Changes in algal composition and environmental variables in the highaltitude Mohale Dam — an important water supply reservoir to South Africa

Abstract

Whereas limited knowledge exists on the ecology and algae present in Lesotho's high-altitude Katse Dam, nothing is published about these aspects in the linked Mohale Dam, completed in 2003. This study investigated the changes in algal composition and associated environmental variables since the construction of the Mohale Dam until the present. Monthly sampling in 2003 and 2006 indicated that dissolved inorganic nitrogen concentrations decreased, while dissolved inorganic phosphorus concentration increased 12-fold, resulting in increases in algal concentration and a shift from green algal dominance in 2003 to cyanobacterial dominance in 2006. Multivariate analyses indicated that cyanobacteria were favoured by high temperatures and low DIN:DIP ratios. Although currently non-problematic, the dominance of cyanobacteria may pose future potential problems, especially if further increases in nutrient concentrations occur. Strains of Microcystis aeruginosa, currently one of the dominant species, are toxic.

Keywords:algae, cyanobacteria environmental variables, Lesotho Highlands Water Project, phytoplankton composition

African Journal of Aquatic Science 2007, 32(3): 265–274