Abstract
Freshwater is essential to life on earth. Even though the planet’s surface is mostly covered by water, only a small portion of that is freshwater and an even more minute fraction is readily available to humans. The ecological importance of freshwater resources is well known, but in spite of this, freshwater systems are under constant pressure from a multitude of anthropogenic factors. Wetlands are among these threatened freshwater systems, and studies suggest that these are among some of the most threatened freshwater aquatic systems which are overexploited and degraded. Hence it is imperative that policy makers and land owners are educated about the ecological services that wetlands provide, which include nutrient uptake, flood control, water storage during times of drought, stream bank consolidation and they have cultural significance in some cases. Research on wetlands has historically been limited, hence this study aims to contribute to the limited knowledge using the upper Braamfontein Spruit catchment in Johannesburg (Gauteng province, South Africa) as a case study of the role of wetlands in densely populated urban areas. The Braamfontein Spruit catchment is ecologically important because it is the longest running stream in the city of Johannesburg, as such it is a popular resource for recreational activities, but most importantly, it is the one of the main tributaries of the Jukskei River, which feeds into the Crocodile River, one of the most ecologically and commercially important rivers in Southern Africa. Water and sediment quality variables, as well as macroinvertebrate assemblages (as indicators of aquatic health) were assessed for each of four streams that comprise this catchment (Braamfontein Spruit itself, plus its three major tributaries, namely the Sand Spruit, Westdene Spruit and Montgomery Spruit). Two seasons (August 2020 – dry season, and March 2021 – wet season) were sampled in order to assess consistency in the results between the wet and dry season. The study aimed to determine how the water/sediment quality variables and macroinvertebrate assemblages vary down the length of each stream. Water quality parameters measured in the streams were compared with the Target Water Quality Range (TWQR) criteria for aquatic systems and the ecological status category of each stream was determined using the SASS5 bioassessment index. These objectives were achieved by sampling water, sediment and macroinvertebrates at various sites along each of the streams. The results indicate that water quality and aquatic health did not deteriorate downstream as hypothesised, and instead showed
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a general tendency to improve at some of the lower sites. The water quality and aquatic health improved from the dry season to the wet season, which supported the hypothesis that pollutants would accumulate in the slow flowing conditions of the dry season and that water quality, as well as aquatic health, improved in the high flow conditions of the wet season. In most cases the water and sediment nutrient concentrations were higher than the TWQR for aquatic systems, which was expected for these urban streams which are surrounded by densely populated areas. The ecological category for all the streams in both seasons was E/F (critically/seriously modified), indicating that the streams in the upper Braamfontein Spruit catchment are highly modified. The objectives of the study in the second data chapter (i.e. chapter 3) were to compare the water, sediment and macroinvertebrate variables for sites above and below each of three wetland systems on the Wesdtene Spruit and Montgomery Spruit to test the hypothesis that water/sediment quality variables and aquatic health would improve from upstream to downstream of the wetlands. The last objective was to compare the physico-chemical variables, as well as the macroinvertebrate assemblages, between the two streams with extensive wetlands, (the Westdene Spruit and Montgomery Spruit) versus those without wetlands (the Braamfontein Spruit and Sand Spruit). Extensive wetland refers to the aquatic vegetation that spans over a large enough area to filter the water that flows through the wetland. The results showed no convincing improvement in physico-chemical variables or macroinvertebrate assemblages downstream of the wetlands, thus not supporting the study hypothesis. However, the hypothesis that overall aquatic health would be better in the two streams with wetlands versus those without wetlands was supported by the water and sediment quality, as well as the macroinvertebrate data, which showed poorer aquatic health in the streams without wetlands compared to those with wetlands.