DIFFUSIVITY IN SURFICIAL SEDIMENTS AND BENTHIC MATS DETERMINED BY USE OF A COMBINED N2O O-2 MICROSENSOR

Diffusional characteristics of two biologically active surface sediments were determined by use of a combined N2O-O2 microsensor. By analyzing changes in the N2O-gradients in these sediments, it was possible to determine the product (phiD(s)) for this species with submillimetre depth resolution, where phi is the porosity and D(s) the substrate diffusion coefficient. The (phiD(s))-value for O2 could be calculated then from (phiD(s))-values for N2O, because the diffusivity of the two molecules were modified in the same way within the sediment. Both sediments exhibited fine-scale horizontal and vertical variability in diffusion characteristics, and this must be accounted for when analyzing microprofile data. The average (phiD(s))-value for N2O at 20-degrees-C for an estuarine surface sediment was 0.93 x 10(-5) cm2 s-1 (at 0-4 mm depth), while the value for the upper 2 mm of a stream sediment covered by a microbial mat was 1.42 x 10(-5) cm2 s-1. Biological inactivation and oxidation by exposure to an O2 atmosphere had no effect on the measured (phiD(s)) for the estuarine sediment; however, the value for the sediment covered by a microbial mat, with dense populations of meiofauna, decreased by 20%. The method presented is ideal for measurements of diffusivity at a high spatial resolution in surficial sediments and densely packed microbial communities.