Pollution-induced community tolerance for in situ assessment of recovery in river microbial communities following the ban of the herbicide diuron

Changes in agricultural practices related to environmental policies such as the European Water Framework Directive should lead to a decrease in pesticide contamination levels in rivers draining agricultural areas. However, there is still a need for biological tools to evaluate resulting ecological changes, namely to evaluate ecological recovery following the improvement of surface water chemical quality. Accordingly, the main purpose of this study was to assess the relevance of a pollution-induced community tolerance (PICT) approach in natural phototrophic biofilms to estimate microbial community recovery following a decrease in diuron contamination due to the ban of the use of this herbicide on vineyards. To this end, we performed a 3-year field study (2009-2011) in the Morcille River (France), located in the French Beaujolais wine region. This river was frequently contaminated by diuron (among other organic and inorganic pesticides), with increasing concentrations downstream. Following the ban on diuron, imposed in December 2008, a progressive decrease in diuron concentrations was observed in the Morcille River. While the mean annual tolerance levels measured at the low contaminated station remained relatively constant over years, a clear and significant decreasing trend was observed at the medium and high contaminated stations during the survey. This temporal evolution revealed a direct link between a change in agricultural practice due to the ban of diuron and an increase in the sensitivity of phototrophic biofilms to this herbicide. This finding brings further new evidence that PICT has great potential to detect microbial community recovery following chemical restoration due to changes in the agricultural use of pesticides.