Abstract
Floodplain ecosystems in Africa are under threat due to direct anthropogenic pressure and climate change. The lower Phongolo River and associated floodplain is South Africa’s largest inland floodplain ecosystem and has been regulated by the Pongolapoort Dam since the 1970s. The last controlled flood release from the dam occurred in December 2014, after which a severe drought occurred and only a base flow was released. The central aims of this study were to determine the historic and present water quality state of the middle and lower Phongolo River and assess the possible effects of the most recent drought may have had. Historic water quality data (1970s to present) were obtained from monitoring stations within the Phongolo River catchment to assess the long-term water quality patterns. Using multivariate statistical analyses as well as the Physicochemical Driver Assessment Index (PAI), a water quality index developed for South African riverine ecosystems, various in situ and chemical water variables were analysed. Key findings included that the water quality of the middle and lower Phongolo River has degraded since the 1970s, due to increased salinity and nutrient inputs from surrounding irrigation schemes. The Pongolapoort Dam appears to be trapping nutrient-rich sediments leading to nutrient-depleted water entering the lower Phongolo River. The nutrient levels increase again as the river flows through the downstream floodplain through input from nutrient rich soils and fertilizers. The drought did not have any significant effect on water quality as the PAI remained similar to pre-drought conditions.
Highlights
Numerous causes exist that may change or degrade river water quality [1]
The Physicochemical Driver Assessment Index (PAI)% indicated that Downstream 2 was in the most unmodified state (96.56%) while in an unmodified, natural state (A rating) both above and below the Pongolapoort Dam (Figure 3; Upstream and Downstream 1 had a lower score (88.39%)
Nutrients have significantly decreased above the Pongolapoort Dam from historic to pre-drought conditions, after 1995, while further below the impoundment, within Ndumo Game Reserve (NGR), nutrients have increased since the 1980s
Summary
Numerous causes exist that may change or degrade river water quality [1] These include natural processes such as natural high and low flows and weathering of soils and rock, and/or human activities such as the construction of dams and the discharge of industrial and domestic wastewater, as well as agricultural drainage [2,3,4]. These phenomena can be exacerbated by climatic conditions such as drought, one of the most prominent effects of which is reduced flow, which places additional stress on freshwater ecosystems already impacted by human activities [5,6]. Reduced river discharge may result in a reduced flooding area and environmental flow, leading to variation in natural abiotic and biotic processes [23] such as nutrient cycling and the migration and spawning of fish [8,24]
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