A Multidimensional Approach to α- and β-Diversity of Fish Assemblages in Tropical Estuaries Under Semiarid Influence

Functional adaptations enable estuarine fishes to exploit habitats along environmental gradients. This study investigated how geomorphological and anthropogenic features of three tropical Brazilian estuaries within semi-arid river basins (Mamanguape, Paraíba, and Gramame) influence the taxonomic, functional, and phylogenetic diversity of fish assemblages during rainy and dry seasons. A total of 11,073 individuals from 118 species and 33 families were recorded. The assemblage was dominated by schooling, zoobenthivorous, fusiform, marine estuarine-dependent, and pelagic traits. We found that: (1) putative seasonality increases taxonomic β-diversity and decreases taxonomic α-diversity, likely due to intensified environmental filtering along estuarine gradients during periods of higher freshwater flow; (2) larger, permanently open estuaries showed β-diversity dominance, whereas the smaller, intermittently closed estuary exhibited α-diversity dominance; and (3) estuaries characterized by smaller mouth widths showed homogenization of species, functions, and lineages, reflecting low functional redundancy and increased vulnerability. Overall, our results show that integrating multiple diversity facets can support stronger connections with conservation priorities for estuarine fish biodiversity, highlighting the value of a multimetric approach in estuaries influenced by a semiarid climate.