Comparative assessment of water quality and cyanobacterial toxin risks in Lake Chivero and Lake Kariba, Zimbabwe

Concerns regarding the degradation of aquatic ecosystems and the production of toxins resulting from water pollution and eutrophication are increasing. The study was carried out to compare water quality and algal communities of Lake Chivero, a hypereutrophic lake, and Lake Kariba, a warm monomictic meso-oligotrophic lake. The study hypothesised that these reservoirs had contrasting water qualities and significantly different phytoplankton communities. A total of 160 water and 160 phytoplankton samples were collected for physico- chemical analysis and assessment of algal community structures. Identification and abundance scoring of phytoplankton was done through microscopy. Cyanotoxin genes were detected with polymerase chain reaction using molecular primers for microcystin and cylindrospermopsin production. Lake Chivero had lower temperatures and transparency but higher conductivity, phosphates and total phosphorus. Dissolved oxygen, ammonia, and nitrate were higher in Lake Chivero than in Lake Kariba in winter. Cyanobacteria species dominated both lakes, showing the highest cellular concentrations across all sampling months. In Lake Chivero, Microcystis spp. dominated with confirmed toxin-producing strains, while Cylindrospermopsis sp. was more prevalent in Lake Kariba. These findings highlight the link between eutrophication and the production of toxins in water bodies, posing significant risks to water quality and aquatic life.