Abstract

Cyanotoxins in surface drinking water sources are known to pose a threat to human health, of which microcystin-LR is the most investigated. The main aim of this study is to assess the levels of microcystin-LR in Luvuvhu River catchment and to assess the physicochemical parameters that may promote the growth of cyanobacteria. The level of microcystin-LR in some of the sampling sites was <0.18 µg/l except for one site (Luvuvhu River just before the confluence of Dzindi and Mvudi Rivers) which had a reading of 2 µg/l during August, 2009. Though the results indicated that some of the sites, especially on the Mvudi River system, had high nutrient levels, alkaline pH and water temperature <24°C, the levels of microcystin-LR were <0.18 µg/l. The production and release of microcystin-LR into water bodies by different strains of cyanobacteria involves a complex relationship between environmental variables. The water quality of the shallow hand dug wells and reservoir water were almost similar. The outflows had slightly high levels of nitrates and no soluble reactive phosphates in comparison with inflows, suggesting that the phosphates were being incorporated into the sediments. This could be a potential danger if the climatic conditions were to change and as this will promote the proliferation of nitrogen fixing cyanobacteria. Maybe, the non availability of phosphorous which is known to be a limiting nutrient in freshwater systems, could have contributed to no cyanobacteria blooms. Thus, the domestic consumption of these surface water sources may be a potential health hazard to the rural communities as it exposes the users to low levels of cyanotoxins over a long term.

Key words: Nutrient enrichment, microcystin, sediments, cyanobacteria.