Assessing water quality using benthic diatoms as bioindicators in the Sabie River (Kruger National Park)

Abstract

The Sabie River’s headwaters originates in the Mpumalanga escarpment and flows eastward into the Corumana dam in Mozambique–the river has a shared tripartite agreement between South Africa, Swaziland and Mozambique and South Africa needs to ensure that the water quality of the Sabie River is up to standard and no conditions are violated. The majority of the Sabie River to the west of the Kruger National Park (KNP) is affected by human activities such as informal settlements, urbanisation, industry, small-scale farming, and discharge from the wastewater treatment plant. The 110 km length of the river within the KNP is the main water supply for wildlife, agriculture, and ecotourism in the area. The study proposed to determine the temporal and spatial changes in water quality along the Sabie-Sand River system, in KNP, using benthic diatom assemblages as bioindicators. In addition, to relate microphytobenthos biomass and composition, using chlorophyll a fluorescence as an index, to environmental conditions. Upstream disturbances and environmental changes affect the functioning and processes of microphytobenthos (MPB) communities within river systems. Biomonitoring provides a direct measure of river health and allows for the understanding of environmental effects on water quality. Biomonitoring adds a valuable component to traditional physico-chemical sampling and is essential for Integrated Water Resource Management (IWRM)and a crucial component for assessing water quality and river health. Physico-chemical variables and benthic diatom collection was done in accordance with the SANParks rangers at five sites along the Sabie River within the KNP. Historic diatom slides (1983 and 1985) were obtained from the South African National Diatom Collection housed at the North-West University. Samples were collected in September and October 2019 (low rainfall season) and March 2020 (high rainfall season). Ten stones were selected for conventional collection of MPB and sampled using the BenthoTorch. Physico-chemical parameters including temperature (oC), dissolved oxygen (% and mg.l-1), conductivity (μS.cm-1) and pH were measured in situ using the YSI Professional Plus multiparameter probe. Water samples were collected in situ in triplicates and filtered (0.45 μm pore size syringe filter). Chemical analysis as well as benthic diatom slide preparation and analysis was carried out at the University of the Witwatersrand. Overall water quality in relation to benthic diatom species assemblages was analysed using OMNIDIA. Water quality decreased after the flash flood in February 2020 –nutrients (ammonium and orthophosphate) and turbidity increased and dissolved oxygen decreased, although it was relatively high (> 6 mg.l-1) throughout the study period and met the Target Water Quality Range (TWQR). The TWQR was met by ammonium (0.007 mg.l-1) during the low flow period (September 2019), however concentrations increased into the high flow period(0.1424 mg.l-1). Orthophosphate concentrations (< 0.005 mg.l-1) in September 2019 characterised the river system as oligotrophic -limited growth of aquatic plants and blue-green algae, however the system exhibited mesotrophic conditions during the wet season (0.054 mg.l-1). Conductivity once corrected for temperature increased from upstream to downstream (103.30 –155.30 μS.cm-1) -the Sabie River met the Resource Quality Objective (RQO) for conductivity (< 300 μS.cm-1). The Sabie River has become more alkaline than what has been recorded in the past. To identify the environmental variables that contributed most to the overall variability in diatom community assemblages, the Canonical Correspondence Analysis (CCA) plot was used. The most important environmental variables in the CCA plot that showed axis ordination and therefore influenced diatom species distribution along the Sabie River were conductivity, dissolved oxygen(mg.l-1), ammonium, and orthophosphate concentrations. It should be noted that the environmental variables were only able to explain 47.1% of the variation in diatom distribution along the Sabie River – further studies need to be conducted to assess what other factors may be influencing diatom distribution and composition. A total of 70 benthic diatom species were identified along the Sabie River within the KNP boundaries (from the 1983, 1985, 2019 and 2020 sampling periods). The five most abundant diatom species sampled during 1983 and 1985 were Achnanthidium minutissimum, Cymbella kolbei, Gomphonema venusta, Navicula heimansoides, and Cocconeis placentula. In 2019 and 2020; Cymbella turgidula, C. placentula, Planothidium rostratum, Encyonopsis leei var.sinensis, and Nitzschia frustulum were dominant. In 1983 and 1985 the dominant species recorded preferred oligotrophic to mesotrophic waters with low to moderate electrolyte content. Whereas the dominant species of 2019 and 2020 preferred oligotrophic to eutrophic waters with low to high electrolyte content. It is evident that the diatom community composition has changed along the Sabie River over the last 36-yearsbecause of changing water quality. The BenthoTorch allowed us to rapidly study the MPB community composition and biomass and as a result the water quality along the Sabie River. Benthic diatom cell density decreased from the low flow periodSeptember 2019 (7280cells.mm-2) and October 2019 (6025 cells.mm-2) to the high flow period in March 2020 (2577 cells.mm-2), however, cyanobacteria density showed the opposite trend increasing from September 2019 (12256cells.mm-2) and October 2019 (9425 cells.mm-2) to March 2020 (21390cells.mm-2). The change in MPB community assemblage as a result of the flash flood could be a cause for concern as an increase in cyanobacteria can decrease the aesthetic value of the river and affect the quality of the water that is consumed by humans and wildlife. The study showed that benthic diatoms are able to indicate temporal (short and long term) and spatial changes in water quality along the Sabie River and it is apparent that diatom composition is influenced by environmental parameters and this finding emphasizes the importance of incorporating benthic diatoms as bioindicators into future water quality monitoring programmes within the KNP. The change in water quality after the flash flood was due to increased surface runoff from higher up in the catchment, as well as increased water velocity that resulted inmixing of riverbed sediments into the water column and biological activity within the river system due to increased water temperatures. The SPI and BDI scores showed that the water quality of the Sabie River has degraded from good quality in 1983 and 1985 (EcoClassification B) to medium quality in 2019 and 2020 (EcoClassification C). KNP management needs to ensure that the Sabie River’s water quality is monitored and managed preferably with the inclusion of benthic diatoms as bioindicators and the BenthoTorch as it was able to provide rapid assessment of the Sabie Rivers water quality – that was supported by physico-chemical and biomonitoring using benthic diatoms. The novel use of the BenthoTorch in the Sabie River means that further studies should be conducted to understand its potential for monitoring water quality in the KNP. All stakeholders within the park and at a catchment level need to be involved in preventing further degradation of the Sabie River –it is apparent that the water quality of the Sabie has degraded since 1983