Agulhas Plain–Heuningnes Estuary

Site description

This complex wetland system lies at the southern tip of Africa, south of Bredasdorp, and consists of the Nuwejaars River and its associated marshes, Zoetendalsvlei, Voëlvlei and Waskraalvlei; and the Heuningnes River and its estuary, which is located in De Mond Nature Reserve. The Heuningnes River mouth is the southernmost estuary in Africa and is formed where the river reaches the sea through a double ridge of sand dunes. This river has two major tributaries: the Kars River, which has its source in the Bredasdorp Mountains and flows 75 km before emptying into Zoetendalsvlei; and the Nuwejaars River, which rises in the Bredasdorp Mountains, Koueberge and Soetanysberg and runs some 55 km from its primary source to Zoetendalsvlei.

The IBA boundary extends inland from De Mond Nature Reserve to beyond the confluence of the Nuwejaars River and Zoetendalsvlei and then in a north-westerly direction to the junction of the R43 and R319, c. 5 km south of Bredasdorp. It turns westward and continues to the small town of Elim, following the northern border of the Nuwejaars Wetland Special Management Area. This special management area, which stretches down to Agulhas National Park, is an innovative approach to land management that combines commercial agriculture with conservation. It covers c. 45 000 ha in total, with a core conservation area of 22 000 ha, and it includes Zoetendalsvlei, Voëlvlei and Waskraalvlei. Beyond Elim, the IBA boundary continues westward past Baardskeerderbos before turning south-west to the coast, meeting it at Franskraal and thus including the smaller Uilenkraals Estuary. The boundary turns east, following the rugged southern Cape coastline past Cape Agulhas, the southernmost tip of Africa, and back to De Mond Nature Reserve. The IBA thus incorporates Agulhas National Park, which was proclaimed in September 1999 and has grown over the years to cover c. 21 000 ha of the Agulhas Plain region.

The wetland systems and inland lakes of the Nuwejaars Wetland Special Management Area and Agulhas National Park are connected by the Nuwejaars River and act as a reservoir, absorbing excess water flow. Although the Voëlvlei system usually drains into the Nuwejaars River, during periods of flooding the opposite occurs and the river’s waters fill Voëlvlei. Then, as the flooding subsides, the flow reverts to its normal direction. The Nuwejaars River empties into Zoetendalsvlei and when this system is full it overflows into the Heuningnes River, which in turn feeds the Heuningnes Estuary. The entire catchment area is relatively small at 1 401 km². The estuary drains the flat, low coastal plain of the Zoetendals Valley. The Nuwejaars River marshes consist of a series of wetlands on the farms Heuningrug and Wiesdrif on the west bank of Zoetendalsvlei. The primary wetland area is found between Heuningrug and Wiesdrif.

Vegetation types, plant species composition and water levels are diverse, producing a mosaic of prime palustrine wetland habitats for rallids. A second important wetland area is the 5-km stretch of the Nuwejaars River close to Moddervlei. Where the river runs in a well-defined channel lined with exotic trees, its banks are narrow and muddy, with little or no fringing sedges. Instead, dense stands of palmiet extend over the water. In other areas, the river broadens out and is bordered by beds of sedges and reeds, with scrub, restios and grass on drier land. On small tributary streams there are pools and patches of flooded vegetation. Many of the drainage channels are poorly defined and run-off into the estuary occurs via vleis and marshes, which constitute large areas of wetland. This area is currently being cleared of alien vegetation.

The Heuningnes Estuary comprises an extensive bay with sand, mudflats and tidal salt marsh. A causeway 1.3 km from the ocean obstructs tidal flow considerably in the upper reaches of the estuary and at 12 km upstream the tidal influence is marginal. A result of the obstruction is that only the final 2-km stretch of the estuary shows typical estuarine features, such as strong tidal activity and clear water with a high salinity. A significant characteristic of the lower estuary, which formed after the mouth was kept open artificially, is a large inner delta. The flatness of the area causes the estuary channel to meander irregularly over the sandy plain between the two dune ridges. Long periods of precipitation can cause extensive flooding, whereas at other times the river flow can be insufficient to keep the mouth open.

Birds

The wetlands of the Agulhas Plain and associated protected areas and the agricultural matrix provide excellent habitat for a wide diversity of birds, including waterbirds, threatened raptors, Palearctic migratory raptors and large terrestrial species such as bustards and Blue Crane Anthropoides paradiseus. The rank reed growth of the wetland system is ideal habitat for the more secretive rallid species. Baillon’s Crake Porzana pusilla is uncommon but probably breeds here; Red-chested Flufftail Sarothrura rufa has been recorded at one pair per 0.25 ha; and African Rail Rallus caerulescens and Black Crake Amaurornis flavirostra are widespread and abundant. Greater Painted-snipe Rostratula benghalensis, of the isolated and declining Western Cape breeding population, is also present, but not in large numbers. The shy Hottentot Buttonquail Turnix hottentottus has been recorded in Agulhas National Park and is likely to be found in other suitable habitat in the IBA, and the secretive Striped Flufftail Sarothrura affinis probably also occurs in the region.

