Stream biogeochemical cycling, including both assimilatory and dissimilatory processes, can be altered via the alleviation of a light limitation in open canopy streams. However, few studies have examined ammonium (NH4+-N) removal via both assimilatory and dissimilatory processes, and assessed the combined effects of light availability and biofilm growth on removal in the same experiment. Here, we quantified water column NH4+-N and nitrate (NO3--N) removal using n=176 short‐term additions in four experimental streams over two years. We developed a Bayesian multilevel model to assess the influence of light availability (via experimental shading) and biofilm colonization on NH4+-N and NO3--N removal rates, while also accounting for the simultaneous decline in NH4+-N and increase in NO3--N via the dissimilatory process of nitrification at the reach-scale. Using this approach, we can describe the effects of biofilm and light availability on both NH4+-N and NO3--N removal rates, while also providing estimates of reach-scale NO3--N production across a continuum of biofilm colonization and diel conditions that would inform the relative roles of these effects on inorganic N fluxes to downstream ecosystems. This method offers a new approach for quantifying nitrogen biogeochemical cycling in streams without the use of an isotopic 15N tracer and informs the variability around removal rates.