samedi 3 mai 2014

Wetlands of Sri Lanka (Draft)

 Adapted from:

The wetlands of Sri Lanka can be very broadly considered under three groups: offshore and marine systems, coastal systems and inland systems.

1. Offshore and Marine Systems

These include shallow sea bays and trait, tnall offshore islands and islets. Some of these sites have received considerable attention because of their fisheries potential.

2. Coastal Systems

These include estuaries, brackish to saline lagoons and mangrove swamps, along with rocky seacoasts, sandy beaches, salterns, saltpans and aquaculture ponds. A considerable amount of information is available on the estuaries and lagoons of Sri Lanka (e.g. Abewickrema, 1960 & 1966; Anon, 1977; Arudpragasam, 1975 & 1984; Fernando, 1983; Marga Institute, 1985a; Norris, 1957; de Silva, 1984), and the mangrove ecosystems have been the subject of many studies (e.g. Aruchelvam, 1968; Flueeler, 1983; Jayewardene, 1985 & 1987; Macnae & Fosberg, 1981a & 1981b; Modenke & Modenke, 1983; Seneviratne, 1978 & 1979; Sivakumar, 1979). Much of this information was recently brought together by the Conference on Critical Habitats (organized by the Coast Conservation Department) and Conference on Coastal Ecoystems (organized by the National Aquatic Resources Agency).


There are some 45 estuaries around the coast of Sri Lanka, belonging to two types: basin estuaries where rivers discharge into relatively shallow basins which in turn open into the sea (e.g. Puttalam Lagoon, Negombo Lagoon and Jaffna Lagoon), and riverine estuaries where rivers discharge into the sea by way of relatively narrow channels (e.g. Kaluganga Estuary and Kelaniganga Estuary). The total extent of the basin estuaries is estimated at 40,000 ha, whereas that of the riverine estuaries is unknown. Sand barrier formation in recent years has transformed some basin estuaries into lagoons (e.g. Koggala Lagoon). In other cases, e.g. Batticaloa Lagoon and Kokkilai Lagoon, sand barriers erase the connection with the sea during a part of the year. Many of the estuaries are under threat from the disposal of industrial effluents and domestic sewage, oil pollution, sand mining, salt exclusion schemes and reclamation for housing developments.
Some hydrographic information is available for the major basin estuaries and a few riverine estuaries, and a little information is available on primary productivity and phytoplankton biomass. Several studies have been carried out on seasonal changes in zooplankton diversity and abundance, and on other invertebrates, particularly annelids, molluscs and crustaceans. A considerable amount of research has been carried out on the fisheries.


There are some 40 true lagoons around the coast of Sri Lanka. They are most common along the southern, southeastern and eastern coasts, where littoral drift causes accumulations of sand as barriers and spits at river mouths through which the freshwater discharge is low. The total area of the lagoons is estimated at about 20,000 ha. Many are seasonal features, formed during the wet season and subsequently drying out during the dry season. In some lagoons, the water becomes hypersaline when sources of fresh water dry up and the connection with the sea is erased by sand barriers (e.g. lewayas in Hambantota). In others, freshwater run-off has a dominant effect and the salinity is very low. In the long term, the lagoons will silt up and provide a barrier against coastal erosion. In the short term, their significance rests mainly upon fisheries, salt production, wildlife, tourism and reclamation of land for agriculture and human settlement.


