| Summary: | The Atlantification of the European Arctic is defined as Atlantic water masses increasing their influence further North. This process, which is induced by climate warming, leads to changes in the physical and biological environment of the marine ecosystems. Decreased sea ice cover, increased sea surface temperature and increased salinity, as well as incoming boreal species, introduce a potential threat to the local organisms. Colony breeding seabirds, such as black-legged kittiwakes (Rissa tridactyla), can be used for biomonitoring of the occurring changes in marine ecosystems. Diet samples can indicate how the communities of prey species are changing and how these variations affect the energy budget for the birds. Calorimetry analyses revealed that the energy content in the diet samples of kittiwakes in Kongsfjorden declined from 2012 to 2020. While Atlantic-type conditions in summer, measured in salinity, increase in frequency, Arctic prey species decrease in frequency of occurrence in the diet samples of the birds. In Atlantic-type summers, the energy content in the diet samples was mostly lower. The median clutch size decreased over time, while in general there were larger clutches in years with Arctic-type conditions. However, not all Atlantic-type summers resulted in low energy content in the diet samples and low clutch sizes. Some Atlantic prey species could be a qualitatively high substitute for Arctic prey species. Especially the Atlantic Mallotus villosus could be a good replacement for the Arctic Boreogadus saida, as energy densities of both species were very similar. The effect of the Atlantification on the kittiwakes appears to be moderate for the given study period. My results indicate that black-legged kittiwakes are suitable biomonitors for the conditions in Kongsfjorden and suggest, that the Atlantification leads to decreased energy availability during breeding season and therefore reduced reproductive success. To be able to learn more about the influence of the Atlantification of the European ...
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