Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobenthos) have been considered primarily for entire assemblages, How P-I responses vary with mat thickness and with community composition has not been considered in detail. We used a combined approach of modeling, microscale determinations of photosynthetic rate and light attenuation, and whole-assemblage O-2 flux measurements to explore P-I relationships. The modeling approach suggested that the onset of photosynthetic saturation and photoinhibition will occur at higher irradiance and that whole-mat photoinhibition (decreased photosynthesis at very high irradiance), biomass-specific maximum photosynthetic rate, and initial slope of the P-I function (alpha) should decrease as assemblage thickness increases or light attenuation increases. Spherical Light microsensor profiles for a variety of stream algae indicated a strongly compressed photic zone with attenuation coefficients of 70-1791 m(-1) for scalar photosynthetic photon fluence density. The O-2 microelectrode measurements showed little if any photoinhibition at 2 and 4 mm depths in one filamentous green algal (Ulothrix) assemblage, with a relatively low attenuation coefficient, and no photoinhibition in a second Ulothrix community, An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xanthophyte (Vaucheria) community exhibited no photoinhibition at all. The microelectrode data revealed increases in alpha over several millimeters of depth (photoacclimation), These data supported the model predictions with regard to the effects of mat optical thickness on whole-assemblage values for alpha and photoinhibition. Whole-community O-2 flux data from 15 intact assemblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or alpha expressed per unit area; the relationships with chlorophyll a were negative when photosynthetic rates were expressed per unit chlorophyll a. None of the whole assemblages exhibited photoinhibition. Thus, the data from the whole communities were consistent with model predictions.
