Functional dynamics of fish assemblages in a tropical estuary during different phases of El Niño‒Southern Oscillation

Climate change has increased the frequency and intensity of El Niño‒Southern Oscillation (ENSO) events, which are climate phenomena involving fluctuations in surface temperatures, wind speed, and rainfall patterns across the globe. These changes affect estuarine fish communities by modifying hydrological cycles and environmental conditions such as salinity and freshwater flow, thereby influencing community composition and functional diversity. Analyzing functional diversity is therefore essential to understanding ecosystem resilience to climate change and anticipating the growing impacts of ENSO. Since ENSO effects vary by phase and geographic location, this study examined how different ENSO phases influenced the functional dynamics of the fish community in the Mamanguape River estuary in northeastern Brazil, using data collected from 2011 to 2016. In terms of environmental conditions, there was a peak in precipitation during the first year of La Niña, followed by the subsequent dry years, with the neutral and El Niño periods being more similar. However, the La Niña phenomenon of 2011 was atypical, with below-average rainfall, and occurred in the middle of a prolonged period of drought that began in the late 1990s, which may have influenced the functional response of the fish community. Unexpectedly, functional redundancy values remained low throughout all ENSO phases, highlighting possible functional vulnerability. The functional diversity of this ecosystem may be maintained because of species complementarity and compensatory effects. These results highlight the importance of analyzing the influence of ENSO in estuaries with different climatic characteristics.