Tools that may be used to characterize the state of aquatic ecosystems and the impacts associated to pesticides are currently requested for the monitoring of ecosystems. Chemical analysis alone is not sufficient for such purposes and biomonitoring is essential. Many bioindicators based on diatoms are commonly used to assess the quality of aquatic environments but have not been developed to specifically address the impacts of pesticides. It is therefore crucial to develop dedicated tools. Our research aimed at performing a field study (analysis of monitoring network data) in order to evaluate the responses of different metrics towards pesticide pressure.

For diatom communities, toxic pressure associated to pesticides, especially herbicides, had a weak effect on species composition and on the various tested metrics. Nutrient and organic matter concentrations (and also typology) are the main influencing factors. When trophic and saprobic levels are controlled, it is possible to identify the effects of herbicides on some metrics. The only responding metric is the abundance of the “high profile” guild, which corresponds to taxa presenting an important contact surface with water flow. Its abundance decreases when herbicide toxic pressure increases. It could therefore be used to assess the impact of herbicides on diatom communities, but in a framework where confounding factors variability (e.g., nutrients, organic matter, river order) is reduced.