There is currently no basin-scale assessment of threats to aquatic ecosystems available for the Limpopo River basin. The following table was developed by UNDP-GEF (2008) for the Orange-Senqu River system. While this summary was developed for a different river basin, the factors and impacts are transferable to the Limpopo River basin.
Factors affecting aquatic ecosystems.
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Factor
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Impact
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Alien species
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Pioneer alien species out-compete indigenous species for space, nutrients and sunlight
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Dams, inter-basin transfers, hydro-electrical flow releases, irrigation and mining abstraction
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Modified flow regime or hydrology
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Pollution from mines and return flows from irrigation
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Reduction in water quality, including nutrient build-up and salanisation
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Reduced flood regime and modified seasonal flows
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Geomorphologic modification of the river channel due to lower flows, resulting in less or no scoured
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Riparian and in-stream vegetation is harmed and continues to deteriorate
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Floating aquatic plants increase with reduced flow
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Changes to the shape of the wetted perimeter of the river channel, with lower water levels causing banks to dry out, temporary exposure of unprotected banks and bank collapse
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Enhanced benefit to pioneer reeds, such as the Common Reed (Phragmites australis), under reduced flow, with increased distribution and patch size, thereby accumulating sediments, blocking channels and resulting in large disturbances when washed out during large floods. These often form reed mats that cause blockages downstream and exacerbate the effect of floods.
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Loss of indigenous trees and gallery forest in the riparian belt because of reduced floods (moisture), reduced seed dispersal, more frequent hot fires because of increase in reed beds and less cooling effect as previously moist riverbanks are drier
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Increased agricultural encroachment into the riparian belt because of reduced flooding and waterlogged soils
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Invasion by alien vegetation, notably Mesquite (Prosopis spp.), exacerbated by a loss of indigenous vegetation and disturbance (e.g., through fires and agricultural activities)
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Changes in species composition and abundance as a result of fertilizers and salts draining into the river, with for example Common Reed (P. australis) and Wild Tamarisk (Tamarix usneoides) increasing and having a negative effect on safsaf willow, Kaapse wilger or Cape Willow (Salix mucronata).
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Source: UNDP-GEF 2008
In addition to eutrophication, a significant threat to aquatic ecosystems in southern Africa (and elsewhere) lies in the invasive species Water Hyacinth (Eichhornia crassipes) and Water Fern (Azolla filiculoides).
