The Orange River mouth is located on the arid Atlantic coast at South Africa's border with Namibia. The nearest town is Alexander Bay. It is a delta-type river mouth, consisting of a series of braided troughs interspersed with sand banks, channel bars and islets, with a tidal basin and salt marshes. Extensive mudflats occur at the mouth, and large areas of intra-fluvial marsh occur upstream of the mudflats. This system is so dominated by fresh water that it has few estuarine characteristics. The major part of the vegetation of the lower section of the river is typical of a low-salinity wetland. Common plant species include the sedges Scirpus and Sporobolus species, saltworts Sarcocornia species and common reed Phragmites australis.
This IBA falls within the Desert Biome, with three vegetation types: Arid Estuarine Salt Marshes (NFEPA Endangered wetland), Alexander Bay Coastal Duneveld (Critically Endangered) and Western Gariep Plains Desert. It is a highly disturbed, modified ecological system as a result of years of degradation due to diamond mining activities, flow regulation of the river, and poor management of the mouth.
Approximately 60% of the landscape is in a near-natural state, and 40% has been degraded or transformed by the cultivation of lucerne, mining activities, wind erosion, roads and sewage ponds.
This IBA is considered to be a critical coastal wetland in southern Africa because of the overall numbers of wetland birds it supports and because of its role as a migration stopover. A total of 253 bird species has been listed (Anderson et al. 2003), of which 102 are waterbirds. When the IBA was assessed, its pentads had been poorly atlased for SABAP2.
Globally significant numbers of Cape Cormorant Phalacrocorax capensis, Kelp Gull Larus dominicanus and Hartlaub's Gull Chroicocephalus hartlaubii, and nationally significant numbers of South African Shelduck Tadorna cana and Cape Shoveler Anas smithii are present. The IBA regularly supports Great White Pelican Pelecanus onocrotalus, Lesser Flamingo Phoeniconaias minor, Greater Flamingo Phoenicopterus roseus and White-breasted Cormorant Phalacrocorax lucidus. Substantial numbers of Damara Tern Sterna balaenarum, Common Tern S. hirundo, Caspian Tern S. caspia, Pied Avocet Recurvirostra avosetta, Chestnut-banded Plover Charadrius pallidus, Black-necked Grebe Podiceps nigricollis, Little Bittern Ixobrychus minutus, African Spoonbill Platalea alba and Maccoa Duck Oxyura maccoa occur.
In summer, large numbers of migrant Palearctic waders, including Curlew Sandpiper Calidris ferruginea and Little Stint C. minuta, stop over. Near Pachtvlei, the terrestrial vegetation surrounding the wetland supports Cape Spurfowl Pternistis capensis, Tractrac Chat Cercomela tractrac, Barlow's Lark Calendulauda barlowi and Cape Long-billed Lark Certhilauda curvirostris.
In 1985 waterbird numbers totalled 26 650 individuals, representing 57 species. By 2003, they had declined by c. 74% (Anderson et al. 2003). When the 2012 CWAC reported only 2 316 waterbirds, the decrease was attributed mainly to the absence of Cape Cormorants and Common Terns. There is no evidence that the numbers of Damara Tern, Lesser Flamingo, Chestnut-banded Plover and Caspian Tern have declined. Despite the drop in the number of birds over time, the species richness of waterbirds has remained relatively constant. A total of 87 waterbird species were recorded during CWACs.
Globally threatened species are Cape Cormorant (1 000; this and the following figures are from CWACs), Damara Tern, African Black Oystercatcher Haematopus moquini, Lesser Flamingo (2 400), Chestnut-banded Plover (100) and Ludwig's Bustard Neotis ludgwigii. Regionally threatened birds are African Marsh Harrier Circus ranivorus (two), Caspian Tern, Great White Pelican (100), Barlow's Lark and Greater Flamingo. Restricted-range and biome-restricted birds include Cape Long-billed Lark, Cape Spurfowl, Namaqua Warbler Phragmacia substriata, Pale-winged Starling Onychognathus nabouroup and Tractrac Chat. Congregatory species include Black-necked Grebe, South African Shelduck, Cape Shoveler, Pied Avocet, Kelp Gull (1 000), Hartlaub's Gull and Common Tern.
Several endemic and threatened fish species, including Namaqua barb Barbus hospes and rock catfish Austroglanis sclateri, are present.
This IBA is being placed under increasing stress due to the increased demand for water from the Orange River for human consumption and industrial and agricultural purposes. The river has become highly regulated by the 23 dams and numerous weirs within its catchment. The Gariep and Vanderkloof dams are also regulated for hydropower generation. Abstraction and regulation have reduced the Orange's annual flow by half. Should further dams be built, worst-case scenarios project periodic drying up of the river that would have extremely negative consequences for the functioning of the mouth wetlands. Agricultural and industrial pollution are increasing the chemical and nutrient load flowing into the IBA. Algal blooms sometimes occur during low flow conditions.
Occasionally, floods severely alter the shape of the river mouth and result in significant vegetation loss, and the system may take several years to recover. Siltation is also a significant threat, as increased erosion inland results in an increased silt load and clogging at the river mouth. A severe threat is further degradation of the remaining salt marshes.
The extensive invasion of alien mesquite Prosopis species along the riverbanks, on the islands and in the upper reaches of the estuary after the 2011 floods is reducing habitat area and quality. Six other invasive alien species are present, and common reed is abundant.
Other threats include the gill-netting of fish and angling pressure in the vicinity of the estuary mouth; the hunting and poaching of ducks and geese; illegal dog-hunting and predation by feral dogs on the floodplain and islands; grazing by domestic and feral cattle; and recreational boating and off-road driving in the vicinity of sensitive breeding and roosting sites.
Climate change is another threat to this IBA. Temperatures are likely to increase and rainfall decrease sharply in arid areas. Fog is an important source of moisture in this desert environment, both for the ecology and for water availability. In some fog-supported ecosystems, regional rises in sea surface temperature as well as changes in local land cover can have an influence on the occurrence and amount of fog precipitation. Such changes potentially have implications for the biodiversity of this region.
This IBA was declared a Ramsar site in 1991, as was the Namibian side of the mouth in 1995. Together they form the Orange River Mouth Transboundary Ramsar Site.
Due to the severe degradation of the salt marsh by mining activities, the Ramsar site was placed on the Montreux Record in 1995. Since 2001 Alexkor SOC Ltd has promised to rehabilitate the area and restore the salt marsh, although it is unlikely that it will return to its former state. A Working for Wetlands project in 2005 completed minor rehabilitation work. A 2013 Alexkor application for restoration activities forms part of the management objectives of the strategic management plan for the site.
This IBA is on track to be declared a provincial nature reserve by June 2015. Negotiations are under way with NAMDEB to acquire the Namibian property for a transnational conservation area. DENC, in collaboration with SAEON Arid Lands Node, re-instated long-term monitoring of the estuary ecology in 2012. The Endangered Wildlife Trust's Source to Sea Programme started a project in 2014 and will assist with avifaunal monitoring and invasive alien plant research. Bird monitoring and an avifaunal management plan are urgently needed.
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Anderson MD, Kolberg H, Anderson PC, Dini J, Abrahams A. 2003. Waterbird populations at the Orange River mouth from 1980–2001: a re-assessment of its Ramsar status. Ostrich 74(3&4): 159–172.
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