Feeding and reproduction measurement in Gammarus fossarum : recommendations for freshwater sediment toxicity assessment

Sediments are a key compartment of aquatic ecosystems, providing habitat for benthic species while acting as both sink and source of contaminants. Sediment quality assessment relies largely on chemical analyses that do not account for contaminant bioavailability or mixture effects. Sediment toxicity bioassays could address these gaps, but standardized test using European species remain scarce. This study aimed to develop two new bioassays on sediments - feeding and reproduction- using a freshwater amphipod Gammarus fossarum, a species widely used in European ecotoxicology. A total of 137 natural sediments, characterized in terms of physical and chemical properties, from four French river basins were tested, alongside a formulated sediment used as a laboratory control and reference. Three key questions were investigated: (1) whether grain size and organic carbon content act as confounding factors on survival, consumption rate of MUG® and fecundity; (2) whether a formulated sediment provides a suitable laboratory control; and (3) whether the two bioassays can discriminate between natural sediments of contrasting toxicity. Results showed no correlation between grain size or organic carbon content and consumption rate or fecundity, indicating that these properties did not confound the interpretation of bioassays. While survival in formulated sediment met established validity criteria, feeding rate and fecundity were significantly altered, questioning its suitability as a reference. A threshold-based statistical approach, derived from the distribution of responses across natural sediments, was therefore proposed as an alternative interpretation method and successfully identified toxic sites. These findings support that feeding and reproduction bioassays with G. fossarum are sensitive and discriminating tools for sediment quality monitoring, under the Water Framework Directive, and dredged sediment management. Further work on validity criteria and interlaboratory reproducibility is needed to support their future standardization.