Occurrence and Seasonal Variation of Picoplankton at Saiysad Freshwater in Taif City, Saudi Arabia

A wadi ecosystem, a wetland characterized by seasonal water flow, is a unique freshwater environment typically found in semi-arid and arid regions. This study investigates the seasonal and spatial dynamics of environmental properties and microbial plankton communities at Wadi Saiysad in Taif City, Saudi Arabia. Using flow cytometry, three distinct picoplankton populations were observed: Synechococcus and heterotrophic prokaryotes classified as low (LNA) or high (HNA) nucleic acid content. Surface freshwater samples were collected from three distinct sites, representing habitats with actively flowing water, biodiverse communities, and human-influenced areas. Interestingly, no significant differences among stations were observed, suggesting that the sampled stretch of Wadi Saiysad receives similar nutrient inputs. Seasonal water temperature reached 24.5 ± 0.57 °C in summer and the pH ranged from neutral to slightly alkaline. Nutrient analyses revealed that Wadi Saiysad is eutrophic and limited by phosphorus. Phytoplankton biomass was dominated by nanoplankton, particularly in summer (46.60 ± 5.33%), while Synechococcus increased significantly with a maximum abundance of 1.32 × 104 cells mL−1 during the cooler months. HNA prokaryotes displayed marked seasonal variation (1.95 × 104–1.78 × 105 cells mL−1) compared to LNA prokaryotes (2.05–8.17 × 104 cells mL−1). This study highlights the urgent need for monitoring and managing the nutrient inputs in Wadi Saiysad to protect its biodiversity and support sustainable use.