Physical processes associated with hypoxia events in a shallow lake, Lake Kasumigaura, Japan, are investigated with long-term mooring observations at the middle of the lake basin. Results show that strong stratification during the summer suppresses vertical mixing resulting in hypoxia in the bottom boundary layer. The dissolved oxygen decreases due to the limited oxygen supply under strongly stratified conditions. The intensity of the stratification is controlled by the ratio between surface wind stress and the surface incoming buoyancy flux. The Monin–Obukhov length scale explains the stratification and shows good agreement with the Froude number and Wedderburn number. The stratification is also enhanced by the heat flux towards the sediments, reaching O (100) W m⁻², during the summer season. In addition to field observations, this study conducted numerical simulations to further investigate mixing structure. Results from numerical simulations indicate that the bottom sediment heat flux significantly contributes to mixing and stratification in the lake. This study suggests that mixing processes associated with winds and heat flux at surface and bottom sediments are the key to understanding hypoxia events in Lake Kasumigaura.

中文翻译：

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与浅水湖缺氧相关的分层和混合过程（日本霞浦湖）

通过对流域中部的长期系泊观测，调查了日本霞浦湖浅湖中与缺氧事件相关的物理过程。结果表明，夏季强烈分层会抑制垂直混合，导致底部边界层缺氧。由于在强分层条件下有限的氧气供应，溶解的氧气减少。分层的强度由表面风应力与表面进入的浮力通量之比控制。Monin–Obukhov长度标尺解释了分层，并与Froude数和Wedderburn数显示出很好的一致性。朝向沉积物的热通量达到O（100）W m ^-2也会增强分层，在夏季。除现场观察外，本研究还进行了数值模拟，以进一步研究混合结构。数值模拟结果表明，底部沉积物的热通量显着促进了湖泊的混合和分层。这项研究表明，与风和表面和底部沉积物的热通量有关的混合过程是了解霞浦浦湖缺氧事件的关键。