Fluvial biofilm functioning is impaired by 1H-benzotriazole under fish-mediated nutrient enrichment.

Microbial biofilms respond to environmental stress and drive key biogeochemical processes in streams, underpinning ecosystem services such as toxicant removal, carbon fixation, and nutrient cycling. 1H-benzotriazole (B1H) is a ubiquitous emerging pollutant with known effects on microorganisms, plants and animals, that frequently co-occurs with nutrient enrichment in effluent-impacted freshwaters. Using fluvial microcosms, we evaluated the effects of B1H below reported toxicity thresholds (nominal: 200µg/L; measured 175±12µg/L, mean±SD) on epilithic biofilm structure and functioning under dynamic fish-mediated nutrient enrichment, using mosquitofish (Gambusia holbrooki) as contaminant recipients and nutrient sources. Structural responses were limited, with B1H increasing protein content in the extracellular polymeric substance (EPS) matrix but producing no significant effects on biomass-related endpoints. In contrast, several functional processes were affected by B1H exposure. B1H particularly reduced phosphorus uptake and biomass-specific community metabolism, indicating lower functional efficiency of epilithic biofilms. Nitrification and heterotrophic carbon utilisation effects were found after prolonged exposure. Progressive fish-mediated ammonium enrichment and phosphorus depletion generated severe stoichiometric imbalance in all treatments that also interacted with B1H exposure to shape biofilm responses. By week 3, exposed and unexposed biofilms converged through partial recovery of some responses in B1H-exposed biofilms, together with declining functional performance in unexposed biofilms under nutrient imbalance. Overall, our findings indicate that contaminant effects on freshwater biofilms depend on the ecological context. B1H illustrates that contaminants can modify freshwater ecosystem functioning through interacting direct and indirect pathways even at concentrations below standard toxicity thresholds, highlighting the importance of incorporating ecological interactions into freshwater ecological risk assessment.