Species dispersal and community assembly facing the Atlantic meridional Overturning circulation

• High-resolution Lagrangian simulations reveal contrasting retention along the Pernambuco Plateau. • Mesoscale circulation enhances local retention in a highly advective AMOC system. • Northern shelf communities are dominated by mass effects and oceanic advection. • Southern plateau communities reflect a balance between species sorting and mass effects. • Dispersal pathways explain biodiversity hotspots in the Tropical Southwestern Atlantic. The strong advective circulation governing western boundary current systems poses challenges to coastal organism to retain their planktonic larvae and assembly in local communities. However, instabilities around the predominant current, generating features such as eddies, may increase retention and thereby enhance local diversity. Here, we have investigated this hypothesis through high-resolution (1/36°) community-based Lagrangian dispersal modelling experiments conducted along the Pernambuco Plateau and adjacent open ocean off Brazil, which is part of the strong Atlantic Meridional Overturning Circulation. In the highly advective north portion of the plateau, we found that retention is reduced and advection from open ocean is intense. Under such circumstances, the assembly of local communities is likely driven by mass effect, which results in homogeneous communities with reduced biodiversity. In contrast, in the south portion of the plateau, eddies and meanders associated with the interaction of the currents with the plateau topography and the seasonal variability of the South Equatorial Current bifurcation increase local retention and reduce advection from open ocean. In such conditions, both, species sorting and mass effect assembling archetypes, likely drive the distinct and rich biodiversity inhabiting the region to the south of the plateau. The dispersal patterns obtained from the Lagrangian experiments align with the known spatial patterns of biodiversity distribution along the Tropical Southwestern Atlantic and provide important insights for regions and taxa for which knowledge is limited or absent. These results also provide elements for the proper definition of biodiversity conservation and management strategies and for a better understanding of the processes regulating community assembly in highly advective systems around the globe.