Diversity of Algae and Potentially Toxic Cyanobacteria in a River Receiving Treated Sewage Effluent: A case of Notwane River (Gaborone, Botswana)

Abstract

Algae and cyanobacteria are important primary producers and can be used to indicate the health status of a water body. The aim of this study was to investigate eutrophication levels of a river receiving sewage effluent, to determine the diversity of algae and cyanobacteria species and to identify species with a potential for toxicity. This study showed that the average phosphates level in Notwane River were 1.45mg/L which was slightly above the 1 mg/L recommended environmental guideline by EPA, while the average amount of nitrates in Notwane River was 5.53mg/L which was within the 10mg/L acceptable limit. 33 species of 29 genera representing four phyla; Chlorophyta, Cyanophyta, Euglenophyta and Bacillariophyta were identified. Although dense blooms of cyanobacterial species whose crude extracts were found to have toxic effects on mice were found at the wastewater stabilisation ponds dumping the sewage effluent, only a few colonies of Microcystis spp. were found at the river. These attributes gave an indication that the river, during the study period, was a relatively healthy river.