Trends in chemical pollution and ecological status of Lake Ziway, Ethiopia: a review focussing on nutrients, metals and pesticides

Aquatic ecosystems contribute to human well-being by delivering ecosystem services, but their protection has been given low priority in Africa. Lake Ziway in the Ethiopian Rift Valley basin provides services including irrigation, drinking water and fish food in the region. This paper reviews the biological resources and spatio- temporal variation of water quality of the lake focussing on nutrients, metals and pesticides. Lake Ziway is under increasing agricultural and urban pressure and is exhibiting deteriorating trends in several water quality and ecological parameters. Nutrients and trace metals, including PO43-, NO3-, NH4+, Ca2+, Cu and Ni of the lake have shown increasing temporal trends in concentration. Spatially, higher values of major parameters (e.g. NO3-, NH4+, K, Na and electrical conductivity) were observed at shoreline sites near floriculture farming. The water quality of the lake exceeded guideline values for drinking water (alkalinity and Fe) and for aquatic life (NH+, Fe, Cr, Cu and Se). The recently reported pesticides in the lake possibly cause ecological and human health effect. Accordingly, agriculture and urbanisation are affecting water quality of Lake Ziway, with likely negative effects on human health and the lake ecosystem function unless appropriate interventions are taken. Our results may be useful in assessing other African lakes subject to similar anthropogenic pressures in their catchments.