Multiple energy pathways structure mercury biomagnification in an Amazonian River food web

Mercury biomagnification in Amazonian rivers is usually assessed through fishes, yet contaminants can also move across riparian food webs through multiple energy pathways. We evaluated how total mercury (THg) is structured across the upper Caqueta River basin, a mining-impacted river floodplain system in the Colombian Amazon. Using stable carbon and nitrogen isotopes, trophic guild classification, biomagnification models, and Bayesian mixing models, we integrated basal resources, macroinvertebrates, fishes, and birds to link mercury exposure with food-web architecture. THg increased with trophic enrichment in fishes, birds, and the combined food web, indicating biomagnification across aquatic and aquatic-associated consumers. Reconstructed pathways showed that phytoplankton and detritus support intermediate fish consumers, which in turn connect basal production to carnivore piscivorous fishes and aquatic predatory birds. Avian results suggest that mercury exposure is not restricted to aquatic predators, as hummingbirds showed unexpectedly elevated THg values that may reflect additional riparian or arthropod-mediated pathways. Overall, mercury transfer in the upper Caqueta emerges from the interaction between trophic position and energy-flow structure, highlighting the need to evaluate contamination magnification through integrated aquatic riparian food webs rather than single consumer groups.