Mangroves are discontinuously distributed along the coastline, and are absent along exposed shorelines, particularly in the southwest, south and northeast. The main mangrove areas are situated in Mullaitivu, Trincomalee, Kathiraveli, Vakarai, Panichankerni, Valaichenai, Batticaloa, Karativu, Komari, Potuvil, Hambantota, Pilinawa, Matara, Galle, Gintota, Muthurajawela, Negombo, Chilaw, Mundel, Puttalam, Kalpitiya and Mannar, and on the Jaffna Peninsula. The total area of mangroves was conservatively estimated at 3,000-4,000 ha in 1969. However, recent remote sensing studies have indicated that there are 6,296 ha of mangroves in the districts of Colombo, Amparai, Gampaha, Trincomalee, Batticaloa and Puttalam alone, and the total area of mangroves is likely to be close to 10,000 ha. The mangrove genera Rhizophora, Avicennia, Excoecaria, Lumnitzera and Aegiceras are distributed island-wide. Xylocarpus granata occurs on the west and the east coasts, Bruguiera cylindrica occurs only on the west coast, Ceriops tagal is absent from Jaffna, and Nypa fruticans occurs only in the southwest. Some studies have been carried out on the zonation and pattern of succession in the mangrove forest, and limited information is available on the fishes and invertebrate fauna associated with the mangroves. Many of the mangrove swamps are under threat from extraction of fuelwood and timber for construction (notably in Jaffna, Batticaloa, Trincomalee, Puttalam and Negombo), and reclamation for housing developments (particularly in densely populated areas such as Negombo). As yet, large-scale transformation of mangrove swamps to brackishwater ponds for aquaculture has n'ot occurred in Sri Lanka. Some 300 ha of shrimp ponds have been constructed in the mangroves (e.g. in Negombo and Chilaw), and a further 1,000 ha have been leased for expansion of aquaculture, but most of this land is situated in areas where mangroves have already been degraded.
Detailed information on the avifauna and other wildlife of the coastal wetlands is only available for a small number of sites. Wetlands such as Bundala and Kalametiya have received a considerable amount of attention, especially from bird-watchers. However, it was not until the early 1980s, when J.D.N. & J. Banks began to carry out monthly censuses of shorebirds at Bundala, that continuous data gathering was attempted at a wetland in Sri Lanka. Recently, the annual mid-winter waterfowl count, organized by the Ceylon Bird Club since 1983, and selected ecological studies by the Field Ornithology Group have greatly increased the amount of information available on the avifauna of other coastal sites. However, there remain many coastal sites, which are still poorly known. 3.Inland Systems The natural freshwater habitats consist of about nine large rivers and 94 small rivers (flowing more or less radially and totalling 4,563 km in length), numerous streams (particularly in the wet zone), extensive marshes, which are either connected to rivers or represent seasonally contracted flood plains isolated from the rivers, and many small permanent and seasonal ponds. Although there are no large natural lakes in Sri Lanka, there are many floodplain lakes of the varzea type, known as villus, which cover a total area of about 12,500 ha. Many of the largest villus are situated in the Mahaweli River system in the east. Here there are about 40 of these shallow, seasonal lakes varying in size from nine to 550 ha. Wilpattu National Park, in the west, also possesses a number of very small freshwater villus along with a unique group of salt villus far inland.

In addition to the natural wetlands, there are numerous man-made freshwater habitats. The most important of these are the tanks or "wewa", which vary in size from a few hectares to 6,500 ha at full spill level. Some of these date back 1,500 years and formed part of an intricate water supply system for rice cultivation. Other man-made wetlands include approximately 2,400 km of irrigation channels and some 833,000 ha of rice paddies, as well as numerous very small rain-filled tanks and flooded areas caused by overspill and seepage from the irrigation channels. Rivers and streams The nine major rivers, about 25 smaller rivers and their numerous tributary rivers and streams drain a total of 103 basins (Fernando & Indrasena, 1969). Thirty-six of the streams in the highlands were investigated in 1970 by the Austrian-Ceylonese hydrobiological mission. The findings of this investigation have given a good general picture of the status of mountain streams in the country (Costa, 1972; Costa & Starmuhlner, 1972; van den Elzen, 1972; Liyanage & Starmuhiner, 1972; Radda, 1973; Starmuhlner, 1974; Weninger, 1972). Because of their short courses, most of the rivers and streams of the uplands and highlands are fast-flowing, with many waterfalls and rapids. The aquatic plant communities are generally very simple. Various species of Podostomaceae occur on rocks, which are mostly covered by rapidly flowing water, while species such as Dicraea elongata and Podostemum subulatus are found in the quieter parts of the rapids. Other common species include Miniathus ceylanicus, Dicraea stylosa, Zeylanidium olivaeaum and Farmeria metzgerioides. In the lowlands, where the flow of the rivers and streams is sluggish, rooted aquatics with floating or submerged leaves occur along the river margins. Dominant species include Nymphaea spp, Blyxa aubertii and Aponogeton crispum. On the banks of the rivers, a belt of Hanguana malayana, Phragmites karka and Brianthus arudinaceus often occurs between the flowing water and adjacent marshes and shallow pools. Elsewhere, the river banks typically support gallery forest with species such as Terminalia arfuna, Mitragyna parvifolia, Madhuca longifolia, Polyalthia longifolia and Diospyros malabarica. Thirty-one species of fishes have been recorded in the rivers and streams (Radda, 1973). The invertebrates include three species of crabs, 1 1 species of prawns and 3 1 species of gastropods (Costa, 1972; Costa & Fernando, 1967; Starmuhlner, 1974). Man-made lakes (tanks and reservoirs) The most common freshwater habitats are the irrigation tanks, of which there are over 10,000 in Sri Lanka. About 3,500 may be regarded as significant water bodies, although only about 60 exceed 300 ha in size. The total area of these man-made lakes exceeds 170,000 ha. The tanks and reservoirs can be classified as follows: (a) shallow and heavily silted, with a relatively uniform depth (e.g. Giant's Tank); (b) shallow with a gently sloping bottom (e.g. Tabbowa Tank); (c) deep, encompassing one valley (e.g. Nalanda Reservoir); and (d) deep, encompassing many valleys (e.g. Senanayake Samudra). There are wide variations in the pH (6.8-7.5), calcium content, nitrate and phosphate levels. Slightly acidic waters are common in the southwestern and hilly regions, while more alkaline waters occur in regions with high calcium levels in the Miocene limestone areas. Ecologically, tanks have enriched Sri Lanka greatly, many of them harbouring a very diverse flora and fauna. The duration of water retention during the year is an important factor influencing the floral composition in a tank ecosystem. Typically, the deeper tanks or reservoirs may have little or no macrophytic vegetation because of the large seasonal fluctuations in water level. Even deep-water tolerant species such as Nymphaea do not do well in the high water conditions. However, some forms of floating or floating-leaved plants may occur in the shallower parts of sheltered coves. At low water levels, grasses and annuals invade the drawdown zones of the tanks, and are usually subjected to heavy grazing. For this reason, the drawdown zones of large tanks tend to have a barren appearance with very sparse vegetation. The deeper water near the centre of the tanks usually has only planktonic species.
 Sometimes, however, this zone is invaded by varcono algae and vascular plants such as Azolla pinnata, Wolf ia arrtiza, Pistia stratiotes, Hydrilla verticillata, Lemna spp and Najas spp. In addition, large areas of water may be covered by naturalized exotics such as Salvinia molesta and Eichhornia crassipes. Some of these species are also found in shallow water along with sedges and asoids. The common species in the shallow, marginal zones are Limnophytum obtasifolium, Hygrorhiza aristata, Typha augustifolia, Xyris spp, Cyperus spp, Ipomoea aquatica, Ericaulon spp, Jussiaea repens, Panicum spp, Asteracantha longifolia and Polygonum spp. Common shrubs on adjacent high ground include Hibiscus tiliaceus, Pandanus zeylanicus, Cerbera manghas and Syzigium spp. The shallow village tanks are swampier in character, with very rich aquatic plant communities. Water levels are very low during the dry season, and many of the tanks dry out completely at this time. The small tanks support a very rich and diverse phytoplankton in which species of Oscillatria, Microcystis, Hyella and Coelosphaerium are common. The zooplanktonic and zoobenthic faunas of the tanks have been well documented by Fernando (l965b, 1969 & 1974) and Fernando & Ellepola (1969). Fifty-nine species of freshwater fishes occur in Sri Lanka, including five, which have been introduced (Fernando & Indrasena, 1969). Most of these occur in the tanks and reservoirs. Approximately half of the species belong to the family Cyprinidae, which includes several very abundant species such as Puntius vittatus, Rasbora daniconius and Danio aequipinnatus. Among the catfishes, Wallago attu, Ompok bimaculatus, Heteropneustes fossilis and Macrones vittatus are abundant. One of Sri Lanka's indigenous fishes, Etroplus suratensis, which occurs in most lakes and estuarine waters, has been the subject of many behavioural studies (e.g. Samarakoon, 1981 & 1983).
Waterfowl are abundant at many of the tanks, reflecting the high densities of the fish populations. Species of cormorants (Phalacrocorax spp) and herons and egrets (Ardeidae) are particularly common. Widespread amphibians and reptiles include species of Rana, Crocodylus palustris, Varanus salvator and Cerberus rhynchops fortoises. The otter Lutra ceylonensis occurs in most lakes.