[en] We investigated changes in biomass, biochemical fingerprints, fatty acid methyl ester (FAME) profile and functional status of the natural aquatic microbial communities upon impact of an Atradex pulse. The Atradex was applied to microcosm tanks at concentrations ranging from 24.5 μg L^-1 to 245 mg L^-1. The biomass of all microbial communities declined to a minimum level on day 4 with the effect being more pronounced in treated groups. Similarity between microbial communities also decreased on day 4 with the greatest change occurring at a concentration of 245 mg L^-1 Atradex. After 8 days exposure to Atradex, microbial communities in all treated groups (except tanks spiked with 245 mg L^-1 Atradex) recovered and showed similar metabolic fingerprints and FAME profiles to those of controls. Our results indicate that exposure to an Atradex pulse at concentration above 245 mg L^-1, may irreversibly change the structure and functional status of aquatic microbial communities. - Atradex at concentration above 245 mg L^-1 may irreversibly change the structure and functional status of aquatic microbial communities