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

Abstract

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) stratification (Wüest and Gloor 1998).Especially in eutrophic and mesotrophic systems with 1 Corresponding author (andreas.lorke@eawag.ch). AcknowledgmentsWe thank C. Dinkel and M. Schurter for their great help in the field. D. McGinnis kindly improved the English. We gratefully acknowledge the helpful criticism of two unknown reviewers.

Is phosphorus retention in autochthonous lake sediments controlled by oxygen or phosphorus?

“…Due to the large pool of reduced material in the sediment and the diffusive boundary layer, oxygen is consumed at the sediment surface, resulting in still anoxic sediments in deeper strata (Müller et al 2002;Lorke et al 2003). In addition, seasonal biogenic calcite precipitation forms sediment layers of several millimeters that may not adsorb much P (Hupfer et al 2000) but that act as a diffusion barrier between sediment and water, further inhibiting downward penetration of DO.…”

Interbasin exchange and mixing in the hypolimnion of a large lake: The role of long internal waves

“…Second, we will demonstrate that Kelvin-type waves in Lake Geneva lead to a turbulent bottom boundary layer (BBL) in the side basin, with a thickness of several meters and turbulence levels comparable with those in the upper mixing layer under strong winds. As recently pointed out by Lorke et al (2003), the presence of such a BBL is of considerable importance for the hypolimnetic oxygen budget because it may completely control the flux of oxygen through the viscous sublayer into the sediment. Additionally, the existence of a turbulent BBL with strong vertical velocity gradients and high diffusivities suggests that shear dispersion may be an important mechanism.…”

“…The model (i) has been thoroughly tested in a large number of unstratified flow cases and shown to be superior to other two-equation models, such as the k-model (Wilcox 1998), (ii) does not require any wall functions to predict turbulent quantities in the law of the wall (Wilcox 1998), (iii) computes correct mixing efficiencies in stably stratified shear flows (Canuto et al 2001), (iv) is in excellent agreement with experimental data for mixed layer deepening by surface-wind stress (Umlauf et al , 2005, and (v) predicts the correct height of a sheardriven turbulent BBL and correct dissipation rates within the BBL (Lorke et al 2002;Umlauf et al 2005).…”

Shear-induced convective mixing in bottom boundary layers on slopes

“…Owing to its pronounced seiching, Lake Alpnach has been the subject of numerous previous studies, emphasizing the structure and dynamics of the internal seiching (Münnich et al 1992;Wüest et al 2000), the seiche-induced BBL (Gloor et al 1994;Lorke et al 2002), as well as the control of sediment-water exchange by BBL turbulence (Lorke et al 2003).…”