The role of stable isotopes and mercury concentrations to describe seabird foraging ecology in tropical environments

Abstract

International audienceNitrogen (δ15N) and carbon (δ13C) stable isotopes and contaminants, such as mercury, have been widely used to characterise foraging ecology of temperate and polar seabirds. In this study, for the first time, we used isotopic signatures and mercury levels of feathers and blood of eight tropical seabird species, that forage in a range-gradient between inshore and offshore areas, to describe the foraging habits of a large tropical seabird community (from two neighbouring islands of the Seychelles archipelago, western Indian Ocean) during both the breeding and interbreeding periods. Overall, we found a high overlap in both δ15N and δ13C signatures among species. The high interspecific overlap in δ15N values was expected, given the similarities in the diet of the species from this community. However, several unexpected results, such as (1) the consistently higher δ15N signatures of white terns (Gygis alba), (2) the large variation in inter-specific differences in δ15N signatures among the sampling groups (season, age, island and tissue) and (3) the consistent low δ15N values of breeding birds during the northwest monsoon (austral summer), suggest that δ15N signatures cannot be used as indicators of seabird trophic levels in this community. The high interspecific overlap in δ13C signatures and the absence, during the breeding season, of a δ13C gradient that follows the inshore-offshore foraging gradient within the community can be explained by the habitat homogeneity of the Seychelles continental shelf and suggest that birds forage mostly within the limits of this "plateau". On the other hand, the similarities in δ13C values between the breeding and inter-breeding periods in species that are known to show post-breeding dispersal, strongly support the hypothesis of a lack of latitudinal variation in δ13C signatures of POM in the central Indian Ocean, and the consequent inaccuracy of δ13C values to track seabird movements within this geographic area. Inter-specific differences in mercury levels seem to be related to prey size, while consistent higher mercury concentrations in one of the studied islands suggest different island mercury-backgrounds and possible segregation in foraging areas between the seabirds of the two islands

Trophic ecology drives contaminant concentrations within a tropical seabird community

“…Our study shows, for instance, that average Hg concentration was much higher in French Guiana compared to another tropical seabird community in Seychelles archipelago (Catry et al, 2008). Further, despite in Catry et al (2008) Hg analyses were performed using whole blood, which reflects relatively similar concentrations than that of red blood cells, Hg concentrations in the Brown noddy were much lower than those of French Guiana (Catry et al, 2008). Similar Hg concentrations have been previously associated with deleterious effects (Costantini et al, 2014;Tartu et al, 2016) with consequences at the population level such as reduction of breeding, hatching and fledging success, and population decline (Goutte et al, 2014a(Goutte et al, , 2014b.…”

“…However, they are similar to highcontaminated seabirds in other areas whether from temperate, subpolar, or polar regions (Supplementary Figure S2). Our study shows, for instance, that average Hg concentration was much higher in French Guiana compared to another tropical seabird community in Seychelles archipelago (Catry et al, 2008). Further, despite in Catry et al (2008) Hg analyses were performed using whole blood, which reflects relatively similar concentrations than that of red blood cells, Hg concentrations in the Brown noddy were much lower than those of French Guiana (Catry et al, 2008).…”

“…These investigations should also clarify whether the negative relationship between high Hg and low As and Se concentrations might lead to side effects due to their potential deficiency, as it has been previously shown in captive animals (Fischer et al, 2008;Wang et al, 2009 (Sebastiano et al, 2016). Archipelago (Catry et al, 2008); Kerguelen Archipelago (Fromant et al, 2016;Goutte et al, 2014); Bird Island (Anderson et al, 2009); Adélie Land Tartu et al, 2014).…”

“…All other measurements were made in red blood cells. Hg concentration in Seychelles archipelago were extrapolated from graphs (Catry et al, 2008).…”

First report of chlorinated and brominated hydrocarbon pollutants in marine bird eggs from an oceanic Indian Ocean island

“…In one of the few studies on pollutants in marine birds from the western Indian Ocean, Catry et al (2008) detected no differences in trophic feeding ecology based on stable isotopes in feathers or blood, nor any differences in mercury levels between the Sooty Tern and Common Noddy, despite the differences (though overlapping) in prey items and foraging ecology. On the other hand, Ramos and Tavares (2010) found that lower levels of mercury in feathers of inshore foraging species from the Seychelles compared with offshore feeding species such as the Sooty Tern.…”