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

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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.