Abstract
The Vaal River has become so nutrient-enriched that algal blooms pose problems. A unique opportunity arose to determine if there were changes in the chemistry en algal composition of the Vaal River after oligomesotrophic Katse Dam (Lesotho) water was imported to augment supplies in the light of growing water demands in the Vaal River catchment area. Algal concentration and composition in the Vaal River during three periods (between 1992 and 1994, 1998 and 2000, as well as 2004 and 2006) were compared to those in the Katse Dam (1998–2006). Some algal species, initially absent from the Vaal River, appeared in the river during and after transfer. Mixed algal assemblages found in the Vaal River before transfer of Katse Dam water gradually changed after transfer to assemblages mainly composed of cyanobacteria. The total algal concentration in the Vaal River Barrage doubled from the period between 1992 and 1994 to that between 2004 and 2006, indicating that the transfer of clear, oligomesotrophic Katse Dam water did not dilute the eutrophic Vaal River water sufficiently in order to reduce algal concentrations. Results showed that continuous downstream pollution and eutrophication of the Vaal River system eliminated the diluting effect of Katse imports. This resulted in changes in algal composition and concentration in the Vaal, characteristic of those associated with increasing eutrophication.
Highlights
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Wat aanvanklik afwesig was in die Vaalrivier, het ná die oordrag van Lesothowater voorgekom. ’n Gemengde algpopulasie wat in die Vaalrivier gevind is voordat Katsedamwater oorgedra is, het ná die oordrag geleidelik verander na ’n populasie wat gedomineer is deur sianobakterieë
Long-term changes in the algal composition of the Vaal River, South Africa – did the Lesotho Highlands Water Project play a role? The Vaal River has become so nutrient-enriched that algal blooms pose problems
Summary
Affiliations: 1Unit for Environmental Sciences and Management, North-West University, Potchefstroom Campus, South Africa. Nadat water oorgedra is vanaf die Katsedam, het ‘n unieke geleentheid homself voorgedoen om die invloed wat die helder, oligomesotrofe water vanaf die Lesotho Hooglande op die algsamestelling en - konsentrasie in die troebel, eutrofe water van die Vaalrivier het, te ondersoek. Hu et al (2008, 2010) en Zhai et al (2010) het aangetoon dat wateroordrag vanaf ’n rivier na ’n reservoir die konsentrasie van fitoplankton laat afneem, maar hulle het ook geredeneer dat intensiewe wateroordragte vir lang periodes moontlik die stikstof- en fosfaatkonsentrasies kan laat toeneen en dus eutrofikasie kan bespoedig. Soos Asterionella formosa, Ceratium hirundinella, Cylindrospermopsis raciborskii en Arthrospira sp., wat nie voor wateroordrag in die Vaalrivier voorgekom het nie, word nou op ’n gereelde basis in die rivier aangetref (Janse van Vuuren & Kriel 2008). Asterionella formosa en Ceratium hirundinella is bekend daarvoor dat hulle reuke en smake kan veroorsaaak, asook filters verstop, terwyl Cylindrospermopsis raciborskii ’n baie
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