Caged Gammarus fossarum for monitoring bioavailable contamination in French rivers

Active biomonitoring using caged organisms is increasingly applied to assess chemical contamination and toxicity in aquatic environments. In France, caged Gammarus fossarum is routinely used to monitor bioavailable contamination in accordance with the European Water Framework Directive (WFD). Building on the approach initially proposed by Besse et al., who developed a method to define regional Bioavailable Background Assessment Concentrations (BBACs) for approximately 30 chemical compounds, this study aimed to establish BBAC thresholds at the national scale for the maximum number of compounds analyzed in caged gammarids by the six French water agencies. The BBAC represents the concentration above which contamination is interpreted as resulting from anthropogenic and/or geochemical sources, and below which it reflects background conditions. Using data from the 2019 nationwide monitoring campaign, encompassing 221 sites and 478 deployments, we determined BBAC values to distinguish natural background contamination from anthropogenic inputs and proposed contamination severity classes to characterize the intensity of site contamination. Among 250 compounds analyzed, 60 met BBAC determination criteria, including metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), perfluorooctanoate (PFOS), and certain herbicides. Contamination severity levels were set for 40 substances, enabling the classification of sites based on bioaccumulation levels for each contaminant. Applying such contamination thresholds facilitated a nuanced interpretation of contamination levels. This study represents the first large-scale application of caged gammarids across metropolitan French rivers for chemical biomonitoring in European continental waters. The BBAC values and contamination severity levels established here provide essential tools for regulatory agencies to assess contamination and support water quality management. These findings highlight the relevance of active biomonitoring with caged gammarids as a robust and standardized approach for monitoring chemical contamination in freshwater ecosystems.