Contrasting long-term changes in taxonomic versus functional diversity of fish assemblages in a subtropical estuary

Climate and anthropogenic disturbances are prevalent in estuarine and marine ecosystems. Alterations to these ecosystems caused by climate change and human impacts have intensified and accelerated the biodiversity crisis, causing losses of species and functions that are fundamental to the functioning of estuarine and marine ecosystems. The consequences of these disturbances are worrying, since estuaries are fundamental to maintaining the diversity of species functions, acting as a source for other ecosystems through species recruitment. Using standardized monitoring data (1996-2019), we assessed the temporal variability of the taxonomic and functional diversity of the fish assemblage of a subtropical estuary using indices of taxonomic diversity (Shannon and Pielou) and functional diversity (richness, evenness, dispersion and divergence), and their relationships with environmental, climatic and anthropogenic variables, using generalized additive models. We investigated the hypothesis that natural disturbances associated with the El Niño-Southern Oscillation phenomenon and anthropogenic impacts related to changes in the morphology of the Patos Lagoon estuary mouth influence the taxonomic and functional diversity of the estuarine fish assemblage. The results suggest marked seasonal and annual variability in taxonomic diversity indices and less variability in functional diversity indices, which seems to be associated in a complex way with the influence of natural, climatic and anthropogenic variables and climatic factors like El Niño and La Niña events. Our findings suggest that anthropogenic disturbances, in synergy with climatic impacts, may drive changes in taxonomic and functional diversity in estuarine fish assemblages in subtropical estuaries.