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

本研究取得了以下研究成果：(1)在琵琶湖约90m深的湖底部署了多次平台，测量了湖底沉积物中的溶解氧浓度(DO)。我们在含氧水和缺氧沉积物之间发现了厚度为2-3 mm的薄层，认为这些薄层对应于粘性亚层。沉积物中DO的回收依赖于水和沉积物之间的温差。海底沉积物一旦缺氧，就不容易完全氧化。(2)我们还部署了ADCP-TP-1200F来测量海底边界层中的平均和湍流流动。在计算了各层的雷诺应力后，发现底泥表面的应力不足以使沉积物重新悬浮。这表明，边界层中的浑浊不是由切变不稳定引起的，而是由海底沉积物的沸腾引起的，如AUV“坦坦”号探测到的热液喷口。(3)建立了一个基于水动力和生态系统耦合方案的三维数值模拟模型，以了解湖底附近低溶解氧水域的形成和维持机制。利用该模型，我们发现由内波引起的振荡流在一个相当大的区域内周期性地移动着缺氧水。这一结果可能是重要的，因为它与低DO水区的产生及其对琵琶湖特有的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.