Trace element bioaccumulation in salmonid-dominated riverine food webs affected by coal mining activity

Mining has the potential to negatively affect receiving aquatic ecosystems by releasing trace elements such as selenium, mercury, arsenic, and cadmium, but dietary patterns in invertebrates and fish can mask effects of mine releases on their exposure. We compared trace element concentrations in water, benthic invertebrates, and salmonid fish at two coal-mine-affected streams with those from three reference streams in the upper Smoky River basin, Rocky Mountain foothills, Canada. The region has a high natural background concentration of selenium, evidenced by concentrations that exceed guideline values (>1 µg/L in water, > 4 mg/kg in invertebrates, > 11 mg/kg in fish egg/ovary) even at reference streams, but streams affected by upstream coal mining had significantly elevated concentrations in fish muscle (p < 0.05). Stable isotope analysis revealed that fish that were more strongly connected to the terrestrial detrital pathway had lower selenium (p < 0.001) and mercury (p < 0.01) concentrations, suggesting greater uptake from the algae-grazer pathway that is exposed to mine runoff. Selenium and mercury both showed evidence of biomagnification in food webs whereas arsenic and cadmium strongly biodiluted, further explaining differences in fish concentrations among sites. The high concentrations of selenium in fish muscle meant that it was the most important of the four elements in elevating risk to human consumers, and mine-affected streams had the highest hazard quotients. Our findings highlight the importance of characterizing the diets of fish with ecological tracers, such as stable isotopes, when investigating contaminant accumulation and point to the need to carefully consider future coal mine development because of consequences for downstream ecosystems and human health.