Rol van Lesotho–Hooglandwater op Vaalrivieralge

Abstract

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. 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