13 páginas, 4 tablas, 5 figuras ; Nitrate (NO3 -) and ammonium (NH4 +) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO3 -:NH4 + ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO3 - or NH4 +. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO3 - and +NH4 +) in five streams. Biomass-specific uptake rates (Uspec) of NO3 - and NH4 + were then measured using in situ additions of 15N-labeled NO3 - and NH4 +. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as Uspec- NO3 - of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. Uspec-NO3 - was one order of magnitude greater and more variable than Uspec-NH4 +, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO3 - and +NH4 +) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and Uspec-NO3 - and for the +NH4 + than for the +NO3 - treatments. In particular, enrichment with NH4 + reduced biofilm Uspec- NO3 - uptake, which has important implications for N cycling in high NH4 + streams. ; This study was funded by the Spanish Ministry of Education and Science through NICON project (ref: CGL2005-7362). MR was supported by a contract with the Spanish Ministry of Science and Innovation through the MED_FORSTREAM project (CGL2011-30590-C02-02). DvS’s work was also funded by a Juan de la Cierva postdoctoral contract (JCI-2010-06397) from the Spanish Ministry of Science and Innovation. ; Peer reviewed