| Summary: | Freshwater ecosystems are increasingly threatened by chemical contaminants that can disrupt fundamental ecological processes. This study investigated the effects of
tebuconazole (fungicide) and ciprofloxacin (antibiotic) on microbial and detritivore
mediated leaf litter decomposition. We tested three hypotheses regarding chemical
effects on decomposition processes, differential impacts between microbial groups, and detritivore contributions using a controlled laboratory experiment. Black alder (Alnus glutinosa) leaf discs were exposed to both chemicals at three concentrations (0, 50, and 1000 μg/L) over 28 days, with and without the freshwater isopod Asellus aquaticus. Decomposition was quantified as organic matter loss using ash-free dry mass measurements. Both chemicals significantly reduced microbial-mediated decomposition, but only at the highest concentration (1000 μg/L). Ciprofloxacin caused significantly greater inhibition than tebuconazole at equivalent concentrations, indicating bacterial communities are more functionally important in aquatic decomposition than fungal communities. Detritivore presence substantially enhanced decomposition rates, with microbial and detritivore pathways contributing nearly equally to total decomposition. These findings challenge the traditional fungal-centric paradigm of aquatic decomposition and demonstrate that bioactive contaminants can selectively impair ecosystem functioning through targeted effects on key microbial groups. The results highlight the vulnerability of bacterial-mediated processes and emphasize the importance of community-level approaches in freshwater ecotoxicological assessments.
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