Rietvlei Wetland: Table Bay Nature Reserve

Site description

Located in the northern sector of the greater Cape Town metropolitan area, this 527-ha reserve lies between Table View and Milnerton Ridge in the suburb of Milnerton, 10–15 km north-east of the city centre. It comprises a fluctuating wetland, which floods in winter and partially dries out in summer when the estuary mouth narrows, and it contains a range of natural and semi-natural habitats: shallow marine waters; estuarine waters; sand/shingle shore; tidal mudflats; salt marshes; coastal brackish lagoons; rivers, streams and creeks; permanent freshwater lakes; and permanent and seasonal freshwater marshes and pools. 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 Much of the wetland that surrounded the Diep River estuary in the past has been replaced by urban development.

Birds

A total of 201 species has been recorded at Rietvlei during SABAP2, of which 102 species are waterbirds and 76 are regularly present. Breeding has been confirmed for 23 waterbird species and is suspected for a further 13. 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. Fluctuating water levels are intrinsic to Rietvlei’s biological value. During peak floods, swimming birds of deep open water abound. As the water recedes they are replaced by birds of marshy habitats, and just before it dries up large numbers of waders can be seen exploiting the shallow mudflats. In a prioritisation of South Africa’s estuaries, 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.

Threatened and near-threatened species found at Rietvlei are Greater Flamingo Phoenicopterus roseus, Lesser Flamingo Phoeniconaias minor, Great White Pelican Pelecanus onocrotalus, African Marsh Harrier Circus ranivorus, African Black Oystercatcher Haematopus moquini and, on rare occasions, Chestnut-banded Plover Charadrius pallidus. At times Rietvlei holds globally significant numbers of Great Crested Grebe Podiceps cristatus, Cape Shoveler Anas smithii, Pied Avocet Recurvirostra avosetta, Hartlaub’s Gull Chroicocephalus hartlaubii and Kelp Gull Larus dominicanus. It also holds large numbers of Yellow-billed Duck Anas undulata, Red-knobbed Coot Fulica cristata, Curlew Sandpiper Calidris ferruginea, Little Stint C. minuta, Ruff Philomachus pugnax, Kittlitz’s Plover Charadrius pecuarius, Caspian Tern Sterna caspia, Swift Tern Thalasseus bergii and White-winged Tern Chlidonias leucopterus, although these species do not reach IBA thresholds. It is locally significant for Marsh Sandpiper Tringa stagnatilis. Peregrine Falcon Falco peregrinus occasionally hunts waders at the wetland.

Compared with the numbers published in Barnes (1998), there is an alarming decline for several of the congregatory waterbirds, notably Curlew Sandpiper, Kittlitz’s Plover, Yellow-billed Duck and Hartlaub’s Gull. Kelp Gulls have increased in number since the Barnes (1998) publication. It is suggested that the City of Cape Town and assistants conducting CWACs be informed of the trigger species for this IBA so that appropriate monitoring may begin, or that these species be focused on during CWACs so that accurate numbers can be established.

IBA trigger species

Globally threatened species are African Black Oystercatcher and Lesser Flamingo, and regionally threatened species are Greater Flamingo, Great White Pelican, Caspian Tern, African Marsh Harrier and Lanner Falcon. Biome-restricted species that are common in the IBA include Cape Spurfowl Pternistis capensis and Cape Bulbul Pycnonotus capensis. Species that meet the 1% or more congregatory criteria are Great Crested Grebe (maximum 52 individuals), Cape Shoveler (maximum 422 individuals), Pied Avocet (maximum 456 individuals), Kelp Gull (maximum 1 019 individuals) and Hartlaub’s Gull (maximum 541 individuals). Yellow-billed Duck (maximum 391 individuals), Kittlitz’s Plover (maximum 107 individuals) and Curlew Sandpiper (maximum 1 061 individuals) meet the criteria for the 0.5% or more congregatory threshold.

Other 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. 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 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.

