1. The denitrifying capacity of epiphyton was used to evaluate differences in the function of epiphytic microbial communities on submersed macrophytes in nutrient-rich freshwater ecosystems. The denitrifying capacity of epiphyton on Patamogeton perfoliatus shoots of different age and with different epiphytic abundances from a eutrophic lake was investigated in laboratory microcosms in the Light and dark. Additionally, differences between epiphyton on shoots of Potamogeton pectinatus grown under different in Situ nutrient and hydraulic conditions were investigated by examining their denitrifying capacity. 2. Denitrification was registered in well-developed epiphytic layers on both mature and senescent shoots in the dark, with activities 3- to 10-fold higher in the epiphytic communities of senescent shoots. No activity was detected on young shoots with sparse epiphyton or on shoots from which loosely attached epiphyton had been removed. Denitrification never occurred during illumination. 3. Even though the epiphytic abundance was similar in magnitude, the denitrifying capacity of epiphyton adapted to high nutrient loadings was about a hundred times higher than that of epiphyton adapted to lower nutrient levels. Additionally, epiphytic abundance and denitrifying capacity were higher at sites less exposed to wave turbulence or water currents, than at sites with more water turbulence. 4. The results illustrate how the hydraulic and nutrient conditions of the surrounding water affect both the quantity and function of epiphytic microbial communities in nutrient-rich freshwater ecosystems.