The application of salt on roads is used to reduce hazardous driving conditions, but can pollute nearby aquatic ecosystems. To determine the effects of road salt on stream periphyton, we collected biofilms and stream water and manipulated sodium chloride concentrations (up to 30000 µS) in the laboratory. Photosynthetic activity was monitored by pulse amplitude modulated fluorometry, and fluorogenic substrates were used to measure leucine aminopeptidase, β-glucosidase, and phosphotase activities. Extra cellular enzyme activities were unaffected by salinity. Elevated salinity rapidly reduced photosynthetic activity. The magnitude of this reduction was dependent on dissolved salt concentrations, and photosynthetic activity recovered quickly after biofilms were placed with salt-free stream water. These results indicate that short-term salt exposure has strong negative effects on autotrophic microbiota, and that these negative effects are reversible, if salt stress is removed.