The Rietvlei Wetland Reserve forms part of the greater 880 hectares (2,200 acres) Table Bay Nature Reserve.
Background
Rietvlei is considered as the most important area for waterbirds in the region and is recognized as an Important Bird Area by BirdLife International. Official recognition of its importance to biodiversity by the South African government was first afforded in 1984, when it was established as a Nature Area. This was followed by its declaration as a Protected Natural Environment in 1989, and the establishment of the Rietvlei Wetland Reserve in 1993. The first formal management plan for the reserve was developed in 1994, and this has served to guide management activities to the present.[2]
History
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 9 metres (30 ft). The ecological consequences were profound and irreversible. A sizable portion of Rietvlei's shallow ephemeral pans was changed into a permanent deep-water lake, which resulted in a total change in ecological character for this portion of the system.[3]
Features
Visitor facilities
Rietvlei Wetland Reserve offers various user activities, including several types of water sport recreation, bird watching, picnic, fishing and outdoor environmental education opportunities. The Rietvlei Education Centre hosts a range of environmental education programmes and utilises the two bird hides and the short footpath for field excursions.
A range of natural and semi-natural habitats exist 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.
The Diep estuary
The Diep River flows through the Rietvlei wetland and the Milnerton Lagoon, which together have generally been considered to comprise the Diep estuary. If the 5 metres (16 ft) contour above mean sea level is used as the estuary delineation, then the Diep estuary entirely encompasses the Rietvlei Wetland Reserve. The Diep River has its origins in the Riebeek Kasteel Mountains north-east of Malmesbury from where it flows for about 65 kilometres (40 mi) south-west towards Cape Town before entering the sea at Milnerton, some 5 kilometres (3.1 mi) north of the Port of Cape Town. It has one major tributary, the Mosselbank, which drains the northern slopes of the Durbanville Hills. Other tributaries include the Swart, Groen, Klein, and Riebeeck, with the Klapmuts being a tributary of the Mosselbank. The total size of the catchment is 1,495 km2 or 154,347 hectares.[2]
Biodiversity
Birds, amphibians, reptiles and mammals
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.[3]
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 richardsonii.[3]
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 Pauridia in winter.[3]
The effects of the nearby Century City Development on 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 effects on this breeding area 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 salt marsh 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. Other non-native species, including stands of Acacia saligna, are being cleared from large areas around the margin of the wetland.[3]
