PART 2: INVASIVE ALIEN PLANTS IN SOUTH AFRICA: WHAT ABOUT WATER QUALITY
The impacts of invasive species on surface water runoff are reasonably understood. This has led to the prioritisation of areas that should be cleared of alien invasions through programmes such as Working for Water, established in 1995. However, there is not much information on the impacts of alien plant invasions on water quality. As such, not much has been done to prioritise clearing on the basis of water quality. Some of the alien invasion impacts on water quality are discussed below:
- The higher evaporation rates of alien vegetation reduce stream flows. This reduction in the amount of water in rivers reduces the flushing capacity of the rivers, in turn increasing nutrients, pollutants and salinity concentrations (Nagler et al, 2008). This reduces the resilience and buffering capacity of hydrological ecosystems.
- Alien invasions change the structure and amount of biomass (carbon and nutrient dynamics) in an ecosystem (Kalff, 2002). This can increase the amount of metabolised nutrients and could escalate natural eutrophication processes. If previously non-woody ecosystems are invaded by woody species, the increase in biomass implies higher fuel loads and could increase the frequency of fires and their intensity. This escalates erosion and sedimentation rates of rivers, streams and wetlands, which has flow consequences to downstream users.
- Invasions of aquatic weeds are also associated with a range of impacts on water quality. Dense mats of these weeds can impede water flow, which increases the rate of siltation in water bodies and inhibit the diffusion of air into water, resulting in lower concentrations of dissolved oxygen (Tellez et al, 2008). Lower oxygen concentrations combine with the above-mentioned problems and accelerate eutrophication (Chamier et al, 2012)
All of South Africa’s accessible water is already allocated for use, with agriculture, mining and industry being the biggest users. The impacts of invasive alien plants on water quality, while not considered to be as important as those on water quantity, should also be addressed.
When managing IAPs around river courses, solutions should integrate issues around quantity and quality, as they are intricately linked (Chamier et al, 2012).
CHAMIER J, SCHACHTSCHNEIDER K, LE MAITRE DC, ASHTON PJ, VAN WILGEN BW,(2012), Impacts of invasive alien plants on water quality, with particular emphasis on South Africa. Water SA, 38(2) pp.345–356.
KALFF J (2002) Limnology: Inland Water Systems. Prentice Hall, Upper Saddle River, NJ.
NAGLER PL, GLENN EP, DIDAN K, OSTERBERG J, JORDON F and CUNNINGHAM J (2008) Wide-area estimates of stand structure and water use of Tamarix spp. on the lower Colorado River: Implications for restoration and water management projects. Restor. Ecol. 16 (1) 136-145.
TéLLEZ TR, DE RODRIGO LóPEZ EM, GRANADO GL, PéREZ EA, LóPEZ RM and GUZMáN JMS (2008) The Water Hyacinth, Eichhornia crassipes: an invasive plant in the Guadiana River Basin (Spain). Aquat. Invasions 3 (1) 42-53.