This reserve lies between Tableview and Milnerton in the northern sector of the Greater Cape Town Metropolitan Area, 10–15 km north-east of the city centre. A range of natural and semi-natural habitats exists in this fluctuating wetland, which floods in winter and dries out in summer when the estuary mouth closes. These habitats include shallow marine waters, estuarine waters, sand/shingle shores, tidal mudflats, saltmarshes, coastal brackish saline lagoons, rivers, streams and creeks, permanent freshwater lakes and permanent and seasonal freshwater marshes and pools.Five distinctive wetland plant communities occur: perennial wetland, reed-marsh, sedge-marsh, open pans and sedge pans. The perennial wetland is characterized by scant aquatic vegetation, dominated by Ruppia, Potamogeton and Enteromorpha. The reed-marsh is dominated by Phragmites, invaded in places by Typha. The sedge-marsh is dominated by Bolboschoenus and Juncus. The open pans are sparsely covered in macrophytes, consisting mainly of Limosella and Salicornia, and the sedge pans are dominated by Bolboschoenus in summer and Aponogeton and Spiloxene in winter. Zooplankton multiply rapidly after winter flooding and disappear in summer as the water dries up. In the estuary there is a range of salinities, resulting in a diverse community of zooplankton. The invertebrate fauna is a vital food source for birds and fish, the most abundant fish in the wetland being Liza richardsoni.
See Box for key species. A total of 173 species have been recorded at Rietvlei, of which 102 are waterbirds and 76 are present regularly. Breeding has been confirmed for 23 waterbird species and is suspected for a further 13 species. The high diversity of waterbirds is due to the wide range of wetland habitats present and the proximity of Rietvlei to the ocean, which allows both freshwater and coastal species to exploit the system. Fluctuating water-levels are intrinsic to Rietvlei’s biological value. During peak floods, swimming birds of deep, open water abound. Birds of marshy habitats replace these as the water recedes, and waders exploiting shallow mudflats occur in great abundance just prior to the wetland drying up. Rietvlei has been ranked as the sixth most important coastal wetland in South Africa for waterbirds, and it supports an average of 5,550 birds in summer; during good years, however, numbers are boosted above 15,000. Phoenicopterus minor, a species of global conservation concern, occurs at the site, but not in globally significant numbers.
Non-bird biodiversity: Urban development and encroachment by non-native plants threaten the herptiles Cacosternum capense (LR/nt), Hyperolius horstockii, Bradypodion pumilum (CR) and B. occidentale, which all live on the wetland fringes.
Pressure/threats to key biodiversity
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. The Blouberg Municipality owns the major part of the wetland; Transnet has recently donated the Flamingo Vlei portion to the municipality. Rietvlei was declared a Nature Area in August 1984, and a management committee was appointed in 1985. In 1989 the area was upgraded to a Protected Natural Environment and Caltex provided funds to the Blouberg Municipality for the express purpose of establishing a nature reserve in 1993. The Caltex donation funded the purchase of most of the Rietvlei Protected Natural Area, specifically the area previously owned by Milnerton Estates. The deal was brokered by WWF-South Africa (formerly the Southern African Nature Foundation). Ownership was transferred to the Blouberg Municipality and the area known as the Rietvlei Wetlands Reserve was established on 27 July 1993. The consolidation of the entire system into a single conservation unit could lead to Rietvlei’s registration as a wetland of international importance under the Ramsar Convention. Two permanent wetland pools at the north-western extremity are to be incorporated into the reserve in the near future.The most obvious and dramatic human-induced modification at Rietvlei was the dredging of the entire north-west section between 1974 and 1976. Seawater was pumped into the pans to facilitate the operation and a vast area was dredged to a depth of 9m. 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 portion of the system. The demise of the nearby Blouvlei, which used to support a large heronry holding 12 breeding species, is cause for considerable concern. Most of these birds used to forage at Rietvlei and would contribute substantially to the large numbers of birds occurring here. The loss of this breeding area to make way for the Century City development will probably result in fewer birds visiting the Rietvlei area. Other threats to the wetland include siltation, which results from erosion, and pollution and eutrophication from fertilizers, pesticides, sewage works, stormwater run-off and livestock manure. Petroleum factories and suburban areas on the margin of the system also pose problems.Vast areas of the mudflats and saltmarsh have been smothered by thick mats of non-native grasses, notably Paspalum vaginatum, resulting in habitat loss for waders, the most diverse and abundant community of waterbirds at Rietvlei. The grasses are virtually useless to birds and also contribute to the siltation problem by binding the soil. The reedbeds would probably benefit from periodic burning and the invasive grass could probably be controlled by the occasional introduction of a few domestic cattle, which would certainly provide an improved range of substrate and habitat conditions. Other non-native species, including stands of Acacia, are being cleared from large areas around the margin of the wetland.