Stream C to N to P ratios aligned with microbial needs enhance biofilm nitrate uptake and subsequent nitrogen loss

Anthropogenic inputs of reactive nitrogen (N) elevate nitrate–N (NO₃-N) levels in streams, potentially shifting their dissolved organic carbon (DOC) to N to phosphorus (P) ratios (DOC:N:P) toward N excess. Meanwhile, changes in riparian vegetation can alter light availability. Together, these factors may influence NO₃-N uptake by photoautotrophs and heterotrophs in surface (benthic) biofilms and by heterotrophs in subsurface (hyporheic) biofilms. Although these compartments may exhibit distinct rates and constraints on nutrient uptake and retention, the extent to which stoichiometric imbalances and light availability govern their macronutrient uptake remains largely unexplored. Here, we present results from a stream mesocosm experiment in which light availability and DOC:N:P were manipulated by adding labile DOC and inorganic P to create a physiologically more balanced stoichiometric composition of stream mesocosm water. We show (I) how the relative (macronutrient ratio) and absolute (particulate organic C, particulate N, and particulate P) macronutrient composition of benthic and hyporheic biofilms changes with different levels of light availability (20 and 90 µmol photons m⁻² s⁻¹) and different water DOC:N:P (350:940:1 and 73:40:1), (II) that benthic NO₃-N uptake rates increased with addition of labile DOC and P, whereas light had only a minor effect, and (III) that higher NO₃-N uptake rates due to labile DOC and P addition in benthic biofilms leads to higher N loss from biofilm biomass. This results in similar N retention times across treatments and highlights the importance of water column macronutrient stoichiometry as a predictor of in-stream N cycling.