Confirming the Primacy of Light Controlling Ammonium Removal in Response to Biofilm Colonization and Shade Using Experimental Streams

The removal of riparian shading alters stream biogeochemical cycling by stimulating algal biofilms through the release of light limitation. Few studies have examined the combined effects of shading and biofilm growth on ammonium (NH₄⁺–N) and nitrate (NO₃⁻–N) removal in the same experiment. We quantified water column NH₄⁺–N and NO₃⁻–N removal using n = 176 short-term additions in four experimental streams over 2 years and assessed the influence of shading (shaded vs. open canopy) and biofilm colonization (early vs. late) on removal using a Bayesian three-level hierarchical model. First, we assessed the roles of biofilm and shade on NH₄⁺–N removal. Shading lowered NH₄⁺–N uptake velocity [V_f; slope: −0.45 (−0.69, −0.22)], while the effect of biofilm was too small to detect [0.06 (−0.18, 0.29)]. The following season, we compared NH₄⁺–N and NO₃⁻–N removal, and added night releases to estimate autotrophic and heterotrophic contributions to removal. Shading reduced NH₄⁺–N and NO₃⁻–N V_f, while the effect of biofilm often depended on shading. NH₄⁺–N V_f was higher than NO₃⁻–N V_f by 51% during the day and 38% at night, confirming preferential NH₄⁺–N removal. NO₃⁻–N V_f declined more between the Late-Open and Late-Shade phases compared to NH₄⁺–N, suggesting a stronger decline in NO₃⁻–N demand than for NH₄⁺–N with shade during late biofilm colonization. We found no strong diel shift in NH₄⁺–N or NO₃⁻–N demand. Results demonstrate the primacy of light on NH₄⁺–N and NO₃⁻–N removal in streams. Understanding how shading and biofilm colonization alter removal is critical as streams are vulnerable to the impacts of land use change.