A method of simultaneously quantifying photoautotrophic (algae and cyanobacteria) and bacterial production in periphyton communities by (14)C-bicarbonate and (3)H-leucine incorporation was investigated and applied to communities subjected to specific intensities of photosynthetically active radiation (400-700 nm). Maximum photosynthetic output (2.23 ± 0.29 (SE) μg C cm(-2) h(-1)) and bacterial production (0.07 ± 0.006 μg C cm(-2) h(-1)) occurred at the highest photon flux density (400 μmol m(-2) s(-1)). Over a photon flux density range of 20-400 μmol m(-2) s(-1), bacterial and autotroph productivity were significantly and positively correlated (r = 0.89). Furthermore, application of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, a photosystem 11 inhibitor, to periphyton films reduced bacterial production by 46%, but it had no such effect on bacteria-only cultures. Therefore, the magnitude of bacterial production in periphyton was coupled to the photosynthesis/metabolism of algae and/or cyanobacteria.