<p>Trophic gradients are key drivers of community organization in freshwater ecosystems, yet taxonomic approaches may overlook important ecological changes occurring at the functional level. In this study, we investigated how zooplankton diversity respond to spatial and temporal environmental gradients in a large tropical reservoir in southeastern Brazil. The data were collected across two major sub-basins (Grande and Sapucaí rivers). A total of 69 zooplankton taxa were recorded. Rotifers showed pronounced taxonomic turnover among trophic states, whereas cladocerans responded mainly through changes in abundance while maintaining relatively stable species composition. Despite these shifts in community structure, traditional taxonomic diversity indices (richness, Shannon, and Simpson) did not vary significantly along the trophic gradient, indicating apparent spatial stability in diversity. By contrast, functional diversity metrics revealed clear spatial and temporal patterns. Functional richness (FRic), functional evenness (FEve), and functional dispersion (FDis) were significantly higher in the Sapucaí River, while FEve varied along the trophic gradient, with lower values under mesotrophic conditions, indicating functional dominance of specific trait combinations. Seasonal reductions in functional divergence (FDiv) and functional dispersion in June suggest strong environmental filtering and convergence toward intermediate trait strategies. Analyses of feeding habits further supported these patterns, revealing reduced abundances of filter-feeding, raptorial, and suctorial organisms in eutrophic environments, indicating functional simplification despite stable taxonomic diversity. Together, these results demonstrate that trait-based approaches provide greater sensitivity to detect ecological responses of zooplankton communities to eutrophication, highlighting the importance of incorporating functional indicators into monitoring and management of tropical reservoirs.</p>