Annual Dynamics of Phytoplankton Communities in Relation to Environmental Factors in Saline–Alkaline Lakes of Northwest China

To evaluate the change trends of plankton in inland saline–alkaline water bodies, this study investigated the ecological restoration and rational development of saline–alkaline lakes in northwest China. From June to October 2023, phytoplankton communities in a high-salinity lake in Alar City, Xinjiang, were analyzed using standard survey methods for inland natural waters. Biodiversity indices were calculated, and redundancy analysis (RDA), Spearman’s correlation analysis, and Mantel test were carried out to assess the functional community structure of phytoplankton and its environmental drivers. In total, 115 phytoplankton taxa belonging to seven phyla were identified. The densities ranged from 23.76 × 105 to 53.54 × 107 cells/L. Bacillariophyta and Cyanophyta were the dominant phyla, accounting for 41.7% and 27.8% of the total taxa, respectively. The dominant species included Microcystis spp., Merismopedia sp., Cyclotella meneghiniana, and other algae. Spearman correlation analysis revealed that salinity, water temperature (WT), Na+, TDS, HCO3−, Cl−, and K+ were key environmental factors significantly influencing phytoplankton community structure. Mantel tests confirmed that salinity (SAL), TDS, DO, and major ions (K+, Na+, CO32−) served as key determinants of spatiotemporal phytoplankton community distribution (p < 0.05). RDA results indicated that WT, TDS, alkalinity (ALK), pH, salinity, and Na+ were the key factors driving seasonal variations in phytoplankton communities. Notably, decreasing salinity and ion concentrations stabilized the phytoplankton community structure, maintaining high-diversity indices. This highlights the positive impact of ecological restoration measures, such as fisheries-based alkalinity control and systematic environmental management, on the health of saline–alkaline lake ecosystems. These findings provide important insights for the sustainable development of saline–alkaline fisheries and the conservation of aquatic biodiversity in arid regions.