National assessment of bioavailable contamination level using caged Gammarus fossarum (Crustacean): towards optimisation and extension of the list of monitored substances

Since 2018, in situ caging of Gammarus fossarum has been used by French water agencies at more than 400 river sites. This active biomonitoring tool helps meet the requirements of the Water Framework Directive by analysing priority substances in biota. It is also used to assess the bioavailable chemical contamination level in waterbodies for a wider range of substances. However, the current list of monitored substances, which is partly determined by the analytical capabilities of the service provider's laboratory, needs to be refined to optimise biomonitoring and better reflect environmental contamination. This study proposes a scientific approach for defining a list of "substances of interest" to be monitored in caged gammarids. A total of 1751 candidate substances were classified using a tiered approach based on a decision tree. This approach categorised substances according to their quantification frequency (QF) in biota (Gammarus, other invertebrates and fish) and sediment. The QF data originates from the monitoring programme of the French water agencies using caged gammarids, as well as from an international bibliographic search focusing on contaminants of emerging concern in the aquatic environment (132 studies were used). This approach yielded a list of 235 substances of interest, for which the QF in biota is at least 20%. Of these, 160 are new compared to the current list and include substances from families that are not currently monitored in caged gammarids, such as pharmaceuticals, personal care products, and rare earth elements. This list includes many molecules currently in use, e.g. pesticides and various industrial products, for which stakeholders can take action. The study also identified 150 "excluded substances" that are irrelevant to analyse in caged gammarids. This study generated datasets on the occurrence of chemicals in freshwater and marine biota and their physicochemical properties. The results of this work can help improve the assessment of bioavailable contamination levels in French continental waters using caged gammarids. The developed tiered approach and its outcomes can be applied in other countries that use gammarids and other species for biomonitoring purposes.