Functional stability despite structural changes in freshwater biofilm communities exposed to an antibiotic and an herbicide - the role of nutrient conditions.

Freshwater autotrophic biofilms play a vital role in primary production and nutrient cycling in freshwater ecosystems but are increasingly exposed to chemical stressors such as antibiotics or herbicides. Although nutrient availability may modulate biofilm sensitivity, its impact on biofilm responses to these stressors remains poorly understood. In four independent experiments, we investigated the functional (ash-free dry weight and chlorophyll a, b and c) and structural (16S/18S rRNA metabarcoding) responses of stream-derived biofilms under low- and high-nutrient levels to chronic exposure (14 days) to the antibiotic ciprofloxacin and the herbicide propyzamide in laboratory stream microcosms. High-nutrient levels strongly increased biofilms functional responses and altered the community composition. Chemical exposure led to pronounced shifts in prokaryotic (ciprofloxacin) and eukaryotic (propyzamide) communities, but without significant effects on functional responses, suggesting functional redundancy and ecological buffering capacity of freshwater biofilms. These results highlight the critical role of nutrient supply in biofilm responses and the need for caution when extrapolating laboratory results to field conditions.