Application of an algal problem index in evaluating water quality in the Vaal River barrage, South Africa

The Vaal River is a vital water source for domestic, industrial, and agricultural use in South Africa's economic heartland, but its water quality is increasingly threatened by nutrient pollution, resulting in nuisance algal blooms and invasive aquatic plant growth. This study applies an Algal Problem Index (API) to assess biological water quality and identify risks associated with algae and cyanobacteria in the Vaal River's upper reaches. Water samples were collected over a 12-month hydrological year from 16 sites, including sites in the main stream, major tributaries, and reservoirs. Cyanobacteria and algae were identified, and analyses of physico-chemical parameters and chlorophyll-a concentrations were conducted. Multivariate statistical analysis was used to evaluate relationships between environmental conditions and algal-related problems. Results revealed 16 taxa, including Microcystis aeruginosa and Dolichospermum circinale, responsible for bad tastes, foul odours, and potential toxin production. Tributaries such as the Riet Spruit and Klip River were major sources of nutrient loading, although low light conditions limited algal growth in certain areas. Elevated API scores increased downstream, reflecting degrading water quality from the Vaal Dam to the Vaal River barrage. This study demonstrated the innovative and practical application of the API as a visual tool integrating multiple biological indicators into a single, easily interpretable measure of water quality. By summarising complex algal-related risks, it can provide water managers with a rapid, practical way to assess ecological conditions, and it can be applied in other catchments facing similar challenges.