Diffusive boundary layers and the oxygen uptake of sediments and detritus1

TLDR

Microelectrodes were used to measure diffusive boundary layer thickness and its effect on oxygen uptake in sediments and detritus. Oxygen gradients were detected within boundary layers 0.2–>1 mm thick, whose thickness varied with flow velocity and surface roughness, leading to diffusion times of 1.2–9 min and acting as a resistance that can render aerated sediment surfaces almost anoxic, thereby explaining the presence of microaerophilic or anaerobic bacteria.

Abstract

The thickness of diffusive boundary layers and their role for the oxygen uptake of sediments and detritus were studied by the use of microelectrodes. Gradients of oxygen were always detectable within the boundary layer, which varied in thickness from 0.2 mm to > 1 mm. The thickness depended on the flow velocity of the water and on the roughness of the solid surface. Oxygen diffused through the boundary layer with a mean diffusion time of 1.2–9 min. The diffusive boundary layer constituted a transfer resistance for oxygen flux across the solid‐water interface which limited the oxygen flux at high uptake rates. Sediments or detritus exposed to aerated water could therefore be almost anoxic at the surface, provided that they had sufficiently high rates of oxygen uptake. This can explain the occurrence of microaerophilic or anaerobic bacteria on exposed sediments where fully oxic conditions would intuitively have been expected.