Linking N:P Stoichiometry to Phytoplankton Community Shifts Towards Bloom-Forming Algae Dominance in Winam Gulf, Lake Victoria

Despite increasing nutrient enrichment in the Winam Gulf of Lake Victoria, the role of nitrogen-to-phosphorus (N:P) stoichiometric ratios in shaping its phytoplankton communities remains unclear. This study examined the influence of N:P ratios, nutrient limitations and over-enrichments on phytoplankton community structure, testing whether deviations from the Redfield ratio (16:1) correspond to shifts in dominant phytoplankton taxa. Data on total nitrogen (TN), total phosphorus (TP), and phytoplankton communities were collected following standard methods (APHA 2017) from diverse stations across the Winam Gulf were analyzed considered in this study. Nutrient data were analyzed using stoichiometric ratios, while phytoplankton communities were evaluated using diversity indices. Correlation and multivariate approaches were used to assess relationships between nutrient conditions and community structure. The N:P ratios (0.18–0.80) were consistently below the Redfield ratio (16:1), indicating strong nitrogen limitation and phosphorus enrichment. Cyanobacteria dominated most stations, with Dolichospermum sp and Cylindrospermopsis sp showing competitive dominance under low N:P conditions. A significant negative relationship was observed between N:P ratios and phytoplankton abundance (Spearman r = -0.87, p = 0.054). These findings suggest that nutrient imbalance influences phytoplankton community structure and promotes bloom-forming taxa in the Winam Gulf. Improved catchment based nutrient management and continued monitoring is recommended to support ecosystem health and fisheries sustainability.