Assessment of biotic health of an effluent dominated river

This study was on Notwane River, in South Eastern Botswana. Notwane is a heavily utilized river, one of two water sources for Gaborone City, receiving effluent from Glen Valley WWTP. The Notwane river has been under increasing pressure from the ever expanding Gaborone population into its riparian zones, as well as decreasing quality of treated waste water as population increases. Previous studies on the water quality of effluent in the river have recommended investigations of the ecosystem health of Notwane over concerns of its threatened status. The study assessed and evaluated current biotic health of a section where the river by-passes Gaborone, utilizing macro-invertebrates and riparian vegetation diversity, abundance and sensitivity. Four field visits were done between January and March 2010. A vegetation assessment tool, VEGRAI was used to evaluate riparian vegetation health. The SASS, ASPT, Simpson's index and FFGs were used for macro-invertebrate data analysis. The ASPT had means of 4.16 and 6.4, downstream and upstream respectively, and Simpson's diversity index had means of 0.4 and 0.6 downstream and upstream respectively. The results for the two zones are variable and suggest higher physical stress downstream as compared to upstream. VEGRAI model indicates moderate divergence of riparian vegetation from natural state, with category C and D for upstream and downstream respectively, and a high of 13% presence of exotic species. Overall, the results suggest the river function processes to be typical of human-disturbed streams. The study contends that existing water resources protection is weak, and recommends initiation of river health monitoring as the best strategy to combat any unavoidable deterioration of riverine ecosystem state, both of the Notwane and other heavily utilized streams in the country.