Breathing sediments: The control of diffusive transport across the sediment—water interface by periodic boundary‐layer turbulence

We performed combined in situ measurements of bottom boundary-layer turbulence and of diffusive oxygen fluxes at the sediment‐water interface in a medium-sized mesotrophic lake. The turbulence was driven by internal seiching with a period of 18 h. This periodic forcing, a prominent feature of enclosed water bodies, led to distinct deviations of the structure and the dynamics of the bottom boundary layer from the classical law-of-the-wall theory. A major feature was a phase lag between the current velocity and the turbulent energy dissipation of approximately 10% of the seiching period (1.5‐2 h). The oxygen flux into the sediment was controlled by the diffusive boundary layer, the thickness of which varied between 0.16 and 0.84 mm during the course of a seiching period, and was strongly affected by the periodic bottom boundary-layer turbulence. The rate of dissipation of turbulent energy in the bottom boundary layer allowed us to define the Batchelor length for dissolved oxygen, which quantifies the smallest scales of oxygen fluctuations and provides an appropriate scaling for the diffusive boundary-layer thickness and the corresponding oxygen fluxes. An analysis of the governing time scales revealed the importance of turbulence in controlling the small-scale spatial heterogeneity of the diffusive fluxes. Higher turbulence causes the diffusive boundary layer (DBL) to follow the sediment topography more smoothly, resulting in an increased area-averaged flux due to the greater effective surface area. After surface zones, the bottom boundary layer (BBL) is the second prime site for animals, plants, and microorganisms in natural waters. From a physical and geochemical point of view, the importance of the BBL is twofold. First, the BBL is a major energy sink for basin-scale currents due to bottom friction and also due to the breaking of propagating internal waves on sloping bottoms (Imberger 1998). Consequently, the level of turbulence is enhanced in the BBL compared with the interior water body. Second, the BBL controls the exchange of solutes and particles between water and sediment. The sediment surface is usually an enormous sink of oxygen due to the processes caused by the decomposition of organic matter. Furthermore, the redissolution and subsequent vertical transport of ions and other solutes supply primary producers with nutrients and affect the stability of the water column by chemical (salinity) strat