Oxygen uptake at the sediment-water interface simultaneously measured using a flux chamber method and microelectrodes: Must a diffusive boundary layer exist?

Oxygen uptake was measured in a laboratory flow system with natural marine sediments and semi-artificial cores, where bioturbation was suppressed. The flux rate across the sediment-water interface was simultaneously determined by flux chamber method and from microelectrode profiles at various flow velocities (laminar and turbulent flow). The total oxygen uptake from flux chamber measurements exceeded the flux calculated according to the concept of the diffusive boundary layer by a factor of 3.9 to 6.8 in both types of cores. Bioturbation or respiration of macrofauna can be excluded in the experiments with the semi-artificial sediment. The analysis of possible mechanisms which might have caused the observed discrepancy demonstrates that transfer in the boundary layer above sediments is not restricted to molecular diffusion: dispersion effects in the top layer of the sediment may cause an effective exchange mechanism which is much more efficient than molecular diffusion. (C) 1998 Academic Press Limited.