ZA093
Wilderness - Sedgefield Lakes Complex


Year of compilation: 2001

Site description
The Wilderness–Sedgefield Lakes complex (WSLC) incorporates the Wilderness National Park and the Goukamma Nature Reserve along the coastal belt east of Wilderness. The entire lakes complex lies below the 5 m contour line on the flat Touw river flood-plain. Three discrete lake systems comprise the Wilderness–Sedgefield Lakes complex. The first, to the west, is the Wilderness lakes system, which consists of a natural channel, the Serpentine, which links the Touw river to its estuary and flood-plain. The channel also runs between Eilandvlei to two other lakes, Langvlei and Rondevlei. The central system is known as Swartvlei, and it consists of a large lake, Swartvlei, which is connected to the sea via the Swartvlei estuary. The last system, which lies to the east, is a single landlocked lake known as Groenvlei. Collectively, they are the only warm-temperate coastal lakes with a marine connection in South Africa.

Whenever a lake’s connection to the estuary is periodically closed by longshore drift, a lagoon, dominated by freshwater, forms. When the connection to the estuary opens, it becomes flooded with seawater. When water-levels are low, exposed sand/mudflats appear on the periphery of most of the water-body. Submerged plants that grow in the lakes, channels and estuaries include Chara, Lamprothamnium, Potamogeton, Najas, Zostera and Ruppia. The fluctuations in salinity and water transparency can sometimes cause total dieback of such plants. The surface waters are clear and open in the central areas. Beds of fringing vegetation, such as Phragmites, Scirpus, Juncus, Typha, Cladium and Bolboschoenus, occur in the littoral zone. The lakes support a diverse community of estuarine macroinvertebrates and fish.

Forests dominated by Podocarpus, Diospyros, Gonioma, Olea, Cassine, Pterocelastrus, Trichocladus and Rapanea border the lakes and comprise most of the terrestrial vegetation within the Wilderness National Park. A 38-km stretch of coastline, including a series of high undulating coastal sand-dunes, is protected within the park and the neighbouring Goukamma Nature Reserve. The Nature Reserve holds dune fynbos.

Key biodiversity
See Box and Tables 2 and 3 for key species. The site supports an average of 10,868 non-passerine waterbirds every month, and the monthly total has ranged between 4,841 and 24,427 waterbirds between 1980 and 1984. The system is important for Palearctic migrant waders and southern African waterbirds, especially ducks (Anatidae), which moult and breed here. The lakes support 72 waterbird species, including good numbers of Thalassornis leuconotus and Anas undulata.The marsh and reedbeds surrounding the lakes hold Circus ranivorus, Rostratula benghalensis, Ixobrychus minutus, Porzana pusilla, Sarothrura rufa, Rallus caerulescens and small numbers of Tyto capensis. The coastline holds notable numbers of Haematopus moquini. Although not present in very significant numbers, up to 6,000 Palearctic waders, consisting primarily of Calidris ferruginea, C. minuta and Philomachus pugnax, can be found within the system during the austral summer. The well-wooded backwaters on Swartvlei and Groenvlei hold small numbers of Podica senegalensis and Alcedo semitorquata.

Non-bird biodiversity: The forests hold several endemic plant species including Gladiolus vaginatus and Satyrium princeps. The endemic and threatened fish Hippocampus capensis (EN) occurs in the Swartvlei estuary. The terrestrial vegetation surrounding the wetland system supports the endemic vertebrates Myosorex longicaudatus (VU), Myosorex varius and Breviceps fuscus.



Pressure/threats to key biodiversity
The Wilderness Lakes are a Ramsar Site. The Wilderness National Park incorporates the entire site except for the eastern fringe of Groenvlei. The adjacent Goukamma Nature Reserve, established in 1960, includes the eastern fringe of Groenvlei, the lower sections of the Goukamma river and its estuary, extensive sand-dunes and 14 km of rocky coastline, beaches and marine reserve. The national park also incorporates most of the terrestrial matrix lying between the lakes.The IBA is threatened by the burgeoning expansion of tourism demands on the coastal zone between Knysna and George, which has become one of the premier holiday resort sectors in South Africa. The lakes are particularly susceptible to high-impact use by tourists. It is vital that development and human activity be monitored and controlled to minimize the anthropogenic impacts on the lakes system. In 1981 a sluice-gate was installed in the Serpentine to manipulate the water-level in the upper Wilderness Lakes system as a means of artificial control. The normal flooding regime of the system was upset because of the development of low-lying properties within the area.

The mouth of the Wilderness Lagoon has to be regularly bulldozed to protect low-lying properties from the threat of flooding. In the early 1980s it became obvious that the artificial opening of the mouth, which had resulted in lower than average water-levels, had caused an expansion of emergent macrophytes, which were interfering with ecosystem processes and disrupting recreation activities. The primary concern within the flood-plain should be to improve water exchange whenever possible. It would appear that flood control and selective dredging might be necessary management practices to improve the flow regime of the estuary.

Several invasive, non-native tree and shrub species represent a major threat to the indigenous flora of the flood-plain. Albizia lophantha, Sesbania punicea and Acacia cyclops have become serious problem species, requiring special attention. Non-native fish, such as Oreochromis mossambicus, have been introduced to the lakes and are causing problems by competing with indigenous fish species. Pollution is also of primary concern, as it may be seriously disrupting ecosystem processes. Effluent from residential and industrial sectors should be monitored to detect changes in pollution levels. It is recommended that partial cutting of fringing reeds and sedges be practised on a biennial cycle in order to reduce the amount of moribund vegetation and improve the habitat for rallids and other birds.


Recommended citation
BirdLife International (2021) Important Bird Areas factsheet: Wilderness - Sedgefield Lakes Complex. Downloaded from http://www.birdlife.org on 08/12/2021.