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Thesis Defense

Influence of Physical and Biological Oceanography on the Structure of the Seabird Community in the Eastern Chukchi Sea

Friday, 10 July, 11:00 am
Adrian Gall, Ph.D. Candidate
Faculty Advisor: Arny Blanchard

Fairbanks—201 O'Neill • Juneau—212 Lena Point bldg.

Processes and responses to environmental change by marine ecosystems are often challenging to quantify because they are hidden under water. Seabirds, however, offer visible evidence of the health and status of marine ecosystems. I studied the community of seabirds at-sea in the eastern Chukchi Sea to improve understanding of the interactions between the biotic and abiotic components of the marine environment. Repeated sampling of systematic transects in the northeastern Chukchi Sea during the ice-free seasons of 2008â2012 showed that the community consisted of ~40 species and was dominated numerically by planktivorous seabirds. In contrast, benthic-feeding species were rare. Seabird abundance varied by up to two orders of magnitude among years and birds generally were more abundant in September than August. Despite these seasonal and interannual variations in abundance, the species composition was similar among years, with anomalies occurring only in years of persistent ice cover. The associations of seabirds with habitat characteristics varied with foraging method and preferred prey. Species that fed primarily by pursuit diving were more abundant in warm, weakly stratified water, whereas surface-feeding species were more abundant in cold, strongly stratified water. Planktivorous seabirds were more abundant within 20 km of thermal surface fronts and in contrast, omnivores were more abundant far from thermal fronts. Relationships between seabird abundance and zooplankton abundance were not as clear as they were for the physical habitat characteristics indicative of processes that aggregate prey. I compared data from this recent period (2008â2012) with data from historical surveys (1975â1981) to evaluate decadal trends in seabird abundance and composition and related those changes to reductions in seasonal ice cover. The seabird community shifted from one consisting primarily of piscivorous seabirds to one consisting primarily of planktivorous seabirds. This shift suggests that zooplankton prey are more accessible now to avian predators as seasonal ice cover has declined. Advective processes that transport oceanic species of zooplankton from the Bering Sea to the Chukchi Sea, together with the local effects of sea ice on physical and biological processes, strongly influence the distribution of seabirds, particularly the planktivorous species. The results of this multi-species and multi-disciplinary study provide a benchmark to assess the ecological consequences of anthropogenic activity against the backdrop of climate change that is affecting the Chukchi Sea.