Bioaccumulation of trace elements by Leach's storm-petrels (Hydrobates leucorhous) in the North and South Atlantic Ocean during the non-breeding period.

Trace elements occur naturally in the environment, but anthropogenic activities can amplify their release, increasing exposure and bioaccumulation in marine predators such as seabirds. Mercury (Hg) in liver, blood, and eggs of Leach's storm-petrels (Hydrobates leucorhous), has been investigated as a short-term exposure indicator of hatching and fledging success. However, chronic Hg intake and exposure to other trace elements during the non-breeding period remain poorly understood. This study assessed geographic variation in trace element exposure and relative trophic position in Leach's storm-petrels from five overwintering locations in the Atlantic Ocean, using secondary feathers (S4s) and geolocation data. Hg concentrations and trophic position (δ15N) varied significantly among overwintering locations, whereas other trace elements varied without clear spatial patterns. Sea surface temperature was positively correlated with Hg concentrations, whereas dietary origin (δ13C) and year were associated with relative trophic position. Colony of origin had no effect on Hg or trophic position, likely reflecting high intra-colony variability in wintering locations. Additionally, Hg concentrations were highest in birds overwintering in oligotrophic regions where δ15N values were low, suggesting lower nitrogen baselines in areas dominated by diazotrophic organisms and enhanced methylmercury availability via sulfate-reducing bacteria. In contrast, storm-petrels overwintering in productive upwelling regions such as the Benguela system exhibited lower Hg concentrations, consistent with Hg biodilution driven by rapid decaying phytoplankton export to deep sediments. These findings provide baseline information on trace element exposure during the non-breeding period of Leach's storm-petrels and inform future studies on migratory carry-over effects influencing adult survival and reproductive success.