Multiple anthropogenic drivers are changing ecosystems globally, with a disproportionate and intensifying impact on freshwater habitats. A major impact of urbanisation are inputs from wastewater treatment plants (WWTPs). Initially designed to greatly reduce nutrient loads, WWTPs increasingly release a multitude of micropollutants (i.e., synthetic chemicals) and organisms (including antibiotic resistant bacteria) to receiving environments. This pollution may have pervasive impacts on biodiversity and ecosystem services. Viewed through multiple lenses of macroecological and ecotoxicological theory, we combined field, flume, and laboratory experiments to determine the effects of wastewater on microbial communities and organic-matter processing using a standardized decomposition assay. Firstly, we conducted a mensurative experiment using 60 sampling locations above and below WWTP discharges in 20 Swiss streams. This showed that microbial respiration and decomposition rates were positively influenced by inputs of treated effluent via warming and nutrient enrichment, but with a notable exception: wastewater decreased the activation energy of decomposition, indicating a “slowing” of this fundamental ecosystem process in response to temperature. Secondly, next-generation sequencing of microbial communities indicated that community structure below WWTPs was altered, with significant compositional turnover, reduced fungal richness, and evidence of negative micropollutant influences. Thirdly, a series of flume experiments confirmed that although diluted wastewater generally has a positive influence on microbial-mediated processes, the negative effects of micropollutants are ‘masked’ by the addition of nutrients. Finally, cotton-strip transplant experiments suggested that wastewater-borne microbes enhance rates of decomposition. Taken together, our results affirm the multiple-stressor paradigm by showing that different aspects of wastewater (i.e., temperature, nutrients, microbes, and micropollutants) jointly influence ecosystem functioning in complex ways. Increased respiration rates below WWTPs potentially generate ecosystem ‘disservices’ via greater carbon evasion from streams and rivers. However, toxic effects of micropollutants may fundamentally alter ecological scaling relationships, indicating the need for a rapprochement between ecotoxicological and macroecological perspectives.
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