We measured biofilm extracellualar enzyme activities and microbial biomass in three headwater streams located in North Central Pennsylvania during the summer of 2007. Morris Run is currently impacted by acid mine drainage (AMD), Lick Creek has undergone remediation for AMD, and Sand Run is undisturbed. Measurements were made on glass fiber filters anchored to the bottom of each stream for one month. Chlorophyll a was highest in Morris, which was dominated by the alga Mougeotia. Phosphatase activity was three times higher in this stream while leucine-aminopeptidase was 25% of that in the other streams. These results demonstrate that Morris was extremely phosphorus limited possibly due to phosphates adsorbing to heavy metals. The ratio of β-glucosidase activity to β-xylosidase was higher in Morris, possibly indicating that heterotrophic bacteria in this stream rely primarily on algal-derived carbon, while bacteria in the other two streams rely more on terrestrial carbon sources. Phenol oxidase activities were lower in Morris and Lick compared to Sand, indicating reduced capacities for microbes in these streams to utilize polyphenolic compounds. These results suggest that low pH and heavy metals associated with AMD influence microbial dynamics in streams and that remediation may restore some, but not all, ecosystem functions.