El Niño Reduces Beta Diversity of Fish Assemblages Across a Large Subtropical Coastal Lagoon

Abstract The El Niño –Southern Oscillation (ENSO) profoundly alters climatological and hydrological conditions, reshaping environmental gradients and ecological dynamics in riverine, estuarine, and coastal ecosystems. We hypothesized that El Niño events would promote homogenization of fish assemblages and reduce β-diversity, whereas La Niña conditions would enhance habitat differentiation and increase β-diversity. To test this hypothesis, we investigated variations in fish β-diversity across a freshwater–estuarine–marine gradient in the Patos Lagoon, southern Brazil, under La Niña , El Niño , and neutral climatic conditions. We analyzed total β-diversity and its local and species contribution components in relation to environmental variability associated with ENSO phases. The results revealed a clear pattern of homogenization between the estuarine and marine environments during El Niño , characterized by increased species sharing and reduced taxonomic differentiation. Species sharing between these regions increased from 10 to 13 species during La Niña and neutral conditions, respectively, to 18 species during El Niño . In contrast, La Niña promoted stronger habitat compartmentalization, resulting in higher total β-diversity (0.558) than El Niño (0.538) and neutral conditions (0.528), indicating greater spatial heterogeneity in fish assemblages. These findings support our hypothesis that ENSO-driven changes in environmental gradients shape fish β-diversity, with El Niño promoting biotic homogenization and La Niña enhancing habitat differentiation. As ENSO events are projected to become more frequent and intense, these processes may have important implications for the biodiversity and structural organization of subtropical coastal lagoons and other estuarine ecosystems.