Assessing the potential health risk of cyanobacteria and cyanotoxins in Lake Naivasha, Kenya

Abstract

This study discerned the causes of cyanobacteria blooms in Lake Naivasha (Kenya). We hypothesized that phytoplankton and cyanobacteria biomass respond to hydrologic cycles, peaking during the wet season, and that microcystin (MC) concentrations are highest following the bloom collapse. Hydrologic loading (inferred from rainfall and lake level changes) and phytoplankton responses in two basins of the lake were monitored over a wet season followed by a dry season between September 2010 and March 2011. Results show that both phytoplankton and cyanobacteria biomass peaked in both basins during the wet season, with associated peaks in particulate MC concentrations. Even though phytoplankton and cyanobacteria biomass were higher in the smaller deep basin, MC concentrations were lower than in the large shallow basin. The high-MC levels during the wet season were followed by a greater MC production per cyanobacteria biomass unit in the dry season in both basins. The timing of the cyanobacteria bloom suggests that its formation was likely controlled by large nutrient influxes from the contributing catchment to the lake associated with intense rainfall following an intense drought, posing a risk to the health of the community due to increased MC levels.