Changes in the algal abundance and composition along the Mooi River in the Potchefstroom area
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Rivers and dams are the main sources of fresh water in South Africa, and the quality of drinking water is rapidly deteriorating. More than 80% of the country's rivers and dams experience pressure as a result of utilisation and pollution. The inhabitants of Potchefstroom are dependent on the Mooi River (specifically Boskop Dam) as its only source of drinking water. The Mooi River originates in the Boons area and flows southwards through Klerkskraal, Boskop, and Potchefstroom dams until it reaches the city of Potchefstroom. From here, the Mooi River flows for 25 km to where it converges with the Vaal River. The Mooi River experiences surface water pollution, as a result of various anthropogenic activities including agricultural activities and effluents from urban, industrial and informal settlement areas. These anthropogenic activities in the catchment contribute to nutrient pollution that stimulates the growth of phytoplankton (algae and cyanobacteria). Excessive amounts of nutrients, particularly orthophosphates, stimulate the growth of harmful cyanobacterial and algal species, and reduce the water quality. Apart from anthropogenic activities, the river is influenced by several tributaries feeding it. The Wonderfontein Spruit enters the Mooi River approximately 3 km downstream from it source and it is influenced by large scale mining, resulting in acid mine drainage and heavy metal (especially uranium) pollution. The second tributary entering the Mooi River is the Gerhard Minnebron, situated in an area where peat mining and mining effluents (via Wonderfontein Spruit) are problematic. The Wasgoed Spruit enters the Mooi River in the city of Potchefstroom and it feeds the Mooi River with urban effluent, sewage effluents, and wastewater from industries. Water samples were collected once a month at eight different sites in the Mooi River, as well as one site in each tributary. Phytoplankton samples were enumerated and phytoplankton was identified to genus level. The main aims of the study were to investigate spatial changes in physico-chemical variables and phytoplankton concentration and composition in the Mooi River, and to relate it to the effect of the inflowing tributaries. Furthermore, the physico-chemical variables were compared to known limits, namely the resource quality objectives and recommended water quality objectives, goals set to strive to a certain desired water quality. The orthophosphate and nitrogen concentrations were used to determine the current trophic status of the Mooi River. This represents the first study on the influence of tributaries on the water quality and phytoplankton dynamics of the Mooi River. Results of the study indicated that the tributaries contributed to elevated nutrient levels in the Mooi River. High nutrient concentrations were the result of agricultural activities and sewage effluents. The mean ammonia and nitrate concentrations for the Mooi River and its tributaries are indicative of mesotrophic conditions, while mean orthophosphate concentrations indicated hypertrophic conditions. Nutrients were positively correlated with green algae, diatoms, cyanobacteria, and euglenophytes. Orthophosphates, in particular, showed a strong positive correlation with the concentration of cyanobacteria. High nutrient concentrations stimulated the growth of phytoplankton, amongst others harmful genera such as Microcystis and Anabaena. These genera are known to produce toxins and also cause taste and odour problems. High concentrations of these genera were accompanied by high turbidity levels and relatively high pH values. Relatively low abundance and diversity of cryptophytes and chrysophytes, groups generally associated with low nutrient conditions, indicate that the Mooi River and its tributaries can be regarded as polluted systems. High TDS concentrations and EC levels in the tributaries, primarily due to irrigation and sewage effluent, had a huge and observable effect on the Mooi River downstream from the points of inflow. Elevated calcium, magnesium, and sulfate concentrations, as a result of mining in the catchment, can be dangerous if consumed by livestock. In general, calcium and magnesium concentrations were high, and can be attributed to the catchment underlined by dolomite - an anhydrous carbonate composed mainly of calcium and magnesium. The Wonderfontein Spruit contributed to elevated magnesium levels in the Mooi River, while Gerhard Minnebron mainly contributed to elevated sulfate concentrations. Mining and sewage effluents could have been responsible for high chloride concentrations in the catchment. Both Wonderfontein Spruit and Gerhard Minnebron contributed to elevated chloride levels in the Mooi River system. Manufacturing industries surrounding the Wasgoed Spruit area could have elevated fluoride concentrations in downstream reaches of the Mooi River. Wonderfontein Spruit also contributed significantly to elevated concentrations of heavy metals such as manganese, hexavalent chromium, and uranium. The Gerhard Minnebron was responsible for higher iron, and manganese concentrations in the Mooi River. Most of the mean values for physico-chemical variables measured in the Mooi River and tributaries exceeded the limits of the resource quality objectives and recommended resource water quality objectives. Phytoplankton concentration in the tributaries did not have a huge effect on phytoplankton in the Mooi River main stream. Similarities between genera observed in the tributaries and the Mooi River can be the result of importation from the tributaries, their preference for high nutrients concentrations or seasonal aspects favouring their growth. It is important that the Mooi River and its tributaries must be monitored regularly, to ensure proper management of the river. Nutrients, especially orthophosphate concentrations, should be reduced. Constant monitoring of phytoplankton dynamics in relation to physico-environmental variables is recommended for future effective management of the Mooi River system.