Wide range of mercury contamination in chicks of southern ocean seabirds

Abstract

Using top predators as sentinels of the marine environment, Hg contamination was investigated within the large subantarctic seabird community of Kerguelen Islands, a remote area from the poorly known Southern Indian Ocean. Chicks of 21 sympatric seabirds presented a wide range of Hg concentrations, with the highest contaminated species containing ~102 times more feather Hg than the less contaminated species. Hence, Kerguelen seabirds encompass the whole range of chick feather Hg values that were previously collected worldwide in poorly industrialized localities. Using stable isotopes, the effects of foraging habitats (reflected by δ(13)C) and trophic positions (reflected by δ(15)N) on Hg concentrations were investigated. Species-related Hg variations were highly and positively linked to feather δ(15)N values, thus highlighting the occurrence of efficient Hg biomagnification processes within subantarctic marine trophic webs. By contrast, Hg contamination overall correlated poorly with feeding habitats, because of the pooling of species foraging within different isotopic gradients corresponding to distinct seabird habitats (benthic, pelagic, neritic and oceanic). However, when focusing on oceanic seabirds, Hg concentration was related to feather δ(13)C values, with species feeding in colder waters (lower δ(13)C values) south of Kerguelen Islands being less prone to be contaminated than species feeding in northern warmer waters (higher δ(13)C values). Within the context of continuous increase in global Hg emissions, Kerguelen Islands that are located far away from anthropogenic sources can be considered as an ideal study site to monitor the temporal trend of global Hg contamination. The present work helps selecting some seabird species as sentinels of environmental pollution according to their high Hg concentrations and their contrasted foraging ecology.

Figure 1. Location of the Kerguelen Islands and of the main oceanic fronts and zones in the southern Indian Ocean.

Abbreviations: STF, Subtropical Front; PF, Polar Front; STZ, Subtropical Zone; SAZ, Subantarctic Zone; AZ, Antarctic Zone .

Figure 2. Relationship between chick feather Hg concentrations (means ± SD; µg•g⁻¹ dry mass) and (a) foraging habitat (chick feather δ¹³C) and (b) trophic position (chick feather δ¹⁵N) of oceanic species.

Filled diamonds and empty circles refer to oceanic and other species, respectively. See Table 1 for species abbreviations.

Figure 3. Feather Hg concentrations (µg•g⁻¹ dry mass) of individual chicks from the six most contaminated species from the Kerguelen seabird community.

See text for toxicity threshold and Table 1 for species abbreviations. BBA illustrates the most contaminated species of the assemblage with all individual values being below the threshold value.