Oxygen microgradients and fluxes were studied in a 0.3-0.6-mm-thick di
ffusive boundary layer (DBL) of aquatic sediments by the use of O2 mic
roelectrodes with sensing tips of 5 mum. One microelectrode was introd
uced vertically from above while another was introduced along the same
vertical axis from below, thus penetrating up through the sediment. M
icroelectrodes introduced from below did not alter the DBL, but those
introduced from above reduced its thickness by 25-45%. The effect was
detected even when the microsensor tip was situated > 1 mm above the s
ediment surface. The reduction of the DBL resulted in steepening the O
2 gradient. In typical coastal sediments, the microelectrode thereby i
ncreased the diffusive O2 flux by 8%; in a highly sulfidic sediment th
e increase was 59%.