Aquatic environments are the final receptacles for many contaminants including pesticides. In an agricultural context, various events (run-off, leaching) transferred these substances to these receptacles. The transfer of these contaminants from soil to water depends on a number of parameters: hydrology, agricultural practices and seasonal fluctuations in pesticide emissions. In artificially drained watersheds, transfers of these pesticides facilitated the contamination of rivers1. This alters freshwater quality and threatens the associated biodiversity2. To prevent ecological deterioration, health diagnosis of wild populations is essential. A number of tools for assessing biological responses to in situ chemical stress have been developed in gammarids, based on multi-level approaches3. The Orgeval basin, located in Seine basin in the north of France (Seine et Marne, 77). It is an 80% drained agricultural basin facilitating the rapid transfer of various types of contamination including pesticides. This observatory is a pilot site for monitoring the impact of the seasonality of pesticide transfers. The aim of the study presented here is to gain a better understanding of the relationship between exposure dynamics and the impact of chemical pressures in an in situ context at different levels of biological organization (from cellular to ecosystem level). The experimental approach based on the use of two major gammarid species from the Seine basin: Gammarus pulex and Gammarus fossarum. G. pulex is a specie characterized by great adaptability to chemical pressures4. Conversely, G. fossarum is more sensitive to organic pollution (5). At our study sites, we use a caging system to measure several biomarkers and the impact of episodic intensive drainage events on these two reference species. At population level, biological traits such as mortality, locomotion, feeding, biomass and amplexus are evaluated. At the cellular level, enzymatic activities involved in various vital functions are assessed. This allows us to take into account the seasonal nature of contamination and to make temporal, spatial and inter-species comparisons. This multi-scale approach will identify the stresses caused by the constraints on gammarid populations and detritivores macroinvertebrates communities (trapped in litterbags). First results show differences in the sensitivity of the biomarkers used, depending on the species, spatial and temporal context. In fact, there is a difference in biological characteristics depending on the sites and the species. In addition, inter-species variation provides information on the sensitivity and complementarity of the two gammarid species. This difference in sensitivity was related to the different campaigns carried out. This highlights the temporal variability of our biological traits, impacted by the seasonality of pesticide transfers. In addition, various abiotic factors affect the biological traits of gammarids following in situ exposure. Finally, the acquisition of complementary data such as pesticide concentrations, enzymatic activities will enable us to assess the impact of these substances on these biological traits. The continuation of this research, with the results of the biochemical analyses, aims to understand how a multi-level assessment coupled with environmental monitoring enables us to assess the overall quality of watercourses.