Blue whales versus humpback whales, frenzy for food or contaminants? Trophodynamics of methylmercury and trace elements in Patagonian fjords.

Baleen whales foraging in the fjords of northern Chilean Patagonia are exposed to contaminant hotspots and high primary productivity. To evaluate ecotoxicological risk, we quantified methylmercury (MeHg) and eight trace elements (Al, As, Cd, Fe, Li, Mn, Se and Zn) across a whale-prey food web comprising blue whales (Balaenoptera musculus), humpback whales (Megaptera novaeangliae), krill (Euphausia sp.), zooplankton (e.g. amphipods) and squat lobster (Grimothea gregaria). Stable isotope (δ13C, δ15N, δ34S) mixing models were used to reconstruct diet and contextualize contaminant pathways. MeHg concentrations increased significantly with δ15N and yielded the highest estimated TMF among the analyzed elements (TMF ≈ 3.303), followed by Se (TMF ≈ 2.372); Al and Zn showed weaker but statistically significant positive relationships with δ15N, while Cd, Li, Mn showed no trophic transfer. These contaminant dynamics translated into species-specific burdens: humpbacks accumulated higher MeHg, reflecting their broader, higher-trophic diet, whereas blue whales exhibited greater absolute daily intake of several trace elements due to higher daily prey consumption. Our results provide a comprehensive ecotoxicological assessment that integrates isotope-based diet reconstruction, contaminant burdens, and trophodynamic patterns in sympatric mysticetes of the southeastern Pacific. More broadly, they show how species-specific foraging strategies mediate differential contaminant exposure risk in productive coastal ecosystems.