Elucidating Sexual and Spatial Influences on the Trophic Ecology of Amazon River Dolphins ( <i>Inia geoffrensis</i> ) and Mercury Contamination

The Amazon Basin hosts an immense biodiversity, including the endemic Amazon River dolphin (<i>Inia geoffrensis</i>). Dietary exposure to mercury (Hg) is one of the threats to this top predator species. This study investigated the relationship between Hg exposure, trophic position and foraging areas in <i>I. geoffrensis</i>. We analyzed Hg concentrations and stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios in 111 blood samples collected in different years. The Hg concentrations ranged from 23 to 582 μg/kg wet weight, while mean δ¹³C and δ¹⁵N values were -29.6 ± 1.7‰ and 9.8 ± 0.5‰, respectively. Narrower isotopic niche areas were observed in the latest year analyzed, suggesting reduced prey availability and/or restricted foraging areas. Males displayed greater niche plasticity, whereas females maintained consistent feeding strategies over time. Spatial segregation between males and females/juveniles appeared to be the main driver of differences in Hg concentrations. These findings raise concerns for species conservation under the increasing frequency of extreme Amazon droughts, which can alter Hg dynamics and degrade habitats. Monitoring <i>I. geoffrensis</i> health is essential not only for the protection of this endangered species, but also as a sentinel for Hg exposure risk in the Amazonian human population, mainly riverine and indigenous communities.