度の水平分布もこのモデルを基本とし, 流入源や重金属濃度の異なる「cloud」どうしの混合として説明できる。

A sedimentological study on the distribution of lake-bottom sediments in Lake Biwa, elaboration of a sedimentation model and an evaluation survey of that model were carried out in order to clarify the sedimentation mechanisms of nutritious substances which lead to the eutrophication of water bodies. These studies are based on the fact that the behavior of nutritious substances in water is very similar to that of suspended sediments in water. The "cloud" model is used to explain the distribution pattern of sediments in the area. Using this model, the correlation between water depth and sedimentation rate is explained. Concentration of suspended materials immediately after a heavy rainfall in the area shows that concentration of suspended materials grows larger in the whole area. In seasons other than thaw the concentration of suspended materials at depths shallower than the thermocline and near the lake bottom is about twice that of other depths, However these suspended materials do not play as important a role as those of other depths. These results lead to the conclusion that the "cloud" model is very attractive. In addition, distribution patterns of heavy metal contents can be explained by this model.