Pharmaceutical use is increasing worldwide due to increased prescription and consumption. Accordingly, pharmaceuticals are detected in aquatic environments at concentrations of ng to μg L-1, due to continual release by wastewater treatment plants. Antibiotics, a class of pharmaceuticals which inhibit bacterial growth and replication, are of particular interest as aquatic contaminants due to their potential to target environmental microorganisms, including those which colonise biofilm communities. Biofilm communities play a role in aquatic ecosystems, as the biogeochemical processes of respiration, primary production and denitrification are microbially mediated. Thus, biofilm function is important in aquatic carbon and nitrogen cycling and ecosystem health. In this study, we cultivated lentic biofilms in a mesocosm environment, and examined the effect of exposure to two antibiotics (ciprofloxacin and doxycycline), alone and as a mix on biofilm function. Biofilm respiration, gross primary production and denitrification rates were quantified on day one and day two following antibiotic exposure to established biofilms. The antibiotic mixture significantly suppressed community respiration and primary production rates at 5.0 μg L-1 (by 67 ± 10 % and 44 ± 5 % respectively). Interestingly, one day after dosage, community respiration and primary production rates were found to recover to control levels. 16S profiling was undertaken to assess community shifts as a result of antibiotic exposure. Our study adds to the understanding of pharmaceutical effects on aquatic biofilm at sub-lethal concentrations. It is essential to ensure resilience of microbial biofilm communities in the face of pollutants to maintain aquatic ecosystem health.