LAUSR

Abstract There is a paucity of data on seabird assemblages at sea in tropical waters, particularly in the western Atlantic Ocean, such as distribution and species occurrence. Here, we describe the seabird assemblage recorded near the Amazon River mouth, the greatest freshwater input in the marine environment globally, in North Brazil. Through two ship surveys conducted between March and May 2015, 219 seabird counts were carried out—118 during navigation and 101 point-counts; 443 individuals of 21 species were recorded. The species composition was almost all migrant taxa from both the Northern and Southern Hemispheres. We accessed the associations between bird assemblages (grouped by seabird families) and environmental variables, through univariate, two-part multiple regression models and canonical correspondence analysis. Seabirds aggregate in two main areas, each related to different mesoscale oceanographic features: the tidal mixing and saline front at the mouth of the Amazon River and the ring-formation zone on the border with French Guiana. For petrels (Procellariidae) and storm-petrels (Hydrobatidae), associations with sea surface temperature and distance from the coast were important. These groups were mainly observed in the ring-formation zone, and storm-petrels also occurred at the 20-m isobaths, where the advection process can cause local zooplankton enhancement. On the other hand, for terns (Sternidae), gulls (Laridae) and skuas and jaegers (Stercorariidae), chlorophyll-a and bathymetry were the most important factors. Their distributions were associated with the mouth of the Amazon River, where primary productivity is high. The oceanographic characteristics of these aggregation zones make them ecologically important areas for meso- and megafaunal marine taxa, particularly seabirds.