Sooty terns as large-scale bioindicator of mercury contamination in marine ecosystems: A pantropical approach.

Mercury (Hg) is a toxic contaminant, which poses serious threats to both wildlife and humans worldwide. Although tropical oceans receive half of the world's atmospheric Hg depositions, its biomonitoring is largely overlooked in these ecosystems. This study provides a comprehensive, pantropical assessment of Hg contamination of marine ecosystems, using the sooty tern (Onychoprion fuscatus) as bioindicator. Total Hg concentrations were measured from feathers collected in 28 colonies across all tropical oceans (2003-2022), from adults (n = 564) and chicks (n = 407), to evaluate longer- and shorter-term contamination. To investigate the role of bird trophic ecology on Hg contamination, bulk stable isotopes of carbon (δ13C) and nitrogen (δ15N) were also analysed, as respective proxies for feeding habitat and diet. Because baselines of stable isotopes may vary at large spatial scales, compound-specific nitrogen stable isotope analyses of amino-acids were performed on a subset of adults (n = 91, 19 colonies), to calculate their trophic position (TP), accounting for both baseline and trophic effects. As expected, Hg concentrations were higher in adults than chicks, because of their shorter bioaccumulation period. Overall, colonies from the Northern Hemisphere exhibited the highest Hg concentrations, consistently with higher historical anthropogenic emissions of Hg in the North. Models revealed that Hg contamination was driven by environmental contamination (colony location, feeding habitat) rather than diet (TP), which remained similar across the pantropical range. Our findings provide important baseline data for tropical marine ecosystems and underscore the need for enhanced monitoring in regions, where gold-mining and changing oceanographic conditions will likely modify future Hg exposure.