Abstract
The combined impacts of various anthropogenic activities, including agriculture, mining,
industrial operations, and the growing population, have placed stress on freshwater
ecosystems. One of the consequences of these activities is the phenomenon of
eutrophication, wherein a water body becomes excessively enriched with nutrients. This
excess of nutrients leads to the proliferation of algal growth and disrupts the balance of
an ecosystem. The Roodeplaat Dam is classified as a hyper-eutrophic system,
signifying an extreme state of nutrient enrichment. This contamination primarily
originates from three main rivers that flow into the dam: Edendalespruit, Hartbeesspruit
(including the tributary Morelettaspruit), and the Pienaars River, with the Pienaars River
being the most substantial contributor of pollutants. The Pienaars River receives a
combination of point source pollution, such as wastewater discharge, as well as nonpoint
source pollution from agricultural runoff and the release of waste effluents from
industrial activities. Numerous studies have emphasized the role played by the three
rivers flowing into the dam as sources of pollution, ultimately affecting both the dam and
its associated biological components. Nevertheless, there remains a notable gap
regarding the extent to which these inflowing rivers and the dam collectively impact the
Pienaars River downstream of the dam. This study aims to address this knowledge gap
by conducting a thorough assessment across seven selected sites, while also
considering seasonal variations. To comprehensively evaluate the health of the
ecosystem, a multifaceted approach was completed. An evaluation of water quality,
encompassing both in-situ and ex-situ water measurements, and the physical
characteristics of sediment were analyzed. Following the Integrated Habitat Assessment
System (IHAS) protocol, a habitat assessment was assessed at each site.
Macroinvertebrates were sampled across various biotopes using the South African
Scoring System version 5 (SASSv5). Subsequently, the SASS results were used to
calculate the Macroinvertebrate Response Assessment Index (MIRAI). Fish diversity
was assessed using the electroshock method for fish collection and the Fish Response
Assessment Index (FRAI). The results of this study showed variations among the
sampling sites. Site Pienaars 3 (PIEN3), situated downstream of the Mamelodi
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Township and Baviaanspoort WWTW, exhibited poor water quality marked by a decline
in macroinvertebrate taxa and fish species. Discharge from the WWTW and township
influences the water quality of the Roodeplaat Dam and the sites downstream of the
dam (PIEN4 to PIEN7). The Principal Component Analysis (PCA) biplot revealed that
sites downstream of the dam were closely associated with concentrations of metals and
nutrients (particularly chlorophyll-a concentrations). From the biological indices, it
became evident that the reference site PIEN1 and site PIEN2 displayed moderate
impairment, with the essential ecosystem functions remaining relatively unaltered. Site
PIEN3, located just before the dam inflow, and PIEN4, positioned after the dam wall,
were classified as seriously impaired, with a decline in basic ecosystem functions. Sites
PIEN5 to PIEN7 were moderately-largely impaired, with an unexpected abundance of
fish species. In essence, this study underscores the need for managing anthropogenic
influences upstream of the dam to prevent the loss of intolerant species and mitigate the
growing contaminant impacts.
Keywords: Biotic integrity, ecological category, SASS5, IHAS, MIRAI, FRAI