Changes in periphytic community induced by environmental exposure to diuron and/or imidacloprid depends on seasonal variation
L'impact induit par une exposition environnementale au diuron et/ou à l'imidaclopride sur les communautés périphytiques dépendent des variations saisonnières
Résumé
Only a little part of agricultural pesticides reach their target organisms; the remaining is transferred by phenomena like leaching to aquatic ecosystems, where they are found in mixtures. Chemical stressors induce direct and indirect changes on the fauna and/or the flora, thus complicating pesticide risk assessment. We exposed mature biofilms for 14 days in aquaria to four treatments: control, the herbicide diuron (priority substance in the Water Framework Directive) and the insecticide imidacloprid (included in the Watchlist), singly and in mixture, at environmental concentrations (5µg/L). Our aim was to understand biofilm community responses when exposed to pesticides, based on changes in structure (species composition, diversity and density) and community tolerance, within a PICT (Pollution-Induced Community Tolerance) framework. We quantified weekly microalgal and micromeiofaunal composition, biofilm dry weight, chlorophyll a concentration and exopolysaccharide content. In the controls, biofilm dry weight increased over time; whereas in the three pesticide treatments the biomass did not increase after 7 days of exposure. PICT approaches based on photosynthesis inhibition revealed pre-exposure to diuron, while imidacloprid didn’t affect directly the microalgae. Contrastingly, the autotrophy index increased with imidacloprid pressure, in agreement with its known target action on animals. These impacts were confirmed by taxonomy analyses. The binary mixture had the same effects on microalgae as diuron, decreasing photosynthetic efficiency and chlorophyll a concentration. Altogether the results of this study highlighted changes in the proportions of microalgae and micromeiofauna reflecting their interactions and exposure history. They also demonstrate that micromeiofauna should not be overlooked in microbial ecotoxicology.