Phytoplankton community structure and functional dynamics in a Mediterranean reservoir under hydroclimatic variability.

Increasing hydroclimatic variability is profoundly altering Mediterranean reservoirs, where prolonged drought cycles restructure phytoplankton communities and threaten drinking water security. North African reservoirs, despite their socio-economic importance, remain severely underrepresented in the functional ecology literature. Here we present the first functional-group-based characterization of phytoplankton dynamics in the Boukourdane Reservoir (Tipasa, northern Algeria), a deep sub-humid Mediterranean system that underwent a severe hydrological crisis in 2021. Monthly sampling at four stations over two hydrological years (April 2023-March 2025) combined taxonomic identification, Reynolds functional group classification, Phytoplankton Assemblage Index (Q), diversity metrics, and redundancy analysis (RDA). A total of 165 taxa across 76 genera and 10 classes were identified, with species richness fluctuating between 25 (autumn 2023) and 66 taxa (winter 2024). Phytoplankton biovolume peaked above 150 mm3 L⁻1 in spring 2023 before declining to 30 mm3 L⁻1 by spring 2025. Despite 24 functional codons recorded, the community was largely dominated by codon L0 (Ceratium hirundinella), consistently accounting for 60-71% of total biovolume. The Q index ranged from 2.18 to 2.89, indicating moderate ecological status throughout. RDA explained 62.5% of the constrained functional variance (p = 0.001), identifying thermal stratification, alkalinity, and dissolved inorganic nitrogen as primary environmental filters. These results reveal a functionally structured community shaped by strong environmental controls, with interannual changes in biovolume reflecting temporal variability in nutrient availability and other physicochemical conditions. The applied functional group framework provides a useful framework for ecological monitoring and early warning in Mediterranean drinking water reservoirs under hydroclimatic stress.