Trophic reorganization and biomass shifts of fishes driven by ENSO dynamics in a tropical estuary

Climatic phenomena, such as the El Niño-Southern Oscillation (ENSO), drive significant interannual environmental variability in coastal ecosystems worldwide. Given predictions that the frequency and intensity of ENSO events will increase due to global warming, understanding how these variations impact tropical estuarine communities over time is critical. However, the effects of the ENSO on the trophic structure of tropical estuarine fish assemblages remain poorly understood. Thus, we evaluated variations in the trophic structure of the ichthyofauna in the Mamanguape River estuary, northeastern Brazil, across different ENSO phases (El Niño, La Niña, and Neutral). Fish species were categorized into functional trophic guilds, and changes in biomass distribution were analyzed along the trophic level (TL) spectrum. Our findings reveal that fish assemblages underwent a trophic reorganization in response to different ENSO phases, with biomass redistribution occurring primarily among species at intermediate trophic levels (TL 3.0 to 4.0). During La Niña periods (characterized by higher rainfall and primary productivity), energy flow was predominantly sustained by zooplanktivorous fishes (e.g., Rhinosardinia bahiensis). In contrast, zoobenthivorous species (e.g., Sphoeroides testudineus) accounted for a larger percentage of biomass under the drought conditions associated with El Niño. Despite the reorganization at intermediate trophic levels, biomass at the lower and upper trophic levels remained stable across all ENSO phases. This indicates that fluctuations within intermediate trophic levels acted as structural buffers, preventing environmental disturbances from affecting the biomass stability of baseline primary consumers or apex piscivores. Finally, this study provides important insights into the ecological mechanisms promoting the functional resilience of tropical estuarine ecosystems faced with extreme climate events, highlighting the importance of integrating functional and trophic indicators into conservation strategies for such environments. Furthermore, considering future climate scenarios, this study shows that supporting long-term ecological monitoring programs is essential for assessing the continued persistence of ecosystem resilience.