Community development alters photosynthesis‐irradiance relations in stream periphyton

The effects of biomass accumulation and community age on the photosynthesis‐irradiance response of periphyton were examined in developmental sequences from three stream locations that differed in canopy cover. (P‐I) curves for four developmental stages from each location were constructed from 14C uptake at six irradiances (22–1,100 µmol quanta m‒2 s‒1) in the laboratory. Photoinhibition in the periphyton from the two shaded sites declined with development, indicating that self‐shading significantly influenced photosynthesis in older communities. Photoinhibition was absent, however, in all of the developmental stages from the high light site. Maximum photosynthesis (Pmax) and slope (α) of the light‐limited portion of the chlorophyll‐specific light response decreased >40% in developing periphyton at all sites. Significant negative correlations between biomass and biomass‐specific Pmax suggested that light or metabolic substrates became progressively more limiting during development. Although there was little community‐level evidence of shade adaptation during development (photosynthesis did not saturate at lower light intensities as development proceeded), periphyton understories appear to retain considerable photosynthetic potential despite significant shading by upper layers.