Conservation issues

Threats

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 principal threats affecting the wetland and estuarine system stem from the surrounding urban development matrix. 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. Changes in water-flow dynamics, particularly those caused by abstraction, may lead to increased salt intrusion into an estuary. In the case of the Diep River estuary, however, the opposite is true. 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.

Water quality is a linked issue, and here the change is mostly negative as a result of increased nutrient loading. 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.

Soil erosion and sediment run-off from agricultural practices in the Diep River catchment add to the problem of reed growth by providing a shallow substrate for the reeds. Dense reedbeds then further slow water movement, which leads to sediment deposition. Increased sediments support further growth, thus creating a positive feedback mechanism. Agricultural activities also lead to increased nutrient run-off from fertilisers, which adds to the eutrophication of the system.

Recreational activities can disturb birds and other fauna, but the restriction of activities to certain areas within the wetland reduces any negative impact. Nature reserves that provide recreational space for local communities have a relatively high perceived value, and urban nature reserves in particular must be seen to be relevant and beneficial to the local people and government if competing land uses are to be prevented from taking their place. Space for recreational activities is the major value that local residents derive and it must be balanced against the biodiversity needs of any given site. This is particularly true for Rietvlei Wetland, where several different recreational activities compete for space. Any recreational use of the reserve must be sustainable in that it is consistent with environmental conservation.

Invasive alien vegetation, including water hyacinth Eichhornia crassipes, pampas grass Cortaderia selloana, Paspalum species and Acacia species, are present in the IBA. The City of Cape Town is conducting alien vegetation eradication programmes that will reduce the encroachment of these species over time. In an urban wetland such as this, an additional threat comes from kikuyu grass Pennisetum clandestinum, which invades from nearby gardens. There are also control programmes in place for this species. Increasing coverage by Typha capensis and Phragmites australis in the wetland, resulting from increased sediments and nutrients as well as reduced salinity, lessens the extent of mud-flat habitat essential for many of the IBA trigger species.

Conservation action

Rietvlei was declared a Nature Area in August 1984 and a management committee was appointed the following year. In 1989 the area was upgraded to a Protected Natural Environment. Caltex provided funds to Milnerton municipality for the express purpose of establishing a nature reserve and most of the Rietvlei Protected Natural Area, specifically the parts previously owned by Milnerton Estates and Transnet, was purchased in a deal brokered by WWF-South Africa. Ownership was transferred to Milnerton municipality and the area known as the Rietvlei Wetlands Reserve was established on 27 July 1993. Subsequently the management of the site was taken over by the Cape Town City Council and various nature reserve fragments were consolidated to form the Table Bay Nature Reserve, which includes the Rietvlei Wetlands IBA. This area is awaiting proclamation as a nature reserve under NEM:PAA and will then be formally protected by legislation.

An integrated reserve management plan prepared in 2011 addresses all aspects of the ecology and infrastructure of the area. The Diep Estuary management plan is also being implemented. Bird counts and other biodiversity monitoring are in place, alien vegetation eradication programmes are operating and opportunities for the rehabilitation of some areas are being investigated.
Current conservation activities include law enforcement; the maintenance of infrastructure such as bird hides, pathways and boardwalks; the eradication of both woody and weedy alien vegetation; and fire management. The reserve employs enough full-time staff to handle these duties, undertake research projects and train interns. The training aims to fulfil the requirements for a diploma and is thus helping to develop future conservation biologists.

Additional conservation measures are intended to protect waterbird populations from disturbance. They include closing the southern section of the lake to boating and prohibiting angling in most of the wetland, which also lessens the likelihood of birds being injured or killed by fishing line and hooks. Fencing the reserve and the establishment of a formal trail system, regulated by means of signage, will help to mitigate disturbance caused by people walking their dogs.

Related webpages

https://www.capetown.gov.za/en/EnvironmentalResourceManagement/Pages/NatureReserve-TableBay.aspx

Contact

If you have any information about the IBA, such as a new threat that could impact on it, please send an e-mail to iba@birdlife.org.za or call BirdLife South Africa +27 (11) 789 1122.

Page last updated

Wednesday, 18 February 2015