Fluxes of reactive chemical species across the sediment-water interface can profoundly influence the dominant biogeochemical cycles in the worlds ocean. However, reliable in-situ measurements of benthic fluxes of many reactive species cannot be carried out without adjustments of stirring rates inside benthic flux chambers to match boundary layer conditions prevailing outside. A simple method to compare flow levels consists of measurements of gypsum dissolution rates inside benthic chambers and on the seafloor. The measurement of the diffusion-controlled dissolution rate of gypsum allows the estimation of the diffusive sublayer thickness and the time-averaged bottom stress on the seafloor. This method had previously been intercalibrated with the stress sensor method in flumes and inside benthic chambers. We describe here free-vehicle deployments of alabaster plates on the bottom of the ocean which gave results consistent with hydrodynamic theory. Errors in the calculated diffusive sublayer thickness were estimated to be about 10--15% for typical deployment conditions in the ocean. Current velocities 5 m off the bottom, which were measured concurrently during two deployments, allowed for comparison with hydrodynamic predictions of diffusive sublayer thicknesses. The values obtained this way agreed within 15%.

The measured mass transfer velocity was found to correlate with the plate dimension L, to the power of 1/3. This confirms the theoretical procedure for extrapolating to infinite plate size when calculating the sublayer impedance of slute fluxes from sediments (where L is large). Typical values of diffusive sublayers thicknesses, corrected to infinite plate size, were 1200 μm for current velocities, U100, of 2 cm s-1, and 500 μm at 8 cm s-1. Furthermore, values of friction velocities calculated from alabaster dissolution were compared with those using stress sensors. Gypsum plate values of u* were 0 and 30% lower than skin friction values of u*, at u* values of 0.6 and 0.2 cm s-1, respectively, possibly as a result of bottom roughness effects. A case is made for the importance of considering boundary layer dynamical conditions when benthic fluxes are to be measured correctly. When rates of oxygen uptake, carbonate dissolution, and nutrient species uptake or release in regions near the sediment-water interface are fast compared with diffusion rates across the diffusive sublayer, overall rates can become mass transport controlled. ¿American Geophysical Union 1991