Tonle Sap (; , ; or commonly translated as 'Great Lake') is a lake in central Cambodia. Belonging to the Mekong river system, Tonle Sap is the largest freshwater lake in Southeast Asia and one of the most diverse and productive ecosystems in the world. It was designated as a Biosphere Reserve by UNESCO in 1997 due to its high biodiversity. The lake has a large seasonal variation in water depth, with the post-monsoon level sometimes being as much as 10 metres (33 ft) higher than during the monsoons. During the 21st century, the lake and its surrounding ecosystems have come under increasing pressure from deforestation, intensive agriculture and fishery, infrastructure development and climate change.
Geology
Tonle Sap Lake is located in the northwest of the lower Mekong plain, formed by the collision and collapse of the Indian Plate and the Eurasian Plate. The lower Mekong basin rests on an ancient block of continental crust that has remained relatively stable since the Jurassic Period, a period of over 150 million years. Large areas were covered by an inland sea during the upper Mesozoic Era, during which thick red-bed sandstones and evaporites were deposited. The lower Mekong plain used to be a bay, and the sea level rose rapidly at the end of the last glacial period. About high, cores from this period found near Angkor contain tidal deposits, as well as salt marshes and mangrove swamp deposits, deposited in caves about 7,900–7,300 years ago. The sediments of Tonle Sap also show signs of marine influence. The current river morphology of the Mekong Delta was developed over the past 6,000 years, while the remaining waters in the northwest corner of the lower Mekong plain formed the Tonle Sap.
The Cambodian floodplain is mostly less than 100 meters in elevation, but a few higher outcroppings are scattered throughout the plain and much of northern Cambodia is characterized by rolling and dissected plains between 100 and 200 meters elevation. The plain is the result of erosion and sedimentation. The sediments vary in depth from at least 500 meters near the mouth of the Mekong to only about 30 meters at Phnom Penh, with bedrock outcroppings in isolated hills above the plain in several places.
Hartung & Koeberl (1994) suggest Tonle Sap could be an impact crater left from an impact that had a relatively low angle, and the possible source of the Australasian strewnfield,Hartung J. and Koberl C., 1994. In search of the Australasian tektite source crater: The Tonle Sap hypothesis. Meteoritics, 29, pp. 411-416. though this hypothesis has been widely discarded.
Hydrology
The Tonle Sap drainage basin is bordered to the southwest by the Cardamom Mountains, with heights of more than , and to the north are the Piandan Mountains with an average height of . The basin covers an area of . The -long Tonle Sap River connects the Tonle Sap Lake with the Mekong River and contributes 9% of the flow of the Mekong.
The size and water volume of the lake varies greatly throughout the year, with a minimum area of about and a volume of about in the dry season, and the water body expands in the rainy season, increasing the depth to . The maximum area is , and the volume is about .
About 34% of the water in the Tonle Sap comes from the rivers that enter the lake, about 53.5% from the Mekong, and 12.5% from precipitation. May to October is the rainy season in the lower Mekong plain, and November to March is the dry season. The annual rainfall is . Almost all the precipitation is in the rainy season. At the end of the dry season, the Tonle Sap Lake has a typical depth of . As the monsoon rain begins, the water level of the river begins to rise, eventually reversing the flow of the river. The water level of the Tonleé Sap increases by about , the flow of the Mekong gradually decreases at the end of the rainy season, and the flow of the Tonle Sap then reverses and begins to replenish the flow of the Mekong. The reversal of the Tonle Sap River's flow also acts as a safety valve to prevent flooding further downstream. During the dry season (December to April) Tonle Sap provides around 50% of the flow to the Mekong Delta in Vietnam.
The hydrodynamic complexity of the Tonle Sap Lake, both in time and space, makes it impossible to measure specific flow, and water level rather than velocity and volume determines the movement of water as it shapes the landscape. 72% of the modern sediments deposited in the Tonle Sap come from the Mekong, while only 28% come from the catchments upstream of the lake. Sediment-bound phosphorus acts as the basis of the food chain through phytoplankton, and internal nutrient cycling plays a crucial role in the productivity of the floodplain and therefore the long-term sustainability of the lake's entire ecosystem.
Volume fluctuation and flow reversal
Inflow starts in May or June with maximum rates of flow of around by late-August and ends in October or November, amplified by precipitation of the annual monsoon. In November the lake reaches its maximum size. The annual monsoon coincides to cease around this time of the year. As the Mekong River is at its minimum flow around this time of the year and its water level falls deeper than