Spatial and seasonal shifts in phytoplankton functional groups in Lake Victoria (Kenyan sector): implications for eutrophication and water quality management

Phytoplankton community structure in Lake Victoria has shifted markedly under increasing anthropogenic pressure from eutrophication, catchment disturbances and climate variability. This study examined spatial and seasonal variation in phytoplankton functional groups, integrating observations with environmental gradients and emerging pollutant pressures within Nyanza Gulf. Cyanobacteria, particularly Microcystis (functional group M), dominated eutrophic inshore waters, while diatoms (groups C, P, and N) and nitrogen-fixing cyanobacteria (Dolichospermum, group H) became more prominent offshore. Functional group distribution was strongly influenced by nutrient enrichment, turbidity and hydrodynamic conditions. Seasonal rainfall enhanced nutrient loading, reinforcing cyanobacterial blooms and associated toxin risks. These findings highlight a transition toward persistent harmful bloom-forming taxa in nearshore waters with implications for water supply, fisheries and ecosystem health in the Lake Victoria Basin.