We tracked biofilm development over 28 days in streams differing in physicochemical attributes (restored prairie [RP] vs. wastewater-treatment plant outflow [TPO]) to assess whether differences in identity of the most abundant algal taxa influenced community structure of bacterial consortia, the quality of organic exudates and denitrification potential (DNP).  Dissolved nutrient concentrations were much higher in TPO (NO₃-N = 12 mg/L; PO₄ = 7 mg/L) than RP (NO₃-N = 0.15 mg/L; PO₄ = 0.13 mg/L).  Algal community structure in both streams was dominated by diatoms and changed considerably over the course of biofilm development, but differed markedly in the dominant taxon after 28 d; Amphora perpusilla comprised ca. 80% of TPO assemblages and Achanthidium minutissimum numerically dominated (ca. 60%) in RP.  DNP rates were greater in TPO, (70 µg N₂O/hr/cm²), than in RP, (44 µg N₂O /hr/cm²). Taxonomic structure of bacterial denitrifiers, assessed via terminal restriction fragment length polymorphism (T-RFLP) of nosZ genes, and the organic ‘signatures’ (via Pyroli\ysis GCMS) of 28-d biofilms also differed between sites.  Our results suggest that the taxonomic content of benthic algal assemblages influences the quality of extracellular organics within biofilms which, in turn, may affect both the structure and biogeochemical efficiency of resident bacterial assemblages.