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A several centimeter thick, [H.sub.2]S-oxidizing microbial mat was encountered in a high-latitude Arctic fjord. Microscopic investigations revealed that the mat contained numerous filamentous bacteria, and fluorescence in situ hybridization showed that the vast majority of these belonged to [gamma]-Proteobacteria and that Abstract Thiothrix spp. was present. The mat grew on a layer of accumulated decaying macroalgae and was characterized by a complex three-dimensional structure consisting of numerous holes, voids and chimney-like structures. Microsensor measurements performed in situ and in a laboratory-established mesocosm showed a highly dynamic [O.sub.2], pH and [H.sub.2]S distribution, especially around protruding structures of the mat. Detailed microsensor investigations documented that anoxic sulfidic water emerged trough the chimneys and indicated a microcirculation, replenishing the emerging water with oxygenated water in areas were the mat was absent. This governed an efficient advective exchange of solutes and presumably allowed the mat to grow to the extensive thickness of several centimeters.

Measurements of the [O.sub.2] microdistribution around relict tubes of the polychaete Spiochaetopterus bergensis revealed an intense passive flushing of the tube structure if exposed to ambient flow. Flushing induced plumes, several microliters large, of [O.sub.2]-undersaturated water that spread downstream in a flow-dependent manner. The plume induced a very irregular interstitial [O.sub.2] distribution around the tubes, with suboxic microniches. Mass balance calculations based on the measured exchange rates estimated the flushing rate of the relict tubes to be on the order of 5-11 [micro]l [tube.sup.-1] [min.sup.-1] at the applied flow rates; this strongly enhanced the benthic solute exchange in the area. Given the high density of several thousand tubes per square meter in the central Skagerrak, this flushing rate has profound effects on the biogeochemistry of the otherwise impermeable sediment, which is unique in the sense that benthic diagenesis is completely dominated by manganese oxidation.