Sediment Oxygen Demand in Lakes: Dependence on Near-Bottom Flow Velocities

Abstract

A model of sediment oxygen demand (SOD) is presented which determines the SOD as a function of flow velocity over the sediment. A quantitative relationship is established between SOD and the velocity and dissolved oxygen concentration in the bulk water. Oxygen consumption in the sediment is expressed as the sum of biological consumption with Michaelis-Menten kinetics, and the chemical consumption assumed to be a first order reaction of oxygen. At very low flow velocities, transport through the diffusive boundary layer is the limiting factor of SOD, and SOD is expressed as a linear increasing function of velocity. On the other hand, when flow velocities are increased, SOD becomes independent of velocity, since the reactions in the sediment are the rate limiting factor. The model also suggests that SOD is an increasing function of dissolved oxygen concentration in the water overlying the sediment and that SOD has no upper limit when DO concentration is large. Combined with the linear theory of internal seiche motion an average SOD in a rectangular, two-layered lake is derived as functions of the wind velocity, aspect ratio of the lake an the depth of the thermocline. The average SOD has a minimum when the thermocline depth is 1/4 of the total depth.