Photosynthesis–light relations of stream periphyton communities

The photosynthesis—irradiance ( P−I ) 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 µ g Chl a cm −2 and taxonomic composition differed. Photoassimilation of 14 C 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 ( I k ) was positively correlated ( P ≤ 0.05) with ambient light, but ranged only from ∼ 100 to 400 µ mol m −2 s −1 among communities. Light‐saturated, Chl‐specific photosynthetic rates (PS Chl M ) ranged from 0.26 to 1.4 µ g C( µ g Chl a ) −1 h −1 . The initial slope ( α ) of the P−I response ranged from 0.80 to 6.4 µ g C (mg Chl a ) −1 ( µ mol quanta) −1 m −2 s −1 . Unlike phytoplankton, α and PS M , (per unit Chl a ) of the periphyton communities were not well related to the ambient light environment and tended to be lower for communities from high‐light habitats. Further, the P−I 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−I relationships.