Agricultural intensification has led to a significant increase in global nitrogen (N) fertilizer use, and a subsequent increase in N runoff to nearby streams. As inorganic N inputs increase, retention and transformation capacities decrease, and the transport of excess N downstream can result in harmful eutrophic conditions. While controls on nitrate removal have been well-studied, the influence of labile carbon availability on reach-scale nitrate removal under varying stream conditions remains understudied. Using experimental streams, we examined how nitrate removal varied temporally (day vs. night), over a biofilm colonization sequence (e.g., early biofilm development, late biofilm, senescence), and with the addition of labile carbon (as acetate). We conducted replicated short-term nitrate additions, alone (+N) and with added carbon (+N+C), in four replicate streams from July-September 2022 (N=48 releases) during early biofilm (EB), late biofilm (LB), and algal senescence (AS). We conducted replicated releases during both day and night conditions to partition the role of autotrophs and heterotrophs on uptake. Nitrate removal was not detectable during daytime +N releases for EB and LB phases; however, we quantified significant nitrate removal during senescence (AS period; 0.1±0.04 mgN m-2 d-1). We suggest nitrate removal was stimulated by organic carbon released during algal die-off which stimulated heterotrophic assimilation. In contrast, for +N+C releases, we measured nitrate removal during both day (0.05-0.8 mgN m-2 d-1) and night (0.1-0.4 mgN m-2 d-1) conditions across all biofilm phases, suggesting that N uptake by heterotrophic biofilms was carbon limited. These preliminary results demonstrate the interplay between autotrophs vs. heterotrophs, and nitrate vs. carbon availability, in controlling nitrate removal in streams, suggesting a temporal partitioning of uptake at multiple scales. Overall, nitrate removal dynamics are complex and driven by multiple environmental controls, but understanding temporal variation in the biogeochemistry will inform efforts to improve water quality in agricultural streams.