Water quality, nutrient dynamics and phytoplankton diversity in cage culture zones of Lake Victoria, Kenya: implications for ecosystem management

Cage culture expansion in Lake Victoria raises concerns about nutrient loading and phytoplankton community shifts. This study assessed physicochemical parameters and phytoplankton diversity in cage and noncage zones to identify factors influencing water quality and ecological health. Triplicate sampling stations were established at Anyanga, Victory Farms and Mfangano. Measured parameters included nutrients (phosphorus, ammonium), chlorophyll a, algal toxins, and standard limnological variables (temperature, dissolved oxygen, conductivity, pH, Secchi depth). Phytoplankton diversity and richness were calculated using Shannon–Wiener and Simpson indices, respectively. Statistical analyses (ANOVA, α=0.05) were performed with Minitab v17. Significant spatial variation was observed for chlorophyll a, temperature, dissolved oxygen, conductivity, pH and Secchi depth at Anyanga (p<0.05), while nutrients at Victory Farm showed no significant differences (p>0.05). Mean dissolved oxygen was 7.41±1.39 mg/L (p=0.0491), conductivity 142.39±63.79 µS/cm (p=0.0005), phosphorus 330.02±311.9 mg/L (p=0.001) and ammonium 319.29±397.35 mg/L (p=0.0008). Mean chlorophyll a at Mfangano cages was 592.9±604.38 mg/L. Diatoms dominated (46% of total phytoplankton) with significant differences among stations (p=0.0001). High turbidity and nutrient enrichment favour cyanobacterial proliferation, posing risks to water quality and human health. These findings provide a basis for ranking cage sites by pollution level and underscore the need for improved management practices to mitigate eutrophication and harmful algal blooms.