Inter-annual fluctuations in mercury contamination associated with environmental variability in two Procellariiformes species of Bass Strait, southeastern Australia.

Mercury (Hg) is a toxic contaminant that biomagnifies along marine food chains, particularly exposing top predators such as seabirds through their diet. Bass Strait, in southeastern Australia, is one of the fastest warming oceanic regions on the planet and is influenced by several currents that create a highly variable physical and biological environment. Here, we studied the feather and blood Hg concentrations in two Procellariiformes species, the common diving petrel (Pelecanoides urinatrix) and the fairy prion (Pachyptila turtur) over four breeding seasons with different environmental conditions on site. We hypothesized that years marked by marine heatwaves would show different blood Hg contamination than colder years, due to differences in prey availability. Both species exhibited significantly higher blood Hg levels during incubation than during chick-rearing, suggesting a carry-over effect from pre-breeding period contamination. Inter-annual variation in blood Hg concentrations was observed in both species, and common diving petrels showed significant differences in Hg and isotopic values between the two colder years of the study. These results suggest that factors other than the increase of sea surface temperature associated with marine heatwaves caused inter-annual variation in Hg contamination. In particular, changes in the relative dominance of water masses may have altered the prey composition in Bass Strait, subsequently leading to fluctuations in Hg contamination. As the East Australian Current is expected to intensify, our results highlight that environmental variability associated with climate change could result in further variation in Hg contamination in seabirds in the future.