IMPACT OF MICROSENSOR-CAUSED CHANGES IN DIFFUSIVE BOUNDARY-LAYER THICKNESS ON O-2 PROFILES AND PHOTOSYNTHETIC RATES IN BENTHIC COMMUNITIES OF MICROORGANISMS

Although O-2 microsensors used for study of benthic photosynthesis hav e tip diameters of less than 10 mu m and stem diameters of about 150 m u m at a distance of 0.5 cm from the tip, they do change the local flo w pattern in the overlying water and thereby reduce the thickness of t he diffusive boundary layer (DBL). The reduced DBL resulted in a lower diffusional resistance towards export of O-2 from communities with a net production of O-2, and insertion of a microsensor in such a commun ity therefore resulted in lowered O-2 concentrations. The total diffus ion flux out of the photosynthetic layer was not significantly affecte d by the presence of the microsensor. The higher upward flux into the water phase was counterbalanced by a reduced downward flux into the se diment. The O-2 profiles and diffusion fluxes were only affected when the surface was smooth, while the effect of microsensor insertion was undetectable when the surface was made irregular by small tufts, bubbl es, etc. In the investigated cyanobacteria-dominated mats, the gross p hotosynthetic rates as measured by the light/dark shift technique were unaffected by the presence of a microsensor.