Impacts

Positive impacts:  

The water resources of the country inherit much of their character from the climate and their flow behaviour mirrors the erratic rainfall patterns (see Pressures). However, the indigenous aquatic fauna and flora of South Africa are well-adapted to the variable climatic conditions. Hydrological variability has strongly influenced the evolutionary character of the biota. Many indigenous species are highly-tolerant of environmental extremes and are reproductively opportunistic as a result.

The level of perturbation caused by natural fluctuations ensure that high biological diversity and habitat integrity are maintained. However, whilst most freshwater systems in South Africa are not affected in the long-term by natural perturbations, they must also endure the increasing human stresses mentioned previously (see Driving Forces and Pressures), to which they are not adapted and which have negative impacts.

Negative impacts:  

Negative impacts on the freshwater environment may be divided into ecological impacts and impacts on human use of the resource.

Ecological impacts:

Ecological changes to freshwater ecosystems occur because of catchment degradation (see Terrestrial Ecosystems section); regulation of flow by impoundments; pollution; over-extraction of water; and the breakdown of natural biogeographical barriers. The primary results of these are extensive habitat loss; a decrease in biodiversity and an increase in invasive and pest species. In extreme cases, negative impacts can result in collapse of the functioning of the natural systems.

A good example of extensive habitat loss is that of natural wetlands. There is little documented information with regards to the extent of wetland loss throughout South Africa, although isolated examples can be quoted:

• The Mfolozi Swamp, forming the largest fluvial plain in South Africa, by 1988 had been reduced through agricultural development to 43% of its previous extent (Begg 1988).
  
• In the Siyaya catchment in northern KwaZulu/Natal, 93% of the wetlands had been lost by 1966 (Begg 1988).
  
• Cape Town City Square was once a wetland, as was Louis Botha Airport in Durban (Begg 1988).
  
• In the semi-arid regions of the country, in the riverine lowlands it was estimated that 90% of the wetland areas had been severely eroded (Department of Agriculture and Technical Services 1972).
  
• The extent of wetlands in the upper Mgeni catchment has been reduced by 66% (D Kotze, Institute for Natural Resources, pers. comm.).
• Wetlands have been reduced by 21% in the Wilge and Klip rivers, in the Vaal catchment.

Kotze et al. (1995) hypothesized the extent of natural wetland loss in South Africa, based on isolated reports and climatic and physiographic information (Figure 3.23). It is estimated that parts of the Western Cape, Eastern Cape and KwaZulu/Natal have less than 50 % of the natural wetlands left. Additionally, riverine habitats have been so changed, that little remains of natural systems in South Africa. Many perennial rivers have become seasonal (e.g. Limpopo, Levuvhu, Letaba);

Figure 3.23 Hypothesised extent of wetland loss, based on the extrapolation of information from isolated reports and using climatic and physiographic information

floodplains that rely on regular flooding which has been attenuated have become less productive (e.g. Pongola) and some estuaries can no longer rely on natural opening of the estuary mouth (e.g. Mfolozi).

Loss of or changes in habitat have resulted in changes in biotic composition. These changes may be characterised into two main types: loss of biological diversity and introduction of invasive species.

The effects of biodiversity loss have been debated extensively, and aquatic ecologists generally agree that long-term biodiversity loss has a negative impact on ecosystem sustainability. An indicator of loss of biodiversity is the number of threatened aquatic plants and animals, known as "Red Data Book" species. Lists of Red Data Book species were compiled for South Africa in the 1970s and 1980s. A list of threatened freshwater estuarine plant and animal species is available in Noble and Hemens (1978). Little information is available on loss of aquatic invertebrates (represented by a single dragonfly species), but 24 plant species, 25 fish species, 6 amphibians, 2 reptiles, 24 birds and 2 mammals are included in the Red Data Book list. Of these, one fern, Christella altissima is now extinct.

Invasive or pest species may be alien to an area, introduced either accidentally or deliberately, or indigenous to an area, but become invasive when habitat changes create perfect conditions for their habits and life cycles. De Moor and Bruton (1988) have compiled an atlas of all known alien and translocated indigenous aquatic animals in South Africa. There are 42 known alien species (16 invertebrates, 23 fish, 1 reptile and 2 birds) in South African waters and at least 74 translocated indigenous fish species with the number of invertebrates unknown . Of these, 37 have a known detrimental effect on the environment (De Moor and Bruton 1988).

Fig 3.24a Distribution of the four most important alien aquatic macrophytes in South Africa - Eichhornia crassipes	Fig 3.24b Distribution of the four most important alien aquatic macrophytes in South Africa - Azolla filiculoides
Fig 3.24c Distribution of the four most important alien aquatic macrophytes in South Africa - Myriophyllum aquaticum	Fig 3.24d Distribution of the four most important alien aquatic macrophytes in South Africa - Salvinia molesta

Some alien aquatic macrophytes have flourished because of changes in flow regime and hypertrophic conditions. This has severe economic implications for South Africa as they cover and choke vast areas of standing and slow-running waters. Of special concern are water hyacinth (Eichhornia crassipes), parrot'sfeather (Myriophyllum aquaticum,) Kariba weed (Salvinia molesta) and the water fern, Azolla filiculoides. Distribution maps of these species are shown in Figures 3.24.

Impacts on use:

Reeds for weaving and fishing

Changes in habitat and quality of the freshwater environment in South Africa also leads to a change in access to the resources available. In some cases it leads to a depletion in harvestable resources (e.g. reeds for weaving, fish, medicinal plants) with direct economic consequences. In other cases, the quality of life of people is affected . For instance, recreational opportunity is diminished, or health is affected (through poor sanitation and increasing pollution (see Social Dimensions section). Additionally, the over-utilisation of surface and groundwaters has led to conflict between users as demand outstrips supply (E Braune, DWAF, pers. comm.)

>     Freshwater: Response

There is also information about Freshwater Systems and Resources in the following reports:
Metropolitan reports:
	Cape Metropolitan Council (1998 edition)		Durban Pilot Study
	Greater Johannesburg Metropolitan Council (1999 edition)		Greater Pretoria Metropolitan Council (1999 edition)