Hydrological dynamics are fundamental drivers of estuarine ecosystem structure and functioning, directly regulating fish assemblage abundance, species richness and functional redundancy. Using a 24-year monthly time series (1996-2019) combined with performance-based metrics of resistance, resilience and functional redundancy, this study provides evidence of the effects of extreme (>150 m3/s) and prolonged (≥5 consecutive months) continental discharge events on fish assemblages in a subtropical estuary of the Southwest Atlantic (32°S, 52°W). Our findings reveal markedly low resistance of shallow-water assemblages to hydrological disturbances, yet highly variable resilience, demonstrating not only immediate ecological disruption but also the capacity for functional reorganization following perturbations. Disturbance intensity and duration exerted differential and lasting impacts on abundance patterns and key ecological functions, leading to both short-term destabilization and persistent structural losses. These alterations have the potential to compromise species recruitment and disrupt critical processes such as larval transport, nursery maintenance and fisheries productivity. The study exposes the heightened vulnerability of estuarine fish assemblages to hydrological stressors intensified by climate change and human pressures, emphasizing the urgent need for proactive management and conservation strategies. Advancing the understanding of ecological resistance, resilience and functional redundancy is crucial to safeguard ecosystem stability and sustain global marine biodiversity.
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