The macroinvertebrate diversity, chemical and physical factors in the Loop Spruit and Mooi River, North-West Province
Abstract
The Loop Spruit, originating near Fochville, forms part of the Mooi River catchment in the North-West Province and ultimately forms part of the Vaal River catchment. This is a relatively small river with an average rainfall of 683 mm per annum, but serves as the main natural water source for many activities in the area including, agriculture. It is also a sink for mining effluent from several mines in the area. These circumstances may have a detrimental effect on the water quality of this river caused by metals originating from mining activities and nutrients produced by agricultural activities. These phenomena may also influence the ecological health, as well as the quality of potable water downstream. There is no State of River Report on the ecosystem health of the Mooi River or Loop Spruit catchment. Furthermore, little research has been done on the Loop Spruit, especially with regard to metal concentrations and macroinvertebrate diversity. The ten preselected sites were selected based on a variety of biotopes present in the rivers, availability of water and accessibility to the rivers. It included sites mostly located in natural areas, but also in areas mostly impacted by anthropogenic activities such as mining and sewage treatment plants.
The aim of this study was to determine the environmental quality and influence on the macroinvertebrates of the Loop Spruit, as well as that part of the Mooi River after the confluence of the Loop Spruit.
In order to determine the metal concentrations in the water and sediment at selected sites within the Loop Spruit catchment, three surveys were conducted during the dry seasons in July 2014, September 2014 and May 2015. Selected abiotic factors, including electrical conductivity, pH, temperature, turbidity, and flow-rate were measured in situ at each site. Metal concentrations in the sediment and water samples were determined using inductively coupled plasma mass spectrometry and were analysed for twelve metals considered potentially toxic to aquatic biota. Particle size determination was also done in order to determine the percentage composition of the total sediment sample. This was done using an Endecott dry sieving system with different mesh sized sieves. The clay particles (< 53 μm) were transferred for X-ray diffraction and scanning electron microscopy analyses. The results showed that quartz was the most abundant mineral with illite, muscovite and kaolinite, to name a few, occurring at different sites in smaller percentages. High percentages of silicon and oxygen were further found at all the sites which could have been due to the high percentage of quartz located at every site, since quartz is composed of SiO2. The same composition is also found in some of the other minerals. The results indicated that mining activities within the Loop Spruit catchment, could contribute to higher concentrations of selected metals in surface water at Site 2 and 4 (in close proximity to mines) while sediment concentrations of some metals were significant at these two sites. A decrease in concentration of these metals occurred downstream from the sites nearest to the mines, but the results also indicate that not only mining, but other anthropogenic activities such as agriculture and sewage treatment plant effluent can contribute to metal concentrations in the Loop Spruit.
To establish the diversity of aquatic macroinvertebrates and its association with selected abiotic factors and biotopes within the Loop Spruit, biota was collected during the three surveys at the ten sites using standard sampling methods in all available biotopes. Abiotic factors, biotope descriptions and vegetation types were noted at each site. The organisms were identified up to species level, whenever possible, otherwise up to genus or family level, using the aid of the guides to Freshwater Invertebrates of Southern Africa. All the specimens were counted and grouped into relevant orders. Sensitivity values were allocated to the families and classified into three classes: tolerant, moderately sensitive and highly sensitive towards organic pollution. Species Richness, Shannon-Wiener and Pielou’s evenness indices were used to describe the community structure of the organisms. A total of 137 taxa within 72 families were collected during this study and the family assemblages were relatively consistent. The results indicated that 16 families occurred most commonly, while the majority of these preferred low to very low water quality regarding organic enrichment. Exceptions to this were Baetidae and Hydropsychidae, which indicate good water quality when represented by two or more species at a specific site. Dytiscidae, Tubificidae and Chironomidae, to name only a few, occurred at a majority of the sites during all three surveys. In contrast to this, 19 of the 72 families only occurred during one survey at less than five of the ten sites. This could be ascribed to several reasons including their preference for high water quality and sensitivity towards organic enrichment. A further temporal variation was noted at some of the sites and also a clear spatial variation. Highly sensitive taxa were represented at only two sites, while moderately sensitive taxa were present, to a lesser extent and tolerant taxa occurring in abundance at several sites. These results indicate that the Loop Spruit is largely organically enriched, enabling the tolerant taxa to thrive, but the impact was not to such an extent as to prohibit the occurrence of moderately sensitive taxa. Possible associations between metal concentrations in functional feeding groups and habitat preference (benthic or pelagic) in selected macroinvertebrate families were also investigated. This was achieved by using inductively coupled plasma mass spectrometry analysis. Caenidae differed significantly (p < 0.05) from all the other families and high concentrations were further found in Simuliidae and Chironomidae, all categorised as benthic organisms. Low concentrations of the majority of metals were found in macroinvertebrates classified as predators. From the results obtained during this study a significant variation (p < 0.05) in metal concentrations were evident between functional feeding groups, as well as between benthic and pelagic macroinvertebrate families. The fact that some metals do not biomagnify within the food chain can be ascribed to the lower concentrations found in the predator families. The lower trophic levels (scraper/grazer, shredder, collector-gatherer and collector-filterer FFGs) had significantly higher (p < 0.05) metal concentrations. The benthic families also had significantly higher (p < 0.05) metal concentrations than the pelagic families in the majority of the metals. Although these metals are all considered as potentially toxic to aquatic biota, these high concentrations may not have had a detrimental effect possibly due to strategies such as elimination, detoxification, as well as metabolisation.
The main aim of the study was successfully achieved through 1) determining the primary lithology and secondary minerals of the area surrounding the study area, in order to establish the metals which originate from mining activities or from natural weathering; 2) determining in situ water quality and metal concentrations in water at each site; 3) determining the physical characteristics and metal concentrations in sediments from the selected sampling sites; 4) determining the aquatic macroinvertebrate diversity within the study area; 5) determining metal bioaccumulation in selected macroinvertebrates from an impacted site; and 6) finally to establish a relationship between measured environmental factors and the aquatic macroinvertebrate community structure. These results can serve as a baseline for future studies in this respect