Submerged macrophytes are regarded as key components of aquatic ecosystems and also profoundly influence nutrient cycling through various physical, chemical, and metabolic processes. Vegetated shallows may act as hotspots for nitrogen (N) transformation in eutrophic rivers. Explosive growing of submerged macrophyte Potamogeton crispus may support enhanced nitrogen processing, but little is known about the variability of denitrification in epiphytic biofilms of P. crispus and the contributions of denitrification to nitrogen removal in rivers. Yi River of Linyi City is strongly influenced by rubber dam and human activities, and its water is rich in nutrition. P. crispus have become the dominant species in the water area in spring and they have been seen under explosive growth from April to May every year. P. crispus and epiphytic biofilms were sampled at explosive growing stages every 7 days in Yi River, and denitrification rates of epiphytic biofilms were analyzed using a ¹⁵N enrichment experiment. The results showed that biomass of P. crispus increased rapidly during explosive growing (from early April to early May) with the maximum biomass of P. crispus at sampling sites up to 1817-3334 g/m². Dry weight, organic matter, total organic carbon, total nitrogen and denitrifying bacteria of epiphytic biofilms increased significantly, which provided abundant material and microenvironment for denitrification. Denitrification rates (as N₂) of epiphytic biofilms significantly increased, the greatest denitrification potential at 6 monitoring sites were 424.18-1728.39 μmol/(m²·h), which was 17.29-29.09 times higher than that of sediments. The contribution to N₂ production by denitrification of epiphytic biofilms was high, with 77%-96% to N₂ (denitrification + anammox) production of biofilms, and 32%-96% to N₂ production by denitrification in river ecosystem. An estimate of epiphytic biofilms N₂ production by denitrification in P. crispus beds, based on continuous observations suggests that these epiphytic biofilms are major hotspots for N removal in Yi River during the later spring months. Scientific management and regulation of P. crispus beds through constructing complex submerged plant communities, moderating harvesting, and regulating water levels for urban river ecosystem service restoration efforts have been recommended.