Photosynthesis-light relations of stream periphyton communities

The photosynthesis-irradiance (P-Z) relationship was examined in 10 intact periphyton communities from sites in eight streams that differed in ambient light, nutrient concentrations, and grazing pressure. Algal biomass ranged from 2.3 to 86 pg Chl a cm -* and taxonomic composition differed. Photoassimilation of 14C was measured in the laboratory at six light levels for each community. Photosynthesis by communities from shaded sites was photoinhibited at the highest light level. The light intensity at the onset of saturation (Zk) was positively correlated (P 5 0.05) with ambient light, but ranged only from 100 to 400 pmol m-* s-l among communities. Lightsaturated, Chl-specific photosynthetic rates (PSChl M) ranged ffrom 0.26 to 1.4 pg C &g Chl a)-’ h-‘. The initial slope (a) of the P-Z response ranged from 0.80 to 6.4 pg C (mg Chl a)-’ (pmol quanta)-’ rr-* s-l. Unlike phytoplankton, (Y and PS, (per unit Chl a) of the periphyton communities were not well related to the ambient light environment and tend.ed to be lower for communities from high-light habitats. Further, the P-Z responses of the communities were better related to periphyton biomass than to the light environment. Self-shading and other aspects of development in a matrix seem to be important determinants of periphyton P-Z relationships. Periphyton is an important component of many lotic systems, influencing nutrient and carbon cycling, invertebrate composition, and other aspects of system character and dynamics (Lock et al. 1984; Meyer et al. 1988). There have been relatively few studies, however, of the photosynthesis-irradiance (P-I) response for intact lotic periphyton communities (Pfeifer and McDiffett 1975; Paul and Duthie 1989). Periphyton communities are sometimes disrupted and suspended in a water column to measure photosynthesis (e.g. Jasper and Bothwell 1986), essentially being treated as phytoplankton. Data for disrupted communities, ‘however, may not reflect performance under natural conditions (see Jasper and Bothwell 1986). Further, the P-I responses of attached algae are not necessarily similar to those characteristic of phytoplankton under