Seabird species assemblages reflect hydrographic and biogeographic zones within Drake Passage
Drake Passage, extending from the southern tip of South America to the northern Antarctic Peninsula, is a dynamic oceanographic region with well-defined habitats delineated by the three strong frontal jets of the Antarctic Circumpolar Current (ACC). Here, we describe seabird species distribution patterns across Drake Passage and test the hypothesis that species assemblages broadly reflect physical characteristics of the hydrographic fronts. Strip-transect seabird surveys were conducted between Tierra del Fuego and the South Shetland Islands (700 km track line) during January–March (austral summer) over 14 years (48 crossings). Locations of the latitudinally variable fronts were assessed using in situ shipboard data on sea surface temperature and salinity; areas of high variance were used to indicate frontal features. We quantified five distinct species assemblages that correspond to biogeographic regions and relate to the positions of the Sub-Antarctic Front, Polar Front and ACC Southern Front. Dense seabird concentrations coincided with regions characterized by highly variable sea surface temperature and salinity, suggesting that associated species assemblages reflect the mesoscale hydrographic surface as indicated by sea surface conditions.