Changes in the water quality of the Wilderness and Swartvlei lake systems, South Africa

Abstract

Measurements of water temperature, salinity, total dissolved solids, pH, dissolved oxygen, secchi disk depth, turbidity and total suspended solids were undertaken monthly in the Wilderness and Swartvlei lake systems between 1991 and 1997. The range of most water quality variables is wider than recorded in earlier studies. Potential causes are discussed for each variable, and include both anthropogenic modification of the quantity and timing of water movements within the lakes, as well as higher sampling and longer duration studies resulting in more accurate recording of natural variability. Historical data on total dissolved solids (1978 to 1990) were combined with data collected in these surveys to assess the occurrence of trends in relation to freshwater inflows. Long-term (19-year) changes in total dissolved solids have occurred in all lakes, with decreases during high runoff periods and increases during low runoff periods. There is little evidence of unidirectional change in total dissolved solids despite increased water abstraction from influent rivers.