Linking phytoplankton traits to environment in four arid North-African reservoirs

Arid and semi-arid reservoirs experience strong hydrological variability, intense evaporation, and seasonal water-level fluctuations, leading to strong changes in salinity and habitat stability compared to temperate systems. As primary producers, phytoplankton are central in freshwater ecosystem functioning and respond rapidly to hydrological instability and physico-chemical disturbances, making them key indicators of ecosystem responses to global change in arid systems. Phytoplankton ecology has been extensively studied in temperate, nutrient-driven systems; however, functional and trait-based approaches remain scarce in arid regions and phytoplankton functional strategies in North African reservoirs poorly understood. To address this gap, we analysed phytoplankton functional assemblages across four arid reservoirs in north-western Algeria using a trait-based approach and multivariate analyses.Functional assemblages were primarily structured by salinity and depth, with temperature acting as a secondary driver, whereas phosphorus and transparency played a minor role. These gradients filtered distinct functional strategies, favouring small, motile and mixotrophic taxa in mineralised and hydrologically unstable (shallow, fluctuating) conditions, and larger taxa in deeper, more stable reservoirs.Our results indicate that, unlike temperate lakes where phytoplankton succession often follows seasonal nutrient dynamics, phytoplankton functional structure in arid and tropical areas is dominated by physical constraints and hydrological instability.