Tolerance patterns in stream biofilms link complex chemical pollution to ecological impacts.

Preventing and remedying fresh waters from chemical pollution is a fundamental societal and scientific challenge. With other non-chemical stressors potentially co-occurring, assessing the ecological consequences of reducing chemical loads in the environment is arduous. In this case study, we comparatively assessed community structure, functions and tolerance of stream biofilms to micropollutant mixtures, extracted from deployed passive samplers at wastewater treatment plant effluents. These biofilms were growing up- and downstream of one upgraded and two non-upgraded wastewater treatment plants, before being sampled for analyses. Our results showed a substantial decrease in micropollutant concentrations by 85%, as the result of upgrading the wastewater treatment plant at one of the sampling sites with activated carbon filtration. This decrease was positively correlated with a loss of community tolerance to micropollutants and the recovery of community structure downstream of the effluent. On the other hand, downstream biofilms at the non-upgraded sites displayed higher tolerance to the extracts than upstream biofilms. The observed higher tolerance was positively linked to micropollutant levels both in stream water and in biofilm samples, and to shifts in community structure. Although more investigations of upgraded sites are needed, our findings point toward the suitability of using community tolerance for the retrospective assessment of the risks posed by micropollutants, to assess community recovery, and to relate effects to causes in complex environmental conditions.

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