The objective of this study was to measure the photo-acclimation response of stream algae inhabiting thin biofilms across a range of light treatments comparing both growth rates and net primary productivity (NPP). Algae were grown on clay tiles incubated in artificial stream channels where light levels were manipulated by layering a neutral-density shade cloth over each channel. We measured NPP and algal growth rates in the early stages of community development and compared assemblages that were either acclimated or unacclimated to a given light treatment. Algal growth rates did acclimate to light treatment, with saturation occurring at light levels that were substantially lower for the acclimated communities. Growth efficiency calculated from algal biovolume increased by a factor of 3. However, algal NPP showed a weaker photo-acclimation response, with only a 30% increase in photosynthetic efficiency. Our results indicate that diatom-dominated periphyton in thin biofilms are probably not light limited in many shaded streams with respect to growth rates, but are light limited with respect to NPP.