Seasonal phytoplankton ecosystem dynamics in response to environmental variables in Winam Gulf, Lake Victoria, Kenya

The Winam Gulf of Lake Victoria has been increasingly subjected to anthropogenic disturbances, which pose significant threats to its ecological health and water quality. This study examined the spatiotemporal variations in phytoplankton diversity, community structure, and abundance in response to changing environmental conditions from February to September 2020. Key physicochemical parameters, including water transparency, pH, chemical oxygen demand (COD), dissolved oxygen (DO), temperature, chlorophyll a, total nitrogen (TN), total phosphorus (TP), silicate, and ammonia nitrogen (NH₄⁺–N), were measured. The phytoplankton community was dominated by eight phyla, with Cyanobacteria (50%), Chlorophyta (28.05%), and Bacillariophyta (14.63%) being the most prevalent. The most common cyanobacteria species identified were Microcystis aeruginosa and Dolichospermum flosaquae. Phytoplankton abundance ranged from 45 to 4900 individuals/mL, peaking in July. Shannon–Weaver diversity indices varied from 1.1 to 4.2, with an average of 1.9, indicating moderate pollution levels in the Winam Gulf. Redundancy analysis (RDA) revealed that significant shifts in the phytoplankton community were statistically correlated with water quality variables, notably NH₄⁺–N, temperature, transparency, and DO. NH₄⁺–N was identified as the key limiting factor for cyanobacterial abundance in the gulf. The dominance of Cyanobacteria, along with their seasonal fluctuations influenced by major environmental factors, underscores the complexity of this lake ecosystem. These findings emphasize the urgent need for continued efforts to mitigate adverse impacts and improve the overall health of this aquatic environment.