Assessing the impact of chloroacetanilide herbicides and their metabolites on periphyton in the Leyre River (SW France) via short term growth inhibition tests on autochthonous diatoms.

The Leyre River is the main tributary to the Bassin d'Arcachon lagoon. Herbicides belonging to the chloroacetanilide class have been found in the river (S-metolachlor and acetochlor) as well as some of their metabolites at higher concentrations. As the environmental toxicity of these molecules is not well known, ecotoxicological tests have been carried out on river periphyton at different levels of biological diversity: from the clone of one diatom species (Nitzschia nana) to the population of the same species (several clones) up to the multi-specific species community dominated by diatoms. Moreover, tests were performed on diatoms coming from an unpolluted upstream site and from a contaminated downstream site, in order to investigate possible tolerance acquisition to pollutants. The method consisted in measuring diatom growth inhibition at different doses of each substance from the increase of chlorophyll-a concentration after 4 days. It resulted that acetochlor was clearly more toxic than S-metolachlor at all levels of biological diversity. EC(50) values estimated from the tests suggest no effect of contaminants on diatom growth or biomass in the river. The toxicity of the metabolites appeared very low compared to that of their parent compounds. No difference in tolerance to the herbicides was demonstrated between summer diatom communities from the two sites in spite of different specific compositions. However, concerning the populations of N. nana isolated in winter following the highest herbicide concentrations in the river (about 0.5 μg L(-1)), the downstream population showed a higher tolerance to acetochlor but there was no co-tolerance to S-metolachlor. Thus, it appeared that acetochlor represents the highest toxic pressure on periphyton among the other contaminants in the Leyre River.