An assessment of the influence of multiple stressors on the Vaal River, South Africa

Abstract

The Vaal River is situated in the mining and industrial heartland of South Africa. It is regarded as a “work horse” river in South Africa and as a consequence it receives treated waste water from the largest metropolitain area in South Africa. It is only with the more frequent occurance of fish kills in the Vaal Barrage area during the past few years that public attention has been drawn towards the decreasing water quality and subsequent deterioration in the aquatic health of the Vaal River system. The aim of this study was to apply a multi-metric approach to assessing the risk of the multiple stressors to fish populations of the Vaal River system. A relative risk assessment approach was applied to divide the Vaal River Barrage into four risk regions. Field sampling was undertaken to validate the predicted risks in each region. The sampling included abiotic (i.e. water and sediment quality) and biotic (fish components) assessment. General water quality parameters (pH, conductivity, dissolved oxygen) together with nutrient, bacteriological and metal concentrations were measured in the four regions. Sediment quality was determined through physical (particle size distribution) and chemical (metal and organic pollutant) analyses. The fish assessment was undertaken at different levels of biological organisation ranging from biomarkers at subcellular levels (cytochrome P450-EROD, metallothionein, acetycholine esterase, antioxidant enzymes, cellular energy), tissue (histopathology), whole organism (fish health index), population and community level. These biological responses were related to environmental exposure through bioaccumulation analyses of metals and organic pollutants in fish tissues. Multivariate statistical analyses were applied to integrate the environmental exposure and effects. The results indicated that those regions that were predicted to be at greatest risk to exposure of multiple stressors did indeed display the greatest disturbance in fish community structures. This was related to decreased fish health as demonstrated by increased oxidative stress due to exposure to metals such as copper and nickel as well as organic pollutants such as PCBs, HCHs and bromated flame retardants. This study clearly demonstrates the importance of the inclusion of higher tier assessment endpoints to elucidate the effects of multiple stressors in aquatic ecosystems. The study further allowed for the identification of specific effect endpoints that need to be included in future monitoring programmes such as viral immunoassays.