Abstract
Duhok Dam is a key multipurpose reservoir in northern Iraq that is increasingly exposed to ecological pressures arising from rapid population growth, climate variability, and localized anthropogenic pollution. In this study, we investigated the seasonal and spatial variability of phytoplankton communities and associated physicochemical variables to evaluate water quality conditions and to assess the reliability of phytoplankton communities as bioindicators of environmental stress. The samples were collected six times over nine months in 2021; 36 samples were collected from six sampling stations representing different pollution sources, across four distinct seasons. Multivariate analyses (PCA) combined with statistical testing revealed pronounced seasonal differentiation in both biological and physicochemical variables (ANOSIM, p < 0.0001). Spring conditions were shaped by precipitation-driven nutrients. This favored mixotrophic phytoplankton, especially Euglena sp. Conversely, summer thermal stratification promoted warm-adapted, pollution-tolerant taxa, such as Oscillatoria sp. and Peridinium cinctum . There was also marked spatial heterogeneity across the reservoir. Sites with untreated domestic wastewater had elevated COD SMn and NO 3 -N concentrations and were dominated by pollution-tolerant diatoms like Nitzschia acicularis . Areas impacted by natural sulfur springs hosted distinct phytoplankton assemblages, including Merismopedia sp. and Navicula sp., highlighting the influence of local geochemical conditions. Overall, our results demonstrate that the phytoplankton community structure responds rapidly and sensitively to both anthropogenic and natural stressors, highlighting its value as an integrative bioindicator of reservoir water quality. This study provides a conceptual basis for incorporating phytoplankton-based biological monitoring into sustainable reservoir management and water quality assessment frameworks in semi-arid regions.