Interspecies comparison of individual- and population-level biomarkers in gammarids caged in a drained agricultural catchment for pesticide ecotoxicity assessment

Subsurface drainage increases pesticide transfer from intensive agriculture to surface waters. Their transfer to surface waters is facilitated by underground drainage networks, but the impact of seasonality still needs to be assessed in situ . This study aims to assess the effects of in situ pesticide on two gammarid species ( Gammarus pulex and Gammarus fossarum ) as well as the influence of confounding factors on their behavioural traits. A series of 18–19-day active biomonitoring campaigns were conducted during the 2022–2023 hydrological season at six sites in the Orgeval Observatory area (northeast France, Seine-et-Marne). The study area is subject to intensive drainage measures implemented through a dense network of sub-surface infrastructure. These sites were selected along a gradient of agricultural pressures, related to pesticide contamination within the hydrographic network, providing a comprehensive overview of the variability in environmental parameters (physicochemical, hydrological and physiographic) likely to affect gammarid behavioural traits. The results demonstrated significant temporal variations in pesticide concentrations related to application period and hydrological regime. Biomonitoring findings indicated sensitivity of both species, primarily driven by physicochemical factors, followed by contaminant levels, particularly those of transformation products and metals. The elevated responsiveness of these biomarkers ( e.g. mortality, amplexus, locomotion, ingestion rate, biomass) to physicochemical parameters as opposed to chemical stresses ( e.g. pesticides and metals) underscores the predominant influence of habitat parameters on the behaviour of both species. Consequently, it is imperative to consider these parameters when assessing the pesticide toxicity in agriculturally impacted aquatic systems to ensure accurate evaluations. • Highlighting the sensitivity of biomarkers to environmental parameters. • Sensitivity of these biomarkers to pesticides, particularly metabolites. • Inter-species comparison in the context of active biomonitoring. • Sensitivity of biomarkers to multiple contaminants (metals, nitrates).