Photosynthesis-irradiance patterns in benthic microalgae: Variations as a function of assemblage thickness anc community structure

Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobe nthos) have been considered primarily for entire assemblages, How P-I respo nses vary with mat thickness and with community composition has not been co nsidered in detail. We used a combined approach of modeling, microscale det erminations of photosynthetic rate and light attenuation, and whole-assembl age O-2 flux measurements to explore P-I relationships. The modeling approa ch suggested that the onset of photosynthetic saturation and photoinhibitio n will occur at higher irradiance and that whole-mat photoinhibition (decre ased photosynthesis at very high irradiance), biomass-specific maximum phot osynthetic rate, and initial slope of the P-I function (alpha) should decre ase as assemblage thickness increases or light attenuation increases. Spher ical Light microsensor profiles for a variety of stream algae indicated a s trongly 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 at tenuation coefficient, and no photoinhibition in a second Ulothrix communit y, An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xan thophyte (Vaucheria) community exhibited no photoinhibition at all. The mic roelectrode data revealed increases in alpha over several millimeters of de pth (photoacclimation), These data supported the model predictions with reg ard to the effects of mat optical thickness on whole-assemblage values for alpha and photoinhibition. Whole-community O-2 flux data from 15 intact ass emblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or alpha expressed per unit area; the relations hips with chlorophyll a were negative when photosynthetic rates were expres sed per unit chlorophyll a. None of the whole assemblages exhibited photoin hibition. Thus, the data from the whole communities were consistent with mo del predictions.