Assessment of the impact of water and sediment quality on the diversity of aquatic macro-invertebrate communities in the Dwars River of the Olifants Rivers system, Limpopo Province

Abstract

Freshwater ecosystems in South Africa are losing their quality and quantity over time due to pollution mainly from mining, agriculture, industries, deforestation, sewage systems, construction of dams, channel modification and over extraction of water. The Dwars River, a tributary of the Olifants River, is of no exception, as recent studies indicated an increase in nutrient input possibly from agriculture. The Dwars River is an important source of water for nearby communities (Ga-Mampuru). The aim of the study was to assess water and sediment quality of the Dwars River using macroinvertebrates as bioindicators of pollution. Water and macroinvertebrates sampling were done seasonally from July 2017 to May 2018. The water quality results indicated that non-toxic constituents such as salinity and EC (Electrical Conductivity) were above permissible limits stipulated by the DWAF (1996) guidelines. More sensitive taxa were found upstream, despite high concentrations of some nutrients and metals in the water column. The high abundance and distribution of macroinvertebrates observed upstream was confirmed by the Canonical Correspondence Analysis (CCA), South African Scoring System (SASS5) and Average Score Per Taxon (ASPT) results during the study. Site 1 was dominated by the most sensitive taxa and this could be due to high dissolved oxygen content and increased heterogeneity of the area. Site 4 was dominated by the most tolerant taxa, according to the CCA, SASS score and ASPT results. This could possibly be due to the nutrients and heavy metals washed from upstream, which get adsorbed by the sediment. The results for species abundance, diversity and richness indicated that Ephemeroptera was the most abundant, while Diptera was the most diverse. Ephemeropterans are known to be indicators of good water quality. Site 1 had the highest number of families and orders while site 4 had the least families and orders. Ephemeroptera, Plecoptera and Tricoptera (EPT) taxa richness and Shannon diversity (H’) index values are high upstream and decrease downstream. Overall, the SASS5 indices, CCA and physicochemical results indicated that the water quality in the Dwars River is deteriorating in most impacted sites