Vertical circulation and formation of anoxic layer. Case study at dredged area in southern basin of lake Biwa.

It is well known that the deficiency of dissolved oxygen or the formation of an anoxic layer is observed in the hypolimnion of a stratified eutrophic lake in a heating period. The authors intended to clarify the mechanism of the formation of this anoxic layer by means of in situ data and a simple numerical model, focussing on vertical diffusivity, the precipitation rate of chemical component estimated by COD and the second order rate constant between chemical component, i. e. COD, and dissolved oxygen, which are the most important parameters for predicting the change of dissolved oxygen concentration in the hypolimnion.The results showed that the consumption of dissolved oxygen in the bottom water is strongly dependent on the precipitation rate of organic matter and rate constant, where the dissolved oxygen is transported from surface to bottom by vertical diffusion. Hence, the faster the precipitation rate is, the more the anoxic layer tends to occur in the hypolimnion. And if the precipitation rate is fixed, the anoxic layer is most liable to occur at the particular rate constant.