The site lies about 70 km to the east of the northern city of Annaba, c.5 km west of the Tunisian border and 10 km east of Lac Oubeïra (site DZ001). It forms part of the complex of wetlands included within the Parc National d’El Kala. The site consists of a marshy basin and a shallow (maximum depth 6 m), seasonal freshwater to brackish, eutrophic lake, bounded on the north by an extensive sand-dune system, through which the lake connects to the Mediterranean Sea via an artificial channel, the Oued Messida. The basin is surrounded by wooded hills, maquis and grazing land, with woodland including Taxodium distichum, Alnus glutinosa, Salix pedicilata, Populus alba and Fraxinus oxyphyla. There is a species-rich alder carr along the northern shore of the lake, which is regarded as one of the most important in North Africa. Most of the lake is covered in dense emergent vegetation, with a band of open water and dense submerged vegetation around the edge. Part of the marsh area dries out for a period of up to three months between August and November. There are isolated clumps of Tamaris sp. and emergent vegetation includes extensive beds of Scirpus lacustris, S. maritimus, Phragmites australis, Sparganium erectum, Iris pseudoacoras and Typha angustifolia. Submerged or floating aquatic plants include beds of Ceratophyllum sp., Ranunculus aquaticus and an invasive exotic, Eichhornia crassipes. Open water and drainage channels also contain Ceratophyllum, Myriophyllum, Sparganium, Potamogeton, Nymphaea (including an Algerian rarity, N. alba, discovered in 1984) and Trapa natans (also nationally rare).Attempts to drain the marsh, starting in the late nineteenth century, appear to have been largely unsuccessful, due in part to the fact that the bottom of the marsh is slightly below sea-level. The lake is one of the most important in the region due to its productivity. Cattle are grazed all around the edges of the lake and marsh, helping to maintain open water at the edges. Other human activities include eel (Anguilla anguilla) fishing and wildfowling, the latter very intensive at times.
See Box for key species. This is considered to be the most important site for breeding waterfowl in eastern Algeria and one of the most important in the Mediterranean. There is a breeding population of hundreds of pairs of Aythya nyroca (estimated at over 600 pairs in 1992). This species also winters on the site, apparently in increasing numbers (less than 20 during/before the early 1990s, 255 birds in 1994, 717 in 1997). Oxyura leucocephala also breeds (the maximum number of nests counted on the site was 28 in 1991, but more than 50 pairs were estimated breeding in the whole El Kala complex, mostly on Lac Tonga, in 1992). Only small numbers of this species were recorded wintering until 1999 when a total of 256 birds were counted. There may be a small breeding population of Marmaronetta angustirostris, but the species was not recorded during specific surveys in 1990. The site is ideal for breeding waterfowl because of the mosaic of dense vegetation and open water and the high productivity of the submerged aquatic macrophytes. Other species recorded breeding include Tachybaptus ruficollis, Podiceps cristatus, Ardeola ralloides, Nycticorax nycticorax, Ardea purpurea, A. cinerea, Botaurus stellaris, Aythya ferina, Porphyrio porphyrio (in hundreds), Circus aeruginosus and Chlidonias hybridus. A recently confirmed, new breeding record for the site is Plegadis falcinellus and other probable breeders include Egretta garzetta, Bubulcus ibis, Ixobrychus minutus, Anas platyrhynchos and A. querquedula.The site is also very important for wintering waterfowl; more than 20,000 waterbirds have been recorded (a total of nearly 34,000 in 1995). In addition to those in the Box, other species wintering in large numbers include Anas penelope (11,481 in 1995), and Anser anser (recorded in thousands in some years), Anas acuta and A. crecca (both recorded as ‘common’), Aythya ferina (7,264 in 1994) and Fulica atra (14,834 in 1997). Wintering ducks are known to move between Lac Oubeïra (site DZ001) which is a more suitable diurnal roost (less vegetation and less hunting disturbance) and this site, where they feed by night.Raptors reported from the site include: Milvus migrans, Circaetus gallicus, Accipiter nisus, Buteo rufinus, Pernis apivorus, Hieraaetus pennatus, Aquila pomarina, Neophron percnopterus, Circus aeruginosus, C. pygargus, Falco peregrinus, F. tinnunculus and F. subbuteo.
Non-bird biodiversity: The mammal Lutra lutra (VU) is present in the lake.
Pressure/threats to key biodiversity
The whole site was designated as a Ramsar Site in 1983 and it lies within the Parc National d’El Kala, designated in the same year, and within the El Kala Biosphere Reserve. The lake has its own microclimate that allows some tropical species of vegetation to persist, although many of these may have disappeared as a result of drainage works over the centuries. Early drainage, including the diversion behind an embankment of a river that previously fed the lake, led to the replacement of much of the open water by dense emergent vegetation. In 1937, attempts to drain the marsh ceased, but in the 1980s the outflow sluice was closed in winter to store water for irrigation and to improve grazing around the lake edge. The elevated water-levels resulting from this may have killed up to 90% of the alder forest and some of the Scirpus lacustris beds in the northern half of the lake. Another potential threat to hydrology and water-levels could come from plantations of exotic poplars and cypresses, which have proved popular in other areas, but which would lower the water-table if planted on the shores of the marsh. Despite its Ramsar status there is reported to be high hunting pressure, especially at weekends.The 1990 Algerian National Report to the Ramsar Convention listed poaching, eel-fishing and abstraction of water for irrigation and domestic supply as potential threats to the ecological character of the site. In summer 1990 the lake dried out completely due to drought and water abstraction. The report of the Monitoring Procedure Mission (to site DZ001) in 1990 also made recommendations for Lac Tonga. These included that consideration should be given to restoring the natural hydrological functions of the lake and that the lake itself should be established as a zone of strict protection within the Parc National d’El Kala, with no hunting and no eel-fishing permitted. It was reported that many birds and otters were being killed in eel-fishing nets. The report further recommended that surface- and groundwater extraction from the lake should be strictly controlled and that conservation management of the site should be carried out as part of a Regional Plan for the wise use and conservation of land and water resources. The site was added to the Montreux Record in 1993 because of concern about decreases in water-supply to the lake and the spread of emergent aquatic vegetation covering open water areas.