The asphyxiation technique: an approach to distinguishing between molecular diffusion and biologically mediated transport at the sediment-water interface [Burrowing macrofauna species, bioturbation]

On the assumption that lack of oxygen will cause the activity of benthic infauna to cease and thereby limit solute transport to that accomplished by molecular diffusion, we have measured the flux of dissolved silicate across the sediment-water interface of sediments incubated in situ under parallel benthic flux chambers. In one chamber the oxygen concentration was allowed to go to zero, and in the other the oxygen and pH remained at ambient values. As long as oxygen was present in the chambers, the measured summer and fall fluxes were 2-10 times those calculated from porewater profiles assuming one-dimensional molecular diffusion. The differences in fluxes predicted from the densities of macrobenthos at two different sites agree well with the flux measurements. After oxygen depletion, the silicate fluxes decreased and approached the values calculated on the basis of molecular diffusion. In one experiment the ratio between the oxic and anoxic fluxes was nearly identical to the ratio between the effective diffusion coefficient, as measured under oxic conditions using $^22Na$ as a tracer, and the whole-sediment molecular diffusion coefficient. In winter $(-1^\circC)$ measured and calculated Fickian fluxes agreed closely, indicating a negligible effect of benthos on solute transport.