Efficient NH4+ oxidation is a critical issue in human-impaired streams receiving high N loads from the effluent of wastewater treatment plants (WWTP). Archaeal (AOA) and bacterial (AOB) ammonia oxidizers are strongly photoinhibited in laboratory cultures, so we expected that light availability would affect the distribution of AOA and AOB and NH4+ oxidation rates at the reach scale. We selected 2 contiguous reaches downstream of a WWTP input in La Tordera river (northeastern Spain) that strongly differed in canopy cover (open and shaded). Against expectations and despite significant differences in light availability, the 2 reaches showed similar abundance of AOA and AOB and similar daily rates of ecosystem respiration, gross primary productivity, and NH4+ oxidation. The abundance of ammonia oxidizers was not correlated with biomass in biofilms protected from light, whereas a positive relationship was found for light-exposed biofilms. This result suggests that biomass accrual could provide light protection to ammonia oxidizers in light-exposed biofilms. The contribution of NH4+ oxidation to whole-reach NH4+ uptake reached up to 89%, indicating a high potential for NH4+ oxidation in the 2 reaches. NH4+ oxidation rates were similar between night and day, but their contribution to whole-reach NH4+ uptake tended to be higher at night than during the day. Altogether, these findings highlight that environmental factors other than irradiance drive reach-scale NH4+ oxidation in this urban stream.