This IBA and the Overberg Wheatbelt IBA (SA115) represent the stronghold for Blue Crane. The birds occasionally roost and feed at open-water pools surrounded by tall sedges in the wetland system and on adjacent agricultural land throughout the region, and large numbers of them regularly undergo synchronised flightless moulting in the wetlands. Caspian Tern Sterna caspia and Kittlitz’s Plover Charadrius pecuarius breed regularly at De Mond Nature Reserve, which is also one of only two protected, confirmed breeding sites for Damara Tern S. balaenarum in South Africa. The pebble slacks behind the beach previously held 15% of the national breeding population of this species. African Black Oystercatcher Haematopus moquini breeds and the threatened Cape Cormorant Phalacrocorax capensis roosts along the entire coastline of the IBA. Bank Cormorant P. neglectus and African Penguin Spheniscus demersus also occasionally roost along this coastline.

Three hundred pairs of Kelp Gull Larus dominicanus regularly nest in the dunes a few kilometres to the north-east of the Heuningnes Estuary, while Great White Pelican Pelecanus onocrotalus, Lesser Flamingo Phoeniconaias minor and Greater Flamingo Phoenicopterus roseus occasionally occur in large numbers, frequenting the salt pans in Agulhas National Park. Substantial numbers of migrant Palearctic and resident waders regularly use the Heuningnes Estuary and other wetlands in the IBA. They include Curlew Sandpiper Calidris ferruginea, Little Stint C. minuta, Common Greenshank Tringa nebularia, Ruff Philomachus pugnax, Common Ringed Plover Charadrius hiaticula, Chestnut-banded Plover C. pallidus and Kittlitz’s Plover. The area hosts important populations of White-fronted Plover C. marginatus, Common Tern Sterna hirundo, which occurs in very high numbers, Sandwich Tern Thalasseus sandvicensis, Whiskered Tern Chlidonias hybrida and White-breasted Cormorant Phalacrocorax lucidus. The historic salt pans on the Brandfontein road and Melkbos Pan near the Rietfontein homestead provide a food source for thousands of waders, especially as the water level drops. The Heuningnes Estuary also often produces rare or vagrant species in early summer when the migratory birds return.

Black Harrier Circus maurus and African Marsh Harrier C. ranivorus, as well as large numbers of African Fish Eagle Haliaeetus vocifer and Booted Eagle Hieraaetus pennatus, are present, and breed, across the Agulhas Plain and its associated wetlands. African Grass Owl Tyto capensis is widespread across the plain and at least one pair has been recorded breeding in De Mond Nature Reserve. Marsh Owl Asio capensis is also likely to occur. Southern Black Korhaan Afrotis afra, Denham’s Bustard Neotis denhami and Secretarybird Sagittarius serpentarius are found in both the agricultural matrix and the protected areas of this IBA. The area is part of the stronghold of the threatened Agulhas Long-billed Lark Certhilauda brevirostris, which occurs only in the south-western Cape, and also supports Cape Clapper Lark Mirafra apiata and Cape Siskin Crithagra totta, as well as other fynbos-endemic species.

IBA trigger species

Globally threatened species are Cape Cormorant, Damara Tern, Chestnut-banded Plover, African Black Oystercatcher, Lesser Flamingo, Blue Crane, Denham’s Bustard, Southern Black Korhaan, Secretarybird, Martial Eagle Polemaetus bellicosus, Black Harrier and Hottentot Buttonquail. Regionally threatened species are African Marsh Harrier, Lanner Falcon Falco biarmicus, African Grass Owl, Greater Flamingo, Caspian Tern, Karoo Korhaan Eupodotis vigorsii, Agulhas Long-billed Lark, Striped Flufftail and Greater Painted-snipe.

Restricted-range and biome-restricted species that are common in the IBA include Cape Spurfowl Pternistis capensis and Cape Bulbul Pycnonotus capensis. Locally common species include Cape Sugarbird Promerops cafer, Orange-breasted Sunbird Anthobaphes violacea, Agulhas Long-billed Lark, Cape Siskin and Cape Clapper Lark, while Southern Black Korhaan, Karoo Korhaan, Hottentot Buttonquail and Striped Flufftail are uncommon.

Other biodiversity

The arum lily frog Hyperolius horstockii, Cape dwarf chameleon Bradypodion pumilum, Cape greater gerbil Tatera afra and aardwolf Proteles cristatus are threatened species thought to occur in the area. It is possible that the South African endemics sand toad Bufo angusticeps, sand rain frog Breviceps rosei, Cape sand frog Tomopterna delalandii, southern adder Bitis armata and silvery dwarf burrowing skink Scelotes bipes may be found in the IBA. African buffalo Syncerus caffer, hippopotamus Hippopotamus amphibius and other large game have been re-introduced. The bays offshore of the IBA are important breeding grounds for southern right whale Eubalaena australis and various other whales, dolphins and porpoises.

