Closer to shore, higher mercury? Interannual variation in exposure and foraging ecology in common murres

Mercury (Hg) contamination poses a significant threat to marine top predators, yet the drivers of short-term variation in Hg exposure remain largely unquantified. We investigated interannual variation in Hg accumulation in adult common murres (Uria aalge) during chick-rearing in the Gulf of St. Lawrence, Canada, over two breeding seasons (2021-2022). Red blood cell (RBC) Hg concentrations, reflecting recent dietary intake, were measured alongside GPS/Time-Depth Recorder (TDR) tracking and stable isotope analysis to quantify trophic position (δ¹⁵N), foraging habitat (δ¹³C, δ³⁴S, GPS), and diving behavior (TDR). RBC Hg concentrations were significantly higher in 2022 (0.774-1.12 μg/g ww) than in 2021 (0.388-0.720 μg/g ww), coinciding with a shift toward higher trophic-level and more coastal/benthic feeding (higher δ¹⁵N; more negative δ¹³C and δ³⁴S), as well as shallower dives and foraging closer to shore and the seafloor. These results highlight strong interannual variability in Hg exposure, potentially reflecting both ecological shifts and variation in environmental Hg availability. We conclude that integrating contaminant measurements with fine-scale tracking and isotopic data provides valuable context for interpreting exposure patterns in marine predators.