Rietvlei Wetland Reserve
Rietvlei is one of the last functioning coastal lakes characteristic of, and restricted to, the Western Cape. Its major water source is the Diep River, which provides a substantial natural water supply. The wetland is a dynamic system, characterised by a seasonal cycle of sudden inundation in winter due to the inflow from the Diep River. This is followed by gradual desiccation during spring and summer. Most parts of Rietvlei are no more than 2 m deep, although the maximum depth is 9 m.
Five distinctive wetland plant communities occur: perennial wetland, reed marsh, sedge marsh, open pans and sedge pans. Zooplankton multiply rapidly after winter flooding and disappear in summer as the water dries up. The range of salinities in the estuary results in a diverse community of 28 zooplankton species that vary from freshwater to marine types. There are more than 84 species of aquatic invertebrates in the freshwater section of Rietvlei and 35 species occur in the Milnerton Estuary; together they make up a vital food source for birds and fish.
In the wetland the most abundant fish is southern mullet Liza richardsoni, while the estuary acts as a nursery ground for white steenbras Lithognathus lithognathus and white stumpnose Rhabdosargus globiceps. Much of the wetland that surrounded the Diep River estuary in the past has been replaced by urban development.
Rietvlei supports a remarkable diversity of birds, with more than 190 species having been recorded here. Of these species 102 are waterbirds, many of them migrant waders that visit these waters in spring and summer. A large bird hide allows their behaviour to be observed undetected. The high diversity of waterbirds is due to the wide range of wetland habitats and the proximity of Rietvlei to the ocean, which enables both freshwater and coastal species to exploit the system. Rietvlei has been ranked as the sixth most important coastal wetland in South Africa for waterbirds. It supports an average of 5 550 birds in summer, but in good years the number is boosted to above 15 000.
The seabird rehabilitation centre run by SANCCOB (Southern African Foundation for the Conservation of Coastal Birds) is also worth a visit, as it allows for close viewing of penguins and other seabirds. The centre is open to public all week, with a nominal fee charged as a contribution towards operational costs.
The sedge pan habitat represents one of the rarest ecological systems in South Africa and is of high conservation value. The open pan, perennial wetland and sedge marsh communities are also extremely valuable ecologically. Urban development and alien plant encroachment threaten Cape caco Cacosternum capense, which occurs in the catchment of the IBA, and the vulnerable Cape dwarf chameleon Bradypodion pumilum, which inhabits the wetland fringes. The vulnerable Cape sand snake Psammophis leightoni and the yellow-bellied house snake Lamprophis fuscus also occur in the IBA, but both live in terrestrial habitats at the margin of the system and are not dependent on the wetland.
The southern African endemic angulate tortoise Chersina angulata, Cape cobra Naja nivea, Cape legless skink Acontias meleagris, silvery dwarf burrowing skink Scelotes bipes and Knox's desert lizard Meroles knoxii are found in the IBA.
The most obvious and dramatic human-induced modification at Rietvlei was the dredging of the entire north-western section between 1974 and 1976. The dredged material was taken to Cape Town Harbour for construction purposes. Sea water was pumped into the pans to facilitate the operation and a large area was dredged to a depth of 9 m. The ecological consequences were profound and irreversible: Rietvlei swapped a sizeable portion of its shallow ephemeral pans for a permanent deep-water lake, which resulted in a total change in ecological character for this part of the system. The northern lobe is now used for a variety of water sports, including power-boating, water-skiing, dinghy-sailing, windsurfing, as well as freshwater angling, while the southern lobe is a bird sanctuary area.
Water quality and quantity are a major concern. In terms of the latter, water abstraction is particularly prevalent in the upper catchment of the Diep River. An increasingly urban environment has led a once-seasonal freshwater flow to become more sustained throughout the year and salt-water intrusion has consequently decreased. Changes in salinity lead to changes in the structure of the invertebrate community, thus altering the availability of food sources for birds.
The Rietvlei wetlands and Diep River receive pollution from nearby industrial areas, where storm-water run-off carries industrial and residential pollutants into the river. Other threats to the wetland include siltation, which results from erosion, and pollution and eutrophication from fertilisers, pesticides, sewage, storm-water run-off and livestock manure. Petroleum factories and suburban areas at the edge of the system also pose problems. The Potsdam waste-water treatment works and the lack of adequate sanitation in the Dunoon, Joe Slovo and Phoenix areas lead to increased nutrients flowing into the wetlands. Recent upgrades to the waste-water treatment works have reduced this threat, although the run-off of raw sewage from the nearby settlements can increase nutrient levels.
Increased nutrients have led to rapid reed growth and changes to the plant and invertebrate communities. Agricultural activities also lead to increased nutrient run-off from fertilisers, which adds to the eutrophication of the system.
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