Abstract
M.Sc. (Aquatic Health)
Many of the municipal wastewater treatment plants in South Africa are not functioning optimally and the ecological impacts of improperly treated effluent on receiving waterbodies requires research attention. In this study, the relationships between water quality variables and aquatic macroinvertebrate assemblages in the Gwaing and Duiwenhoks rivers (Western Cape) were evaluated above and below municipal wastewater discharge points during May and August 2018. Human activities have modified the water quality of these rivers and, in particular, the ecological impacts of wastewater discharge on these systems have recently come under question. The hypothesis was tested that water quality deteriorates downstream of the wastewater discharge point at each river and that macroinvertebrate assemblages are affected by this change in water quality. A SASS5 macroinvertebrate sampling protocol using a sweep net together with water quality measurements (in situ physico-chemical variables and ex situ measurement of nutrients and microbial parameters) was conducted at two sites in the Gwaing River (one upstream and one downstream of the discharge point) and four sites in the Duiwenhoks River (two upstream and two downstream of the discharge point). A clear and significant deterioration in key water quality variables was observed downstream of the wastewater discharge for both streams, with pH, conductivity, total dissolved solids and E. coli being elevated downstream, whilst dissolved oxygen was reduced downstream. Macroinvertebrate richness was higher upstream of the discharge than downstream for both rivers on both sampling occasions. Multivariate PERMANOVA analyses revealed significant differences (P<0.05) in macroinvertebrate assemblage composition between the upstream and downstream sites for both rivers. The Gwaing and Duiwenhoks rivers were found to be moderately polluted and both displayed an increasing pollution gradient from upstream to downstream of the wastewater discharge points. This was further confirmed by the ecological health categories derived from the SASS5 scores throughout the study period for both rivers that generally indicated category A conditions upstream (unmodified, natural) and category D conditions downstream (largely modified). This study suggests that the malfunctioning wastewater treatment works are negatively affecting the water quality and biotic assemblages of both rivers.