Effects of post-fire contamination in sediment-dwelling species of riverine systems

Highlights: • Wildfire runoffs promote sediment contamination with metals in downhill waterbodies. • Post-fire runoff contaminants accumulate in river sediments and impair benthic species. • In-situ exposures to this contamination induced post-exposure feeding inhibition. • Wildfire effects are better discriminated by in-situ than by laboratory bioassays. • Long-term effects may remain due to contaminated sediment resuspension. Wildfires are an important environmental problem in forested watersheds and can significantly alter water quality. Besides the reported ecotoxicological effects on pelagic species, the accumulation of post-fire contaminants in river sediments can also impair the benthic species. In this study, three sediment-dwelling species, Chironomus riparius, Atyaephyra desmarestii and Echinogammarus meridionalis, with different sensitivities, habitats, behaviours and/or feeding strategies, were exposed to water and sediments, in in-situ and in laboratory. Four sites were selected in a partially burnt basin (Alfusqueiro river basin), within and upstream the burnt area. The sites within the burnt area showed higher metal burden in both water and sediment, as well as changes in water physico-chemistry, consistently with the typical effects of incoming post-fire runoff. Both in-situ and laboratory exposures to water and sediments affected by the wildfire induced post-exposure feeding inhibition in the three tested macroinvertebrates. In fact, laboratory and field bioassays have produced generally consistent post-exposure feeding inhibition responses, but the most impactful response could be recognised after in-situ bioassays at the river site within the burnt area, where the species respond to the physico-chemical fluctuations during the exposure period. This comparative perspective supports the importance of using in-situ bioassays as a more realistic approach when dealing with complex and intermittent natural samples such as those affected by post-fire runoff. Overall, our results reinforce the awareness about the negative effects of wildfires on benthic biota, with significant feeding depression and consequent reduction in the available energy budget to ensure successful detoxification, growth and reproduction signalling potential trophic and functional disruption at the ecosystem level. In addition, the duality conditions of sediments as a sink and source of contaminants reinforce concerns, as the exposure of benthic organisms may persist in the long term, even after runoff income ceases due to the resuspension of contaminated sediments.