Hobson, Keith A.2007-05-082013-01-042007-05-142013-01-042007-052007-05-14May 2007http://hdl.handle.net/10388/etd-05082007-131255Arctic ecosystems are vulnerable to human-induced changes such as increases in contaminant levels and climatic warming. To predict effects of these changes, it is important to understand trophic relationships among Arctic organisms and how they change in response to time and environmental perturbations. Seabird diet can reflect relative availability and abundance of planktivorous fish and zooplankton in remote areas. The measurement of naturally occurring stable isotopes presents an alternative approach to evaluating dietary patterns of seabirds at both an individual level and at a larger, ecosystem level. Relative changes in δ¹³C values provided an indication of changes in consumption of benthic vs. pelagic prey and changes in δ¹⁵N values provided an indication of changes in trophic level.I investigated trophic positions of four seabirds (Thick-billed Murres (Uria lomvia); Northern Fulmars (Fulmarus glacialis); Black-legged Kittiwakes (Rissa tridactyla); and Glaucous Gulls (Larus hyperboreus)) at Prince Leopold Island, Nunavut, 1988 – 2003, using my own and previously measured stable isotope measurements in blood samples. Trophic level and space use among years differed within and among species and may be related to ice conditions and species-specific foraging strategies. The species with the most flexible foraging methods, Thick-billed Murres, varied their foraging location and trophic level the most. In 2002, fewer chicks than average for all species were fledged and Thick-billed Murre chicks were lighter than in other years; however, only murres showed a concurrent decrease in the proportion of fish in their diet. Adult body condition of murres in 2002 was positively correlated with trophic level. Breeding season dietary patterns of Thick-billed Murre adults and chicks were examined on Coats Island, Nunavut, Canada, in 2004. Adult trophic level increased slightly through the breeding season and δ¹³C values indicated a switch from benthic to pelagic foraging locations. Chick and adult murres did not differ in either δ¹⁵N or δ¹³C values; however, within a family (two parents, one chick), chicks were fed at or slightly below adult trophic level. I found little variation in stable isotope values which suggests adult murres did not preferentially select prey for either themselves or their chicks. Finally, stable isotope analysis was used to investigate winter foraging ecology of three species of alcids (Thick-billed Murres; Common Murres (U. aalge) and Razorbills (Alca torda)), off Newfoundland, Canada, 1996 – 2004. Thick-billed Murres fed at a higher trophic level than Common Murres. Razorbill δ¹⁵N values were highly variable and overlapped those of both murre species. I found no significant differences in δ¹³C values among the three species confirming a common spatial feeding pattern. Both murre species became depleted in ¹³C during winter suggesting foraging location or prey species shifted from nearshore to offshore. For Common Murres, hatching-year individuals fed at a higher trophic level and foraged farther offshore than after-hatch year birds. For Thick-billed Murres, I contrasted trophic level determined for the breeding colony at Prince Leopold Island with those determined for winter over four years and found considerable inter-annual variation in patterns of seasonal difference in trophic level. However, the proportion of lower trophic level (amphipod) vs. higher trophic level (fish) prey was generally greater in the winter than the summer.en-USice cover analysisisotopic monitoringtrophic level calculationserially sampled individualsdietary modellingtext