Unraveling effect pathways and biomagnification: The impact of a polyfluoroalkyl fungicide on an aquatic decomposer-detritivore system

Leaf litter breakdown is a key ecosystem process in streams driven by detritivorous macroinvertebrates and microbial decomposers, but it can be disrupted by anthropogenic stressors. In this context, we examined the effects of the polyfluoroalkyl (PFAS) fungicide fluopyram on the detritivore Gammarus fossarum and leaf-associated microbial decomposers, considering both waterborne and dietary pathways using three experimental designs. First, G. fossarum was exposed to fluopyram (0-60 mg L-1) via water while feeding on microbially colonized leaf discs for seven days. The median lethal concentration of fluopyram for G. fossarum was 3.7 ± 5.7 mg L-1, with a 50% reduction in feeding at 1.5 ± 0.3 mg L-1. Next, G. fossarum was fed leaf discs colonized in the presence (50-15,000 µg L-1) or absence of fluopyram over 14 days (dietary pathway). Fluopyram adsorbed to leaf material (2.7-221.0 µg g-1), decreased the number of released conidia (∼43-73%) and reduced fungal richness (∼77-98%) but did not affect G. fossarum's leaf consumption in a concentration-dependent pattern. Finally, G. fossarum was exposed to fluopyram directly via water (150 µg L-1), indirectly through leaf discs microbially conditioned in the presence of fluopyram (150 µg L-1), or via both pathways simultaneously. Fluopyram accumulated in G. fossarum tissue (up to 5.1 µg g-1 dry weight) but did not affect consumption, excretion, or growth (<7%). Our findings highlight the susceptibility of fungal decomposers to the PFAS fungicide and suggest a potential bioaccumulation risk for higher trophic levels.