The capacity of stream biofilms to transform and assimilate N in highly N-loaded streams is essential to guarantee the water quality of freshwater resources in urbanized areas. However, the degree of N saturation experienced by urban streams and their response to acute increases in N concentration are largely unknown. We measured changes in the rates of NH4 1 uptake (UNH4) and oxidation (UAO) resulting from experimental increases in NH4 1-N concentration in mature biofilms growing downstream of a wastewater treatment plant (WWTP) and, thus, naturally exposed to high N concentration. We investigated the responses of UNH4 and UAO to NH4 1-N increases and the abundance of NH4 1 oxidizing bacteria and archaea (AOB and AOA) in epilithic and epipsammic biofilms. UNH4 and UAO increased with increasing NH4 1-N concentration for the 2 biofilm types, suggesting no N saturation under ambient levels of NH4 1-N. Thus, these biofilms can contribute to mitigating N excesses and the variability of NH4 1-N concentrations from WWTP effluent inputs. The 2 biofilm types exhibited different Michaelis–Menten kinetics, indicating different capacity to respond to acute increases in NH4 1-N concentration. Mean UNH4 and UAO were 5 higher in epilithic than epipsammic biofilms, coinciding with a higher abundance of AOA1AOB in the former than in the later (76 104 vs 14 104 copies/cm2). AOB derived from active sludge dominated in epilithic biofilms, so our results suggest that WWTP effluents can strongly influence in-stream NH4 1 processing rates by increasing N inputs and by supplying AOA1AOB that are able to colonize some stream habitats.