Annual trends in mercury contamination are associated with changing trophic niches of giant petrels

Annual variation in prey availability can influence seabird diets and hence their exposure to pollutants, including mercury (Hg). Among seabirds, those species that scavenge carrion of marine mammals and other top predators may be especially vulnerable to accumulating high Hg concentrations. In this study, total Hg (THg) concentrations and carbon (δ13C) and nitrogen (δ15N) stable isotope values were measured in chick feathers of northern giant petrels Macronectes halli and southern giant petrels M. giganteus at Bird Island, South Georgia (2013-2020). Both species are opportunistic predator-scavengers which feed mainly on penguins and Antarctic fur seal Arctocephalus gazella carrion, and to lesser extents on marine prey and other seabirds. THg concentrations were not significantly different between northern giant petrels and southern giant petrels (means ± SDs, 2.49 ± 0.92 μg g-1 dw and 2.34 ± 0.85 μg g-1 dw, respectively), but concentrations in both species declined signficantly over time, as did δ13C and δ15N values. Annual feather THg concentrations of giant petrels were positively correlated with the number of dead Antarctic fur seal pups and their mortality rate at Bird Island, but not with population sizes or breeding success of penguins. Accordingly, these results suggest a shift away from carrion (associated with the decreasing size and productivity of the Antarctic fur seal population) and towards the consumption of prey from lower trophic levels (e.g., Antarctic krill Euphausia superba), with a corresponding reduction in dietary Hg exposure. Future work should investigate the consequences of changing prey availability for diets and pollutant exposure of other marine predators within the South Georgia and Scotia Sea marine ecosystems, given the ongoing environmental changes in the region.