Mediterranean reservoirs face escalating water quality degradation due to compounding pressures from climate change and anthropogenic activities. In this context, we investigate the Sidi Chahed reservoir system (Morocco) where tributaries Oued Mikk & egrave;s and Oued Mellah face severe pollution challenges. The watershed's semi-arid climate exacerbates ecological threats from agricultural and domestic discharges. This study focuses on the spatio-temporal dynamics of nutrient concentrations (sulfates, orthophosphates, nitrates, nitrites, ammonium) and organic pollution indicators (BOD, COD) to assess their impact on eutrophication risks and ecosystem health. Water samples were collected from six stations over a complete hydrological cycle, covering spatial (upstream-downstream) and temporal (seasonal) variations. The results indicate significant spatio-temporal variability in nutrient and organic matter levels, driven by agricultural practices and domestic discharges. Elevated nutrient concentrations, particularly orthophosphates (0.14 +/- 0.14 mg/L) and nitrates (2.10 +/- 1.34 mg/L), were observed in Oued Mikk & egrave;s, reflecting intense anthropogenic pressure. Organic pollution levels (BOD: 10.26 +/- 5.23 mg/L; COD: 31.77 +/- 14.33 mg/L) were higher in winter and autumn, reflecting combined agricultural runoff and wastewater inputs. Water temperatures range from 9.7 degrees C in winter to 32.4 degrees C in summer, with pronounced thermal stratification during hot months, a phenomenon common in Mediterranean reservoirs. Data analysis shows higher organic input in winter and autumn. ANOVA confirmed strong seasonal variability for nutrients (p < 0.01), while spatial patterns were significant for sulfate and ammonium (p < 0.05). Spearman correlations revealed sulfate-BOD linkages (r = 0.593, p < 0.01) and temperature-driven oxygen decline (r = - 0.511), highlighting key pollution pathways. These findings highlight the urgent need for adaptive management strategies, including phytoplankton monitoring programs, agricultural runoff containment through riparian buffers, and wastewater treatment optimization. Such targeted interventions, aligned with Morocco's National Water Plan, would mitigate eutrophication risks while addressing the identified spatio-temporal pollution patterns characteristic of Mediterranean reservoirs under climate stress.