Study on the formation and maintenance mechanisms governing micro-structure of low dissolved oxygen water areas in a lake

我们从这项研究中得到了以下结果；(1)在琵琶湖水深约90m处多次部署湖底平台，测量湖底沉积物溶解氧浓度（DO）。我们在缺氧水和缺氧沉积物之间发现了2-3mm厚的薄层，并得出结论，这些层对应于粘性亚层。沉积物中DO的回收取决于水和沉积物之间的温差。(2)我们还部署了ADCP-TP-1200F来测量底栖边界层（BBL）的平均流量和湍流流量。通过对各层的雷诺应力计算，我们发现底部沉积物表面的应力不足以使沉积物重新悬浮。(3)基于水动力学和生态系统耦合方案，建立了湖底低溶解氧水域的三维数值模拟模型，以了解湖底低溶解氧水域的形成和维持机制。利用该模型，我们发现由于内波引起的振荡流动周期性地在相当大的区域内移动缺氧水。这一结果可能很重要，因为它与低DO水域的产生及其对琵琶湖特有底栖动物的影响密切相关。

We had the following results from this study ;(1) We deployed a lake bottom platform several times at about 90m depth in Lake Biwa to measure the dissolved oxygen concentration (DO) in the bottom sediments. We found thin layers of 2-3mm thickness between oxic water and anoxic sediment and conclude that these layers correspond to viscous sub-layers. The recovery of DO in sediments depends on the temperature difference between the water and sediment. It is not easy to oxidize bottom sediments completely once they become anoxic,(2) We also deployed an ADCP-TP-1200F to measure mean and turbulent flows in benthic boundary layers (BBL). After calculation of Reynolds stress in the layers, we found the stress on the surface of bottom sediments was not great enough to re-suspend sediments. This suggests that turbidity in the BBL is not caused by shear instability, but originated from bottom sediment ebullition such as hydrothermal vents detected by the AUV "Tantan",(3) We developed a three dimensional numerical simulation model based on a hydrodynamics and ecosystem coupling scheme to understand the formation and maintenance mechanisms of low dissolved oxygen water areas near the lake bottom. Using this model, we found the oscillation flow due to internal waves periodically moved the anoxic water over a rather wide area. This result can be important, because it is deeply related with the generation of low DO water areas and their influence on benthic animals endemic to Lake Biwa.