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Multiscale processes through dynamic boundaries drive the local contributions (LCBD; uniqueness in species composition and abundance) and species contributions (SCBD; individual species abundance variation) to fish beta diversity in coastal seascapes. We partitioned LCBD into LCBD.sub.Repl (replacement) and LCBD.sub.AbDiff (abundance difference), and used model-based approaches to investigate whether seascape size influences environmental and spatial drivers of LCBD, and effects of species (distribution, prevalence, and coastal dependence) on SCBD. Three large bays (bay level) and three zones per bay (zone level), southeastern Brazil, were seascape models during 1 year. At the bay level, more locations had higher LCBD, related to species composition, lower water transparency, and much less to flood/high tides and higher number of estuaries. LCBD.sub.Repl was also prevalent and stronger positive effects of transparency and mangrove cover reinforced the importance of environmental filtering and alternative habitats driving local contributions to replacement processes in larger seascapes. Species richness influenced positively LCBD primarily at the zone level, and negative effects of vegetal cover and primarily human settlements suggested species loss under lower heterogeneity. Also, more similar LCBD.sub.Repl and LCBD.sub.AbDiff evidenced the increasing local contributions to abundance differences in smaller species pools. At both levels, larger- and finer-scale spatial effects evidenced possible influences of dispersal and biotic interactions, respectively, on LCBD.sub.Repl and LCBD.sub.AbDiff. SCBD was positively related to species prevalence at locations and distribution at both levels. Therefore, multiscale mechanisms dependent and irrespective of seascape size support the critical importance of habitat availability, complexity, and variety to beta diversity of coastal fishes., Author(s): Ellen Martins Camara [sup.1] , Magda Fernandes de Andrade-Tubino [sup.1] , Taynara Pontes Franco [sup.1] , Leonardo Mitrano Neves [sup.2] , Luciano Neves dos Santos [sup.3] , Francisco Gerson [...]