Mercury bioaccumulation and biomagnification in southern hemisphere riverine food webs: Environmental drivers across a 3,700 km Chilean latitudinal gradient

Mercury (Hg) and methylmercury (MeHg) in freshwater food webs is a worldwide concern, commonly interpreted through broad geographic gradients derived largely from Northern Hemisphere datasets, yet controls on dynamics in Southern Hemisphere rivers remain poorly resolved. Our multi-basin, multi-matrix investigation of total mercury (THg) and MeHg across a ∼3700 km Chilean latitudinal gradient using integrated abiotic (water, sediments) and biotic (benthic macroinvertebrates, trout) matrices and stable isotopes (δ¹³C, δ¹⁵N), alongside environmental and land use variables. Sediment THg ranged from 0.01 to 0.11 g/g μ dry weight, while water concentrations spanned 1.72-28.93 ng/L, with northern basins exhibiting the highest levels. MeHg concentrations in macroinvertebrates (0.002-0.011 μg/g wet weight) and trout (0.006-0.234 μg/g wet weight) were elevated in mining-impacted areas. The absence of a clear latitudinal gradient underscores the primacy of local-scale drivers over broad geographic patterns. Trophic Magnification Factors (TMFs) ranged from 1.66 to 5.04, confirming biomagnification across all sites. TMF variability was positively associated with pH and mining land use but negatively linked to agricultural land use and total nitrogen. Model selection (GLM/AICc) identified pH as the strongest predictor of TMF, with additional support for PO₄^3-, while dissolved oxygen best explained sediment THg (negative association) and Cu dominated water-column THg. Collectively, these findings highlight the complexity of MeHg dynamics and emphasize the decisive role of local environmental and anthropogenic factors. This study provides novel Southern Hemisphere findings, reinforcing mercury monitoring under international frameworks.