Macrophyte dynamics in Mediterranean rivers: a first insight from the upper Oum Er-Rbia Basin (Middle Atlas, Morocco)

Macrophyte dynamics in rivers are expressed through spatiotemporal gradients that reflect the functioning of freshwater ecosystems. In Mediterranean rivers, characterized by irregular low-flow periods and sudden floods, these gradients strongly influence the distribution and structure of aquatic vegetation. In Morocco, however, the ecology and distribution of aquatic macrophytes remain poorly documented, particularly in the Upper Oum Er-Rbia Basin. This study analyzes the spatiotemporal dynamics of macrophyte diversity along this basin using thirteen monthly surveys (June 2018–June 2019) conducted at fourteen sites in five rivers spanning spring-fed, perennial, and temporary streams. Macrophyte diversity and evenness were quantified using Shannon–Weaver (H′) and Pielou’s (J′) indices and evaluated with a multivariate framework: PERMANOVA and NMDS to test community differentiation between rivers and months, PCA to summarize hydrochemical and hydromorphological gradients, and CCA to relate species composition to environmental drivers. The results reveal marked spatial structuring, with upstream–downstream organization and strong river-specific assemblages. Geographic position within the basin emerges as a key determinant of macrophyte diversity, while hydrochemical status and hydromorphological conditions—including flow velocity, water depth, channel substrate, and disturbance pulses associated with floods and low-flow phases—shape within-river trajectories of composition and cover. This work provides the first basin-scale ecological assessment of macrophyte distribution in the Upper Oum Er-Rbia and highlights the importance of integrating spatial and temporal processes when diagnosing and managing Mediterranean river ecosystems.