A theoretical investigation of the diffusive boundary layer in benthic flux chamber experiments

Studies of sediment dynamics often use semi-enclosed chambers that are partly pressed into the sediment. To mimic the oceanic turbulence in benthic chambers, the water is stirred by a rotating magnetic-bar. The stirring device creates random turbulence as well as a rotating circulation in the chamber. Close to the sediment surface, the rotating circulation vanishes and it is replaced by an inward flowing viscous boundary layer. This near-surface flow is important for the concentration of a solute at the sediment surface and thereby also for the exchange of properties between the chamber and the sediment. In this study we investigate the influence of the near-sediment flow structure on the sediment–water exchange in benthic chambers. We apply an advective–diffusive model to calculate the near-bottom distribution of solutes. The exchange process associated with large-scale flow is quantified by calculating a diffusive boundary layer thickness, which admits a comparison with the diffusive boundary layer thickness in a turbulent environment. Qualitatively, the model describes available measurements of exchange rates, indicating that the laminar boundary-layer flow controls the water-to-sediment exchange of solutes to the lowest order. According to the model, the diffusive boundary-layer thickness is proportional to the radius of the chamber and the inverse square root of the circulation velocity. Further, we argue that the size of the stirrer is important for the structure of the diffusive boundary layer and thereby the exchange rates.