Department Environmental Toxicology

The overarching aim of this field study was to examine causal links between in-situ exposure to complex mixtures of micropollutants from wastewater treatment plants and effects on freshwater microbial communities in the receiving streams. To reach this goal, we assessed the toxicity of serial dilutions of micropollutant mixtures, extracted from deployed passive samplers at the discharge sites of four Swiss wastewater treatment plants, to in situ periphyton from upstream and downstream of the effluents. On the one hand, comparison of the sensitivities of upstream and downstream periphyton to the micropollutant mixtures indicated that algal and bacterial communities composing the periphyton displayed higher tolerance towards these micropollutants downstream than upstream. On the other hand, molecular analyses of the algal and bacterial structure showed a clear separation between upstream and downstream periphyton across the sites. This finding provides an additional line of evidence that micropollutants from the wastewater discharges were directly responsible for the change in the community structure at the sampling sites by eliminating the micropollutant-sensitive species and favouring the tolerant ones. What is more, the fold increase of algal and bacterial tolerance from upstream to downstream locations was variable among sampling sites and was strongly correlated to the intensity of contamination by micropollutants at the respective sites. Overall, our study highlights the sensitivity of the proposed approach to disentangle effects of micropollutant mixtures from other environmental factors occurring in the field and, thus, establishing a causal link between exposure and the observed ecological effects on freshwater microbial communities.

1. A major challenge in environmental risk assessment of pollutants is establishing a causal relationship between field exposure and community effects that integrates both structural and functional complexity within ecosystems.
2. Pollution-induced community tolerance (PICT) is a concept that evaluates whether pollutants have exerted a selection pressure on natural communities. PICT detects whether a pollutant has eliminated sensitive species from a community and thereby increased its tolerance. PICT has the potential to link assessments of the ecological and chemical status of ecosystems by providing causal analysis for effect-based monitoring of impacted field sites.
3. Using PICT measurements and microbial community endpoints in environmental assessment schemes could give more ecological relevance to the tools that are now used in environmental risk assessment. Here, we propose practical guidance and a list of research issues that should be further considered to apply the PICT concept in the field.