Reconstructing the long-term history of water quality and availability using fossil diatoms at an agricultural site in the Cape lowlands

Master Thesis


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The Berg River is a pivotal source of fresh water for domestic, industrial and agricultural use as well as for in stream ecology, therefore knowledge of what impacts this rivers water quality and assessing whether ecological resilience has been surpassed are of the utmost importance. Since diatom assemblages are inextricably linked to the chemical, physical and biological characteristics of their environment, they were chosen for this study to investigate long-term changes in water quality and availability and suggest potential drivers of such changes at an agricultural site (Rhenostervlei Farm) adjacent the Berg River in the Cape lowlands through fossil diatom analysis of a sediment core (RV3). Diatoms were extracted along the length of the RV3 core, chemically and physically treated to remove unwanted material and then were mounted on a slide to be counted. Twenty abundant diatoms were chosen as environmental indicators for the analysis. Their abundances were plotted against depth and age and changes in water quality and availability were inferred based on their autecological characteristics. The most prominent shift in the indicator diatoms at Rhenostervlei Farm as shown by the stratigraphic diagrams, CONISS analysis and the PCA occurred at the onset of the 20th century CE and was characterised by a shift from a saline, dry and nutrient-poor environment (ca. 1790-1890 CE) to a more turbid, nutrient-rich environment with increased freshwater influence that was prone to periodic flooding (ca. 1890-2011 CE). Through the chronological analysis and the interpretation of the historical record (climate and land-use) as well as other environmental proxies (sediment accumulation rate and macro-charcoal), the causes of the detected change in water availability and quality at the floodplain site were likely related to land-use change in the form of agricultural intensification at Rhenostervlei Farm and potentially in the Berg River catchment as a whole. This could have involved burning, clearance of natural vegetation, soil disturbance and fertilizer use - all of which contributed to increased surface runoff, erosion and nutrient and sediment loading into the site. Furthermore, water extraction and diversion in the 1950s could explain the decreased flooding signal (i.e. lower abundance of Aulacoseira granulata). Although no evidence of a catastrophic regime shift was identified, if land-use practices continue to intensify (e.g. increased fertilizer use) and future climate change interacts and influences the agricultural alterations to hydrological systems, we may expect increased vulnerability to global change and unexpected ecological outcomes such as regime shifts. In order to improve the interpretation of fossil diatom records in terms of changing water quality and availability, a study with multiple proxies should be undertaken to help infer environmental conditions in a complex environment that has many potential drivers, such as the Berg River.