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

Diffusional characteristics of two biologically active surface sedimen ts were determined by use of a combined N2O-O2 microsensor. By analyzi ng changes in the N2O-gradients in these sediments, it was possible to determine the product (phiD(s)) for this species with submillimetre d epth resolution, where phi is the porosity and D(s) the substrate diff usion coefficient. The (phiD(s))-value for O2 could be calculated then from (phiD(s))-values for N2O, because the diffusivity of the two mol ecules were modified in the same way within the sediment. Both sedimen ts exhibited fine-scale horizontal and vertical variability in diffusi on characteristics, and this must be accounted for when analyzing micr oprofile 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 de pth), 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 mea sured (phiD(s)) for the estuarine sediment; however, the value for the sediment covered by a microbial mat, with dense populations of meiofa una, 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.