Trophic ecology structures mercury bioaccumulation across marine tetrapods: Implications for cross-taxa biomonitoring

Mercury (Hg) concentrations in marine tetrapods vary widely across taxa, yet predicting contaminant burdens across ecologically diverse species remains a key challenge for environmental monitoring. Here, we evaluated how trophic ecology structures Hg bioaccumulation across marine tetrapods, testing the relative importance of ecological and taxonomic factors in explaining contaminant variability. Hepatic Hg concentrations were quantified in 151 individuals from 18 species (seabirds, cetaceans, and sea turtles) from the southwestern Atlantic, integrating δ¹⁵N as a proxy for trophic position. Hg concentrations spanned more than three orders of magnitude (0.29-2237 mg.kg⁻¹ dw), with the highest burdens consistently observed in oceanic species and the lowest in neritic species. Hg increased significantly with δ¹⁵N (R² = 0.25, p < 0.001), consistent with broad-scale biomagnification patterns. However, comparative models showed that ecological guild explained substantially more variation in Hg concentrations than trophic position alone, indicating that habitat use and ecological function strongly influence contaminant exposure. Ecological grouping explained substantially more variability in Hg than taxonomic classification (adjusted R² = 0.56 vs. 0.37), while variance partitioning revealed considerable overlap between taxonomic and ecological components of Hg variability. Overall, Hg bioaccumulation patterns were primarily structured by ecological guild, although species-specific traits contributed additional variability within guilds. Median Hg concentrations followed a clear ecological gradient, from oceanic cetaceans and pelagic seabirds to coastal species and sea turtles. Our results highlight the value of functionally equivalent species as sentinel taxa for Hg monitoring, providing a scalable framework for cross-taxa biomonitoring and environmental risk assessment in marine ecosystems.