From cell to ecosystem function: Which biological levels best inform pesticide and transformation product ecotoxicity through active biomonitoring?

The aquatic environment receives a significant proportion of diffuse pollution from agricultural activities, particularly pesticides and their transformation products (TPs). In agricultural catchments, subsurface drainage networks increase the transfer of pesticides and TPs from agricultural soils to surface waters. However, the impact of seasonality of pesticides and TPs, which are closely related to hydrological functioning of artificially drained catchments, remain poorly studied. The aim of this study is to assess in situ the relevance of different biological levels (e.g. cellular, individual, population and ecosystemic functions) in determining the ecotoxicity of temporal variability of pesticides and TPs on macroinvertebrates notably gammarids. A series of 18–19-day active biomonitoring campaigns were conducted during two hydrological seasons (2022–2023 and 2023–2024) at six sites within the Orgeval catchment (ORACLE observatory, northeast France, Seine-et-Marne), a study site representative of intensive agricultural and artificially drained catchments. Active biomonitoring campaigns are based on multi-parameter monitoring (i.e. physico-chemical, chemical, and hydrological) and the deployment of caged gammarids and litter bags. These campaigns enable the observation of a wide range of biological responses, encompassing levels from the cellular to the ecosystem. This observation encompasses enzymatic activity, biological traits and the degradation of litter. The results demonstrated significant temporal variations in pesticide and TP concentrations related to the application period and hydrological regime. Biomonitoring findings indicated high sensitivity across multiple biological levels, specifically the individual and cellular levels, driven by contaminant levels, particularly TPs and pesticides, as well as physicochemical factors. These results underscore the importance of accounting for confounding factors when assessing pesticide and TP toxicity in agriculturally impacted aquatic systems, to ensure accurate environmental risk assessment. Overall, the strong responsiveness of these multiple biological levels to TPs highlights the need to assess their impact as part of active chemical pressure analyses.