Conservation issues

Threats

Agricultural activities have transformed much of the fynbos and renosterveld in this region and impoundments and the abstraction of water along watercourses have led to irreversible damage to the wetland habitats. Fortunately, the current approach to agricultural production takes these threatened vegetation types into consideration and rehabilitation is under way in certain areas. Agricultural activities are also responsible for pollutants such as chemical pesticides and fertilisers entering the ecosystem, and overgrazing can lead to erosion across large areas. High levels of nutrients in the river and wetland systems and the abstraction of water may alter water-flow regimes in the estuary and lead to occasional outbreaks of Escherichia coli.

Invasive alien vegetation represents a major threat to the biodiversity of the Agulhas Plain. This vegetation was actively planted in the past to facilitate land stabilisation, but it has encroached over large areas, transforming habitat and causing water loss in the wetland systems. In addition, fires occur more frequently than is natural, leading to a reduction in the abundance of certain fynbos elements.

Residential developments may impact on the Heuningnes Estuary in future, but for now they are contained within nearby settlements. The expansion of residential areas at Struisbaai and Arniston may encroach on agricultural land, which provides habitat for birds such as Blue Crane, a prolific species in the area. Human disturbance in this mixed-use landscape could prevent large terrestrial species from breeding, while illegal actions such as off-road driving within the protected area boundaries are likely to cause sensitive species such as Damara Tern and African Black Oystercatcher to abandon their nests, resulting in decreased reproductive rates for these threatened species.

This is one of the most interesting and important wetland systems in South Africa in terms of vegetation diversity and its great variety of seasonal and permanent palustrine habitats. At lower latitudes, any African rallid of palustrine wetlands could occur and breed in an area such as this. Unfortunately, such habitats are now extremely rare in Africa as a result of being modified or even destroyed.

Most of the catchment is on privately owned farmland. Several impoundments in the form of irrigation dams and weirs have been constructed on the Kars River. Any further damming of the tributaries could have serious implications for the flow of fresh water into the Heuningnes estuarine system. The causeway in the intertidal zone, 1.3 km upstream of the mouth, obstructs tidal exchange and has altered the character of the upper estuary, which now resembles a slack and turbid river. When the estuary was in its natural state, water would dam up behind the dune barrier, forming a 2.5-km-long lagoon between the dune ridges. During floods, the barrier would be breached, opening up the mouth. The estuary mouth probably moved considerably, shifting by up to 2 km along the beach. Whenever the mouth closed, extensive flooding occurred in the agricultural hinterland, destroying crops and causing the widespread loss of topsoil.

The sand dunes on either side of the estuary and the dune north of it were stabilised by being planted with alien marram grass Arenaria ammophilla, which prevents sand from migrating across the river and blocking the channel. The width of the mouth has been reduced and the mouth is confined artificially by brushwood barriers and dune afforestation. These barriers prevent the mouth from migrating on a large scale and the formation of a natural sand spit. The considerable stabilisation of the estuary and the alteration of its natural flow may have affected how it functions. The construction of road protection levees has reduced tidal activity, resulting in the colonisation of salt marsh by coastal dune plants. Above the salt marsh, the floodplain vegetation is heavily grazed.

Conservation action

The IBA consists of a mix of protected and unprotected land within an agricultural matrix. Approximately 21 000 ha fall within Agulhas National Park, another 45 000 ha are in the Nuwejaars Wetland Special Management Area, and the remainder of the formally protected land is in CapeNature’s De Mond Nature Reserve. The critical habitats – the Heuningnes Estuary and the Agulhas Plain wetland system – are therefore under some form of protection. De Mond State Forest was established in 1975 to preserve the Heuningnes Estuary and adjoining coastal fynbos vegetation. The estuary and dunes on either side are a Ramsar site, designated in 1986.

Like all gazetted protected areas, Agulhas National Park and De Mond Nature Reserve are guided by detailed management plans and annual plans of operation. However, conservation action is not limited to the formally protected areas, and the well-managed Nuwejaars Wetland Special Management Area, soon to be gazetted as a Protected Environment, contributes substantially to conservation in the region, as does the Agulhas Biodiversity Initiative, which helps to coordinate the activities of multiple organisations at a landscape scale. Environmental education and awareness campaigns have played a role in the work of groups such as the Overberg Crane Group and Agulhas Biodiversity Initiative to reduce the threats to biodiversity from agricultural production. Together with a more enlightened approach to sustainable farming, they have helped provide a platform for conservation and agriculture to co-exist.

The actions undertaken in formally and informally protected areas include the eradication of alien vegetation; the restoration and rehabilitation of degraded land; the re-introduction of species, including large mammals, that enhance the functioning of the ecosystem; and fire management, law enforcement and infrastructure maintenance where necessary. The Heuningnes Estuary Management Forum implements a management plan for the estuary. A number of skills development, tourism and cultural preservation projects are also under way to improve the socio-economic conditions of people living in this region.

Related webpages

http://www.sanparks.co.za/parks/agulhas/

http://www.nuwejaars.com/index.html

http://www.westerncapebirding.co.za/overberg/routes.php

http://www.sanparks.co.za/parks/agulhas/

http://www.nuwejaars.com/index.html

http://www.westerncapebirding.co.za/overberg/routes.php

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

Tuesday, 10 March 2015