In marine western boundary systems, strong currents flowing coastward spread oceanic water masses over the continental shelves. Here we propose to test the hypothesis according to which oceanic cnidarian species may dominate western boundary system regions even in coastal waters. For that purpose we use a set of data collected above the shelf, slope and around oceanic seamounts and islands in the Western Tropical South Atlantic. Samples were acquired with a plankton net with 300 μm mesh size over 34 stations during an oceanographic cruise carried out in October 2015. Results reveal a diverse cnidarian assemblage in the area, extending the known distribution of many species. In addition, the Fernando de Noronha Chain and most of the narrow continental shelf presented a typical oceanic cnidarian community, dominated by holoplanktonic siphonophores. In this western boundary system, this condition was likely driven by the strong currents, which carry the oceanic tropical water and associated planktonic fauna toward the coast. A specific area with reduced influence of oceanic currents presented typical coastal species. The pattern we observed with the dominance of oceanic plankton communities up to coastal areas may be typical in western boundary systems characterized by a narrow continental shelf.

The dispersal of marine organisms can be restricted by a set of isolation mechanisms including hard barriers or hydrological features. In the Western Atlantic Ocean, the Amazon River discharge has been shown to act as a biogeographical barrier responsible for the differences in reef fish communities between Caribbean Sea and Northeast Brazil continental shelves. Here, we compare the diversity of all Animalia phyla from biogeographic ecoregions along the Tropical Western Atlantic continental shelf to test the hypothesis that the Amazon River plume spatially structures species diversity. For that, we used beta diversity estimators and multivariate ecological analysis on a database of species occurrence of the whole animal kingdom including 175,477 occurrences of 8,375 species from six ecoregions along the Western Tropical Atlantic. Results of the whole animal kingdom and the richest phyla showed that the Caribbean Sea and Tropical Brazil ecoregions are isolated by the Amazon River Plume, broadening and confirming the hypothesis that it acts as a soft barrier to animal dispersal in the Western Tropical Atlantic. Species sharing is larger northwestwards, in direction of the Caribbean than the opposite direction. Beyond species isolation due to local characteristics such as low salinity and high turbidity, our results suggest the dominant northwestward currents probably play a major role in animal dispersion: it enhances the flux of larvae and other planktonic organisms with reduced mobility from Brazil to Caribbean and hinders their contrary movement. Thus, the Amazon area is a strong barrier for taxa with reduced dispersal capacity, while species of pelagic taxa with active swimming may transpose it more easily.