Human activity has significantly increased inorganic nitrogen (N) in streams and rivers and can also influence the relative proportion of reduced (ammonium) and oxidized (nitrate) N forms in these ecosystems. N is a key element for organisms and its availability can either limit ecosystem production or promote eutrophication, which in turn may affect the capacity of the ecosystem to retain and transform this element. In this talk we will present results from a study aimed to assess the N-uptake response of biofilm communities to increased N availability. Therefore, we examined the relationship between N uptake by biofilm communities and N concentration; and further, we evaluated how N uptake varied with ammonium and nitrate as N sources. The relationship was also compared between biofilm communities grown in low- and high-N streams. In each stream, we measured N uptake by biofilms of in-stream flumes using ¹⁵N-tracer experiments, exposing epilithic biofilm communities to ¹⁵N and manipulations of ambient N concentration via fertilization. We hypothesized that N-uptake responses would vary depending on the N form considered and the ambient concentrations to which the communities were exposed. Results support this hypothesis and indicated a clearer positive response to ammonium than to nitrate increases.