Mercury in Feathers from Birds of the Southeastern Pacific: Influence of Location and Taxonomic Affiliation

Hugo Ochoa-Acuna¹, Maria S. Sepúlveda², Tim .S. Gross², and Stephen M. Roberts<sup>1

1</sup>Center for Environmental and Human Toxicology
University of Florida, Gainesville, FL 32611-0885

 ²United States Geological Survey
Florida Integrated Science Center, Gainesville, FL 32653

Presented at the Society of Toxicology, 40th Annual Meeting, San Francisco, California.  March 25 – 29, 2001.
 

Background

Mercury (Hg) is a persistent substance that comes from natural and anthropogenic sources.  Hg that enters aquatic systems is converted to methylmercury by aquatic biota, bioaccumulating in aquatic food webs.  As a consequence, humans and wildlife are exposed to Hg, primarily through the consumption of contaminated fish. Because they are highly visible top predators, seabirds may be used to monitor spatial and temporal patterns of Hg contamination in marine ecosystems.  Several studies in a variety of seabirds have shown that 70% of the Hg body burden is contained in the plumage.  Seabirds usually renew their plumage every year after breeding, although some species have summer and winter plumages and therefore have two more or less complete molts per year.

Feather samples have been used to assess Hg body burdens in several seabird species. These studies have demonstrated large differences among species in the Hg content of feathers, even among co-occurring species. Furthermore, high Hg levels have been reported from seabirds inhabiting remote islands where anthropogenic releases of Hg are low.  The objectives of this study were to explore possible differences in Hg loads in feathers among different seabirds collected at various locations along the coast of Chile in the Southeastern Pacific.

Hypotheses and Methods

Our objective was to evaluate if there were differences in the concentration of Hg in feathers with respect to location along the Chilean coast, and whether the expected differences among species were due to differences correlated with taxonomic affiliation, or alternatively, due to differences in feeding strategies.

We expected to see lower Hg levels in birds inhabiting the southern part of Chile and the offshore Juan Fernández Archipelago because anthropogenic impacts due to urban and industrial (mining) emissions are concentrated in the central and northern parts of the country, respectively.

We obtained feathers as part of a sampling effort to obtain specimens for the American Museum of Natural History. We collected feathers from 106 birds belonging to 22 species, mostly seabirds. In the laboratory, multiple growing feathers were weighed and then liquefied with sulfuric and nitric acid. Aliquots of the feather digestates were then heated with potassium permanganate and potassium persulfate. Total Hg concentration was determined with a cold vapor atomic absorption spectrophotometer. The method detection limit was between 0.020 and 0.025 ppm.

Influence of Taxonomic Affiliation

This study found large differences among taxonomic groups. However, our results suggest that these differences are due to differences in feeding strategies, irrespective of taxonomic affiliation. The figure to the left shows feather Hg loads among bird orders. The figure to the right shows differences among petrels and shearwaters (Order Procellariformes). With the exception of the diving petrels (Pelecanoides sp.) all the species in this group are pelagic in distribution and migratory. 

It is clear from the figure to the left that coastal species have much lower Hg loads than the mostly pelagic petrels and shearwaters.  The low Hg loads of diving petrels (figure above) further confirms the hypothesis that different feeding strategies, and not taxonomically related differences in metabolism and excretion of Hg, is the reason behind the different Hg loads observed among species.

Although a large proportion of the global release of Hg is due to human activities, its entry into food webs is mediated by its methylation in anoxic waters below the thermocline. Recently, it has been shown that Hg loads among small fish (i.e, preyed by seabirds)  increases with the depth at which they spend the day (Monteiro et al., Mar Ecol Prog Ser, 141:21-25, 1996).  Most of these species (mainly fish of the family Myctophidae) migrate to the surface at night where they become available to seabirds.

This higher Hg load in mesopelagic fish that migrate to the surface at night may be the reason why the nocturnal gadfly petrels (Stejneger's Kermadec's, and Juan Fernández petrels, Genus Pterodroma) have higher Hg loads than the diurnal shearwaters (Puffinus sp.).

Influence of Location

Contrary to expectations, we did not find higher mercury loads in birds inhabiting the central and northern areas of the country.  We found that differences among locations within species or genus did not follow a clear pattern. For example, Peruvian diving petrels, Pelecanoides garnoti inhabiting the northern part of Chile had similar mercury loads than Magellanic diving petrels, P. magellani collected near in the Beagle Strait.  On the other hand, sooty shearwaters, Puffinus griseus collected off the central port of Valparaiso had significantly higher mercury loads than sooty shearwaters collected near the southern port of Concepcion.

Diving petrels are a highly anomalous group of petrels in that they are non-migratory and coastal in distribution. Sooty shearwaters, on the other hand, are highly migratory, breeding in the islands of southern Chile and Argentina and migrating to the northern hemisphere during the austral winter.

The lack of difference in Hg in feathers among the two diving petrels suggest that mercury in the coastal marine ecosystem does not seem to be significantly influenced by point sources such as mining activities and urban centers.

On the other hand, differences among the pelagic, migratory sooty shearwaters suggest that birds found at these two locations might come from different breeding and feeding grounds, feeding on prey with different Hg loads.

Influence of Location: Franklin's Gulls

Among the gulls, the Franklin's are one of the world champion long-distance migrants. After breeding in North America, they undergo a complete molting of their feathers, then fly south following the Gulf and Pacific coasts all the way to southern Peru and Chile. After spending the winter there, they undergo another complete feather molt, and follow the same migration route in reverse.

Because of this unique molting pattern, feathers collected in Chile will reflect exposure to mercury that occurred during the breeding season while in North America; whereas feathers collected from breeding adults will reflect exposure to mercury occurring while in wintering in Chile and Peru.

Our results, coupled with those of Burger (Auk, 113:399-407, 1996), provide evidence that Franklin's gulls are exposed to higher levels of Hg in their diet while breeding in North America, than while wintering in Chile.  Wintering birds have significantly higher feather Hg loads than breeding birds. In addition, feather Hg levels chicks (which reflect exposures during the rearing period from local food sources) are comparable to those of wintering birds.

Acknowledgments

Permission to obtain the samples was granted by the respective agencies (Servicio Agricola y Ganadero, SAG; Servicio Nacional de Pesca, SERNAP).  We also thank the Corporación Nacional Forestal (CONAF), the Chilean Navy, and Dr. Gary Nunn for assistance during the field activities.