Influence of radial pressure gradients on solute exchange in stirred benthic chambers

Bromide was used as a conservative tracer for evaluating the effect of stirring-induced radial pressure gradients in 2 typical benthic chamber designs. One chamber was square, w ~t h side lengths of 30 cm (volume 9.0 l), and the other was cylindrical, with a diameter of 19 cm (volume 2.5 l). It was demonstrated that radial pressure gradients associated with chamber water rotation could induce advectlve porewater transport In both chambers. The intensity of the advective porewater transport was a function of sediment pel-meability and stlrring rate. Stirrrd at 12 rpm, solute transport chdnged from diffusive to advective at a sediment permeability >2 X 10-'' in the square chamber and >5 X 10.'' m-' in the cylindrical chamber. The srdimrnt permeability at which solute exchange was still controlled by diffusion could be increased h). almost 1 order of magnitude in the square chamber by decreasing the stirring rate to 7 rpm. The sediment permeability values can be used as a guide to when benthic chamber experiments can be performed without introducing a serious stirring-induced artefact.