LAUSR

Abstract Phytoplankton is an important role player in the ecology of rivers and streams, and sensitive to changes in water quality. This study aimed to determine how pollution influenced phytoplankton composition in two streams and to confirm the dominant phytoplankton Classes and genera at each stream as indicators of the water quality. The water from the Vaal River in the Middle Vaal Catchment of South Africa is heavily polluted by the time it is abstracted for treatment by Midvaal Water Company. The Koekemoerspruit is an upstream tributary of the Vaal River and is considered a possible source of pollution. Physical and chemical water quality data were collected for 21 variables together with phytoplankton samples at five strategically selected sites over a two-year period (November 2012 to October 2014). The Vaal River showed extreme total chlorophyll concentrations associated with high pH levels compared to the Koekemoerspruit in which high ammonia, nitrate and nitrate and orthophosphate concentrations confirmed severe organic pollution. A total of 86 algal genera were identified and grouped into seven phytoplankton classes. Average phytoplankton cell concentrations of 1,410,069 cells/mL and 417,931 cells/mL were determined for the Vaal River and the Koekemoerspruit respectively. A redundancy analysis summarised and confirmed that water quality had a definite effect on the phytoplankton assemblages for the Vaal River and the Koekemoerspruit (p-value of 0.08). Both streams were eutrophic but Chlorophyceae dominated in the Vaal River (49%) and Cyanophyceae were predominant in the Koekemoerspruit (44%). The dominant presence of phytoplankton genera such as Scenedesmus spp. (42%) and Nitszchia spp. (84%) confirmed the main water quality characteristics of the two streams.