Advection of Atlantic water masses influences seabird community foraging in a high-Arctic fjord

Abstract Hornsund is a typical high-Arctic fjord, with the usual predominance of water masses of the Arctic origin, and associated lipid-rich zooplankton, which is an important food source for many seabird species. The fjord hosts one of the largest concentrations of breeding little auks worldwide, as well as several large colonies of black-legged kittiwakes, Brunnich's guillemots and northern fulmars. However, in summer 2014, for the first time in the history of almost two-decades long, comprehensive hydrographic studies, the large-scale advection of a relatively warm and saline Atlantic Water was observed in the fjord. These changes in the properties of water masses were associated with significant inter-annual changes in seabird community numbers and composition in the fjord. The advection of Atlantic water masses to Hornsund in 2014 clearly promoted the domination of Boreal black-legged kittiwakes, while rather cold summer seasons of 2015 and 2016 were characterised by the significant proportion of the Arctic little auks. Also, the use of different foraging habitats by seabirds changed between the studied years, with the higher importance of the land-based glacier bays and non-glaciated coastline for the kittiwake in 2014. Our results suggest that with the progressing warming in the Arctic, significant changes in the numbers and structure of seabird community are expected, which in turn will influence the functioning of the high-Arctic fjord ecosystems.