Trophic Ecology and Mercury Contamination of Seabird Communities in the Southern Ocean

Abstract

This thesis examines inter- and intraspecific variation in the trophic ecology and mercury (Hg) contamination of seabirds in the Southern Ocean, with a focus on the seabird community at Bird Island, South Georgia. In the opening chapter, I outline the key themes of the thesis, including the various methods of studying seabird diets and drivers of Hg burdens. Next, in Chapter 2, I analyse variation in the diets of black-browed albatrosses Thalassarche melanophris and grey-headed albatrosses T. chrysostoma since the mid-1990s. This study revealed long-term dietary shifts in both species, which were partly related to proxies of prey availability and had repercussions for breeding success in grey-headed albatrosses. Chapter 3 is the first conventional diet study of northern giant petrels Macronectes halli and southern giant petrels M. giganteus for >35 years at South Georgia. Penguins were the most important prey, and although I found no interspecific differences in overall diet composition, there was variation between sexes, by month and among years. In Chapter 4, using stable isotope ratios of carbon and nitrogen in feathers, I demonstrate a high degree of intraspecific variation in the trophic ecology of grey-headed albatrosses during the non-breeding period. I then analysed Hg contamination in their feathers (Chapter 5), which were the highest recorded in the genus and showed a threefold increase over the past 25 years. Hg concentrations were much lower in feathers moulted in Antarctic than sub-Antarctic waters, and contamination also increased with trophic level. Importantly, Hg burdens were higher in failed than successful males. In Chapter 6, I analysed Hg in blood of brown skuas Stercorarius antarcticus lonnbergi from three breeding populations, revealing geographic differences in the transfer of Hg to skuas in the Southern Ocean, sex differences in contamination levels and relationships with trophic ecology. The final chapter summarises the main findings of the thesis and highlights how emerging techniques may enhance future research on foraging ecology and pollutants.