Spatial changes in algal assemblages promoted by water quality in the Sabie River catchment
The Sabie River catchment forms part of the bigger Inkomati Catchment Management Area under the management of the Inkomati Usuthu Catchment Management Agency, in the Mpumalanga Province and covers about 6 320 square kilometres. The headwaters of the Sabie River and its tributaries such as the Sand River and Marite River arise from the upper Drakensberg escarpment, flowing eastwards into the Lowveld through drastically changing topography, through the Kruger National Park (KNP) into Mozambique where it becomes part of the Inkomati River system. The water of the Sabie River system is vital to the economy of the communities in the area and plays a role in agriculture and ecotourism. It is imperative to monitor and manage the Sabie-Sand sub-catchment’s water quality as it is also important to the ecosystem health of the KNP. The Inkomati River system is an international shared watercourse with Mozambique and therefore South Africa has an obligation to meet the international water quality requirements in the Mpumalanga area, with a further obligation to ensure high-quality water sharing between the three co-basin areas. This study proposed to measure the relationship between water quality and spatial and temporal changes in the algal composition of the Sabie River and its main tributaries namely the Marite and Sand Rivers as well as in the Inyaka Dam. Observing algal assemblages has been important in environmental assessments both to indicate changes in environmental conditions that might impair or threaten ecosystem health as well as to determine if algae themselves are causing problems. Therefore, during this study, the changes in the algal assemblages were determined with specific regard to genera diversity, as well as changes in assemblages during different environmental conditions experienced during the study period. The water quality and algal assemblages of the Inyaka Dam, which is situated in the Marite River, was also included in this study as it is the main water source of the Inyaka Water Supply Scheme and therefore does not only impact on the Sabie River system but directly influences the communities depending on it as a potable water resource. This study was a collaborative project between the North-West University (NWU), Rand Water, Inkomati-Usuthu Catchment Management Agency (IUCMA) and South African National Parks (SANParks). Sampling commenced in January 2016 and continued until July 2017. Four sampling occasions were undertaken per year, so as to occur seasonally (four times in 2016 and three times in 2017). Sampling took place during the 3rd week of each of the following months; January, April, July and October of 2016, and January, April and July of 2017. Surface grab samples were collected at various sites in the Sabie River catchment. The following variables were measured in situ at each sampling site with an YSI multi-meter (YSI 556 handheld field instrument): pH, temperature (°C), dissolved oxygen (%), dissolved oxygen (mg/L) and specific conductivity (mS/m). Chemical analyses were carried out by Rand Water’s Analytical Services. Concurrently with the water quality parameter determinations. Planktonic algal cell enumeration was done at the North-West University according to standard laboratory procedures and a sedimentation technique. The study area received 888 mm rainfall during 2016 compared to 1073 mm during 2017. It was therefore decided to compare the data obtained during the dry year of 2016 to that obtained during the high rainfall year of 2017. Overall 86 algal genera were identified in 2016 and 88 in 2017 from 6 different phyla which include 14 (15 genera during 2017) genera from the phylum Cyanophyta, 33 (35 genera during 2017) from the Bacillariophyta, 32 (30 genera during 2017) genera from the Chlorophyta, 1 genus from the Chrysophyta, 3 genera from the Dinophyta and 3 (4 genera during 2017) from the Euglenophyta. The Bacillariophyta was the dominant phylum at most sites for both sampling years. Genera such as Achnanthes, Achnanthidium, Cocconeis, Cymbella, Gomphonema, Gyrosigma, Navicula and Nitzschia occurred frequently at most sites. Chlamydomonas, Desmodesmus, Monoraphidium and Scenedesmus were the most frequent genera from the Chlorophyta. Genera such as Trachelomonas and Euglena were the dominant Euglenophyta while the Chrysophyta genus Dinobryon, was mostly dominant in the Inyaka Dam. The algal cell concentrations were higher during 2016 (24801 cells/ml) compared to in 2017 (1613 cells/ml). During the 2016 sampling occasions, the impacts of the drought were clearly visible. Site 7 in the Sand River experienced very low flow and the invasive plant species Azolla filiculoides was observed at this site. Increased wildlife was also witnessed especially at site 9 in the hippopotamus pools, along with the invasive plant species Pistia stratiotes. During the drier period there was limited dilution of pollution. This was evident at site 12 where increased nutrient concentrations (maximum ammonia of 11 mg/l and max total nitrogen of 38 mg/l) were measured. The algal composition and increased chlorophyll-a, at this site reflected the higher nutrient concentrations. Site 12 was the site most impacted upon by the non-functioning wastewater treatment plant (WWTP) and rural settlements especially during the dry year. The extremely high E. coli concentrations are also an indication of that the impact that the surrounding communities and WWTP have on this site. Site 12 showed the greatest overall improvement during the higher rainfall year. The %DO increased from lethal to sub-lethal according to the target water quality range (TWQR) and the ammonia, total nitrogen, total phosphorus and ortho-phosphate concentrations all decreased during 2017. The higher rainfall experienced during 2017 relieved the drought conditions experienced as follows: the water levels improved at all the sites and the invasive species (Pistia stratiotes and Azolla filiculoides) were flushed downstream. The total nitrogen and ammonia concentrations decreased and there were clear improvements in the %DO at most sites. The turbidity measured at all the sites were higher during the higher rainfall period due to the increased runoff. The aluminium and iron concentrations increased significantly in the sites located in the Sand River and in the Marite River. This may be reason for concern since these metals can become toxic to users. The high E. coli concentrations observed at all the sites (except for sites 1 and 11) are reason for concern as it poses a serious health risk, especially to the surrounding rural communities that might come in contact with sources of faecal pollution. Most of the physical and chemical water quality parameters determined complied with the recommended TWQR. Nutrients such as the total nitrogen, total phosphorus (phosphorus & ortho-phosphate) did however not comply with the set resource quality objectives (RQO) for the Sabie River, Marite River and Sand River. Heavy metals concentrations, namely aluminium and zinc, also exceeded the TWQR. The Inyaka Dam is the most important site concerning the aesthetic value of the drinking water. This site can face some taste and odour problems in future due to high iron concentrations present as well as emergent chlorophyll-a and possible related geosmin and 2-MIB concentrations. The total nitrogen and total phosphorus concentrations can potentially support high algal and plant productivity and this study area might experience bloom formation in future since problematic algae such as Anabaena, Oscillatoria and Cylindrospermopsis were also found in the study area.