Estimating turbulent diffusion in a benthic boundary layer

Abstract

In aquatic environments, the benthic boundary layer (BBL) is the transition zone for dissolved solutes that are released or consumed from the sediments. The exchange of solutes between the sediment and the overlying water column depends on the turbulent transport in the benthic boundary layer. In situ measurements of turbulent diffusion in natural benthic boundary layers are scarce and are usually derived from the logarithmic law of the wall (log‐law). Based on G. I. Taylor's turbulent diffusion theory, we derived a simple approach to estimate turbulent diffusivity from acoustic Doppler velocimeter (ADV) data. The approach was applied to ADV data collected over a period of 155 h in the BBL of Lake Constance, Germany. The calculated turbulent diffusivities agreed well with those derived in parallel from flux‐gradient measurements. In addition, turbulent diffusivities were calculated from several established approaches, including those based on the logarithmic law of the wall. The log‐law failed to predict plausible diffusivities whenever the boundary flow exhibited decreased current velocities or stable density stratification. Under these conditions, the turbulent diffusivities calculated from the flux‐gradient measurement and from Taylor's theory were as low as 10−6 m2 s−1. These decreased turbulent diffusivities have the potential to control the solute exchange between the sediment and the water column and can result in low oxygen concentrations in the bottom water. Reliable measurements of turbulent diffusivities in the BBL are therefore important to investigate and predict hypoxia in bottom waters and sulfide efflux from the sediments.