Late Holocene diatom community responses to climate variability along the southern Cape coastal plain, South Africa
In recent decades an emphasis on understanding the long-term variability of ecosystems and their responses to environmental changes has been undertaken. This has mainly been achieved through the identification and analysis of fossil material in lake sediments. In the South Africa, the relative paucity of available sites and the semi-arid nature of the region have led to incomplete or discontinuous palaeoclimatic records. However, lakes along the southern Cape coastal plain have shown promise. With this in mind palaeo-records from neighbouring rainfall regimes were used to evaluate the spatial and temporal patterns of climatic fluctuations during the late Holocene. Diatom analysis was conducted on sediment cores retrieved from coastal lakes at two localities, namely the Wilderness Lake Complex, near Knysna, as a representative of the year-round rainfall zone, and Princessvlei on the Cape Flats, as a representative of the winter rainfall zone of the southern African coastal zone. Additionally, oxygen isotopic analysis was performed on the silicate frustules from Princessvlei. On the basis of this evidence, it can be stated with some confidence that salinity is the driving mechanism governing the state of the Wilderness lake systems, which in turn is a response to sea level changes and climatic fluctuations, in particular moisture availability. Alternatively, the Princessvlei system is governed by changes in nutrients and water turbidity, which have been shown to be proxies for moisture availability and wind. This makes moisture availability a primary controlling factor over the environment along the southern coastal plain, with shifts between wet and dry periods occurring rapidly. Notable shifts in precipitation coincide with global climatic phenomena, including the Medieval Climate Anomaly (~1200 - 700 cal yrs BP) and the Little Ice Age (~700 - 125 cal yrs BP). Marine transgressions during the late Holocene are also recorded in the diatom assemblage particularly in the Wilderness region, as is the system’s recovery during the subsequent regressions. However, anthropogenic forcings during the last few centuries have resulted in considerable modifications in the natural functioning of all systems, altering flow dynamics and nutrient influx. The outcome of this study shows that diatom analysis is a powerful tool in the reconstruction of past environments under conditions wherein other typical palaeoenvironmental proxies may be less adequately represented.
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Department of Environmental and Geographical Science