Ecological Degradation Along an Upstream–Downstream Gradient in the Ziz Stream (Morocco): Evidence from Physicochemical and Macroinvertebrate Indicators

This study investigated spatial and temporal variations in morphometric characteristics, physicochemical parameters, and benthic macroinvertebrate assemblages in the Ziz stream (Morocco), aiming to assess water quality and identify key environmental drivers of ecological structure. A multi-seasonal field survey was conducted across eight sampling stations (S1–S8), integrating in situ measurements and laboratory analyses of hydro-morphological, chemical, and biological variables. Results revealed significant spatial heterogeneity in morphometric parameters, including channel width (H = 26.19, p < 0.001), depth (H = 26.58, p < 0.001), and flow velocity (H = 25.66, p < 0.0001). Physicochemical variables also varied significantly among stations (p < 0.001), with marked downstream increases in temperature (up to 25.29 °C), conductivity (up to 1710 µS/cm), nutrients, and organic pollution indicators (BOD5 up to 67.87 mg/L; COD up to 138.37 mg/L), while dissolved oxygen decreased (down to 5.25 mg/L). Macroinvertebrate assemblages comprised 7168 individuals, 65 taxa, and were dominated by Diptera (39.94%) and Ephemeroptera (23.97%). Taxonomic richness ranged from 7 to 31 taxa, with the highest values upstream (S2) and the lowest downstream (S6). IBMWP scores indicated a clear degradation gradient from very good water quality (Class I, S1–S2) to poor conditions (Class IV, S5–S6). Simpson’s diversity index ranged from 0.63 to 0.93, declining significantly in polluted sites. These findings highlight a pronounced upstream–downstream ecological gradient driven by anthropogenic pressures.