Litter breakdown for ecosystem integrity assessment also applies to streams affected by pesticides

International audienceWhile the impact of various anthropogenic alterations, e.g. nutrient enrichment, has been docu- mented on leaf litter breakdown - a key process for stream ecosystems -, our objective was to assess the response of this process to pesticides in agricultural streams. We hypothesized the impairment to be correlated with the pesticides contamination gradient, and the invertebrate decomposers to be more affected than microbial ones. Alder total breakdown rate was found to strongly decrease along the pesticide con- centration in 12 French streams, only due to inverte- brate-driven breakdown (as determined in coarse- mesh bags) since microbial-driven breakdown (fine-mesh bags) remained unchanged. Coherently, litter-associated shredder taxa richness and abundance together with SPEARpesticide (a specific indicator based on invertebrate traits) were greatly reduced, whereas pesticide toxicity did not affect litter-associated fungal biomass and taxa richness. Consequently, the presence of pesticides compromised leaf breakdown, as microbial decomposers did not compensate for the invertebrate decomposers decline. This occurred while pesticides concentrations even in the most contaminated stream were under the European Union’s Uniform Principles thresholds for targeted species. Our study showed that litter breakdown, particularly the ratio of total to microbial-driven breakdown rate, is a pertinent proxy to assess the functional integrity of pesticide-contaminated streams