Zeekoevlei AFR-17

Lat.-34.062 Lng.18.516 Alt.5.2
Riparian Nation(s) South Africa
Surface Area 3 km2 Mean Depth 1.9 m Volume 0.005 km3
Shoreline 12.6 km Catchment Area 80 km2 Residence Time 0.24 yr
Frozen Period None Mixing Type Polymictic Morphogenesis/Dam
Related Info/Site

Description

Zeekoevlei is the largest (256 ha) freshwater lake in South Africa, situated on the Cape Flats near Cape Town in the Western Cape Province. The Cape Flats are a low-lying, sandy region which was originally submerged beneath the sea and which separate the mountainous Cape Peninsula from the mainland. The prevailing climate is a Mediterranean, winter-rainfall type. Zeekoevlei is an important regional recreational venue. The lake is a shallow (mean depth 1.9 m), polymictic and hyper-eutrophic urban-impacted system, which has year-round non-limiting concentrations of nitrogen and phosphorus, with mean annual value of 3.6 and 0.55 mg l-1 respectively. Nutrient loading from the catchment occurs during the winter via the Big and Little Lotus Rivers. Of these, the Big Lotus River delivers in excess of 80% of the annual hydraulic flow, and non-point source catchment pollution results in mean annual concentrations of 2.2 mg l-1 N and 0.64 mg l-1 P in this influent water. The lake overflows only during the winter into a canal leading to the sea. Lake levels fluctuate by an average of 0.5 m between winter and summer.

The vlei is fringed by dense growth of the emergent aquatic macrophytes Typha capensis and The vlei contains no submerged macrophytes but is subject to isolated infestation by the floating water hyacinth, Eichhornia crassipes. Phytoplankton diversity is poor and the phytoplankton assemblage is dominated year round by the blue-green alga Microcystis. Growth of this alga completely masks the periodic characteristics of the sub-dominant taxa of Chlorophyta and Bacillariophyta. Typical mean annual chlorophyll a and Secchi disk transparencies are, respectively, 200 micro l-1 and 0.25 m. Maximal growth and water blooms of Microcystis are experienced during the spring.

The prolific growth of blue-green algae, coupled with restrictions of the natural hydraulic flushing of the lake as consequence of the construction of an outlet weir in 1948, have resulted in massive accumulations of organically rich sediments. These currently comprise 21% of the volume of the lake 1,100,000 m3, and are thought to play an integral role in the internal nutrient cycling and self-sustaining nature of the system during the dry summer season. Since 1990 Zeekoevlei has been the focus of a management investigation conducted by the Cape Town City Council to ascertain viable options for the lake's future rehabilitation (Q).

Photo of Zeekoevlei
Photo: W. R. Harding