Effect of land use on water quality and phytoplankton community in the tropical Khami River in semi-arid southwest Zimbabwe

The water quality and phytoplankton community assemblage of the Khami River, a tropical river sub catchment in semi-arid southwest Zimbabwe impacted by agriculture and urban land use, were examined in March 2015. Conductivity, sulphates, total dissolved solids and salinity were higher at urban sampling points than at agricultural sampling points. In contrast, agricultural sampling points were more turbid, and had a greater content of nitrates than urban sampling points. The phytoplankton community was dominated by Cyanobacteria, mainly Microcystis aeruginosa, with densities of up to 20 times higher at urban than at agricultural sampling points. There was an increasing dominance of Cyanobacteria (M. aerugionosa and Merismopedia glauca) and Chlorophyta (Eudorina elegans, Spirogyra sp. and Pediastrum duplex) and a decreasing importance of Bacillariophyta along the agriculture-to-urban gradient. Given the increasing scarcity of water in southern Africa, not only do our findings confirm the importance of land use types as drivers of water quality and phytoplankton community composition and structure. However, they also show that Cyanobacteria, Chlorophyta and Bacillariophyta can be indicators of changes in water quality, factors that will prove pertinent to management for effective water quality management using phytoplankton composition as bioindicators.