We have implemented two bottom boundary layer mixing sub-models in a one-dimensional bulk mixed-layer thermodynamic and dissolved oxygen model to diffuse the effects of the sediment oxygen demand from the bottom boundary condition in small lakes. In the first sub-model, bottom mixing is calculated following a mixed layer approach, whereas in the second sub-model the dissolved oxygen flux is computed from Fick's Law. The second sub-model results in better prediction of dissolved oxygen in two

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... ll Canadian Shield lakes, compared to the mixed-layer approach. While appropriate for the upper mixed layer, the bottom mixed-layer model produced excessive near-bed mixing. Bottom mixed layer approaches have been successful in large lakes, suggesting a Reynolds number dependence for mixed-layer development.