Spatio-temporal variability of five surface water quality parameters in the Swartvlei estuarine lake system, South Africa

Abstract

Variability of salinity, temperature, pH, dissolved oxygen and turbidity in the temporarily open/closed Swartvlei estuarine system, measured from 1991 to 2013, was investigated at various temporal (seasonal, estuarine open/closed phase, long-term) and spatial (inter- and intra-waterbody) scales. Longitudinal pH and salinity gradients, and seasonal variability of temperature and dissolved oxygen, were typical of many southern African estuaries. Differences to comparable systems occurred with respect to temperature range, and within-system variability of salinity, pH and dissolved oxygen. The influence of differing marine and freshwater inflows, and of submerged macrophyte composition and biomass, on such variability are discussed. No significant long-term changes occurred in salinity or pH up until 2006. Significant freshwater flooding in mid-2006 and late 2007, and following increases in open-mouth conditions, resulted in significant decreases in pH in both Swartvlei Estuary and Swartvlei Lake, and increases in salinity throughout the system. Long-term declines in turbidity occurred in Swartvlei Estuary throughout the study period. No long-term changes in water temperature were recorded. Long-term data indicate that the Swartvlei system is not undergoing a rapid deterioration in water quality, but rather exhibiting both short- and long-term fluxes characteristic of estuarine systems.