The influence of land use-impacted tributaries on water quality and phytoplankton in the Mooi River, North West Province, South Africa

Background: Effluents from agricultural activities, metal and peat mining, urban and industrial areas, informal settlements and wastewater treatment plants all influence tributaries feeding the Mooi River. All these land uses ultimately result in a deterioration of water quality in the Mooi River.Objectives: The main objective was to relate the effects of inflowing tributaries to water quality and phytoplankton assemblages in the Mooi River.Method: Physico-chemical environmental variables were compared with water quality objectives and known limits to assess general water quality in the Mooi River over a one-year period. Water quality and phytoplankton in tributarieswere compared to that at sites upstream and downstream from their inflows to determine the extent to which the tributaries affected the Mooi River. Multivariate analysis assisted in the interpretation of phytoplankton and physico-chemical data at the different sites.Results: Seven phytoplankton phyla were identified in the Mooi River. Diatoms and green algae were most dense and diverse. Cyanobacteria were responsible for various problems, especially in the Wasgoedspruit where they dominated and reached high densities. Lowest phytoplankton density and diversity were found in the upstream and downstream sections of the river, while maximum density and diversity were found during summer in the middle reaches. The inflow of tributaries, especially the Wasgoedspruit, had a pronounced effect on water quality downstream. Phytoplankton density and diversity were less affected than chemical variables, especially nutrient concentrations.Conclusion: Various land use activities surrounding the Mooi River’s tributaries contributed to a deterioration of water quality in the main stream of the Mooi River.