<p>This study addresses the knowledge gap regarding the spatial and temporal distribution of zooplankton functional groups (ZFGs) and the environmental factors influencing them, to evaluate the ecological status of the Sidi Mohamed Ben Taiba reservoir (SMBT), a semi-arid reservoir in north-western Algeria. Morphological, physiological, and behavioral categories were selected as response traits. Physicochemical variables were considered key indicators in shaping functional traits. A total of 46 zooplankton species were classified into five functional groups based on their biological and ecological characteristics. The results showed that temporal variations in environmental variables strongly influenced these functional groups. Non-metric multidimensional scaling analysis (NMDS) confirmed that the distribution of ZFGs in the SMBT Reservoir follows a strict environmental gradient governed by a “water quality” axis (transparency, salinity, and pH) and an “organic load” axis. G1 (algivorous cladocerans) and G3 (herbivorous rotifers) exhibited a successful coexistence strategy through niche partitioning among their genera in response to varying nutrient conditions. G2 (copepods) and G4 (predatory rotifers) showed a clear preference for clear-water conditions characterized by higher alkalinity and electrical conductivity. The most notable finding was the behavior of G5 (detritivorous rotifers). Despite its proximity to the organic matter (MO) vector in the NMDS ordination, the GLMM analysis revealed a significant negative relationship between its abundance and MO concentration. At Station S4, the most polluted site, G5 was entirely absent despite high detrital availability, confirming its reliability as a bioindicator of reservoir ecosystem quality. This functional trait–based approach provides a deeper understanding of aquatic ecosystems and offers a forward-looking perspective on ecosystem quality and sustainability.</p>