Spatio-temporal variability of surface water quality parameters in a South African estuarine lake system

Abstract

Long-term (20+ years) water quality datasets for estuaries are rare, especially for smaller systems. Monitoring of salinity, temperature, pH, dissolved oxygen and turbidity has been undertaken since 1991 in the intensively utilised, modified, and managed temporarily open/closed Touw Estuary and its associated three interconnected estuarine lakes, Eilandvlei, Langvlei and Rondevlei, of the Wilderness Lakes System, South Africa, a national park and Ramsar site. Spatial variability of salinity, pH and turbidity was pronounced in the Touw Estuary but largely absent in the lakes. Significant differences in median salinity, pH, dissolved oxygen and turbidity occurred between lakes, with reverse salinity and pH gradients frequently occurring. Seasonal variability in temperature and dissolved oxygen occurred in all waterbodies. Significant long-term declines in salinity have occurred in the more inland lakes, with decreases in turbidity and pH also occurring in some waterbodies. Water chemistry of the Wilderness Lakes is changing from that of an estuarine to a lacustrine system. Both biological and physical features were driving water quality changes, including reductions in river inflow, reduced marine connectivity, constriction of flow between waterbodies, and declines in submerged plant biomass. Management actions are proposed relating specifically to addressing the apparent causes for water quality changes.