Longitudinal trends in water chemistry and phytoplankton assemblage downstream of the Riverview WWTP in the Upper Olifants River

Abstract Aquatic ecosystem resilience is an integral part of sustainable development for numerous economic, social and culture reasons. The regulation of nutrient inputs into rivers and the capacity of rivers to retain and transform excess nutrients is an important aspect of ecological resilience of aquatic systems and the management of river water quality. The primary production by algae in the selected Olifants River reach, dominated by a cobble or bedrock substrate, had been highly effective in assimilating phosphate concentrations (PO 4 ) during low river flow regimes in winter months. However, the assimilation of PO 4 by phytoplankton was temporary and caused changes in the phytoplankton community structure, reducing the diversity and degrading habitat downstream. The ecological impact of the Riverview wastewater treatment plant (WWTP) was detectable over a distance of 40 km downstream from the plant. The chloride concentration below the WWTP in July during low flow regimes decreased from 33 mg L −1 to 21 mg L −1 at 0.1–40 km downstream from the WWTP. During benthic algae mat senescence and flood disturbance during the high flows of the summer months, displaced nutrients accumulated in these filamentous algae mats and was then displaced further downstream to Lake Loskop where they fuelled the development of cyanobacterial blooms in the riverine zone of this man-made lake.