We investigated the environmental impact of diffuse pesticide pollution on natural river biofilms in the River Morcille, France, during 2 seasons in pristine and contaminated stations. Prokaryotic and eukaryotic community compositions were assessed by denaturing gradient gel electrophoresis, while microalgal community composition was determined by high pressure liquid chromatography pigment analysis. The sensitivity of microphytobenthos to the herbicide Diuron was investigated in the laboratory by short-term photosynthesis inhibition assays. Spatial changes in prokaryotic and eukaryotic species compositions were found in spring and in winter. Community structures were significantly different between contaminated areas and those that were uncontaminated or less contaminated. Associated changes in biodiversity were not found. Community tolerance towards Diuron (based on EC50 values for photosynthesis) was significantly lower in upstream than in downstream photoautotrophic organisms. Pesticide concentration increased along the downstream gradient. These results strongly suggest contamination-driven changes in biofilm community structure and in the tolerance of the photoautotrophic community, confirming the Pollution-Induced Community Tolerance (PICT) hypothesis.

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SUMMARY1. 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.

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