Multiple coastal habitats structure taxonomic and functional diversity of fish assemblages in tropical estuaries with contrasting salinity regimes

Multiple coastal habitats play a fundamental role in maintaining biodiversity and in the spatial structuring of fish assemblages, influencing taxonomic and functional diversity in response to the dynamics of estuarine environmental conditions. This study evaluated the taxonomic and functional diversity of fish assemblages in distinct habitats of two tropical estuaries under different salinity regimes in Northeastern Brazil. The study was conducted in the Tubarão (hypersaline) and Mamanguape (positive) river estuaries, encompassing sampling in the Seagrass (SE), Sandflat (SA), Mudflat (MU), and Mangrove fringe (MF) habitats. Taxonomic diversity was analyzed through species richness (S), Shannon-Wiener diversity (H′), and Pielou’s evenness (J′), and functional diversity was analyzed through functional richness (FRic), functional evenness (FEve), functional divergence (FDiv), Double Principal Coordinate Analysis (DPCoA), and community-weighted mean (CWM). Environmental conditions (salinity, transparency, and temperature) differed among estuaries, habitats, and in the interaction between these factors (PERMANOVA, p ≤ 0.001), with higher mean values of salinity and transparency in the hypersaline estuary and greater environmental variation among habitats in the positive estuary. Taxonomic diversity differed among habitats in both estuaries (PERMANOVA, p ≤ 0.01), although J′ differed only in the hypersaline estuary. Functional diversity also differed among habitats for FRic and FDiv in both estuaries (PERMANOVA, p ≤ 0.01), whereas FEve did not differ. DPCoA indicated differentiation in taxonomic and functional composition among habitats, a finding related to distinct forms of occupation in taxonomic and functional spaces. CWM indicated variations in functional composition among habitats, associated with the use of the vertical habitat, body morphology, and ecological and feeding guilds. Overall, the results revealed a spatial pattern of habitats in structuring the taxonomic and functional diversity of fish assemblages, with differentiated responses under distinct regimes of estuarine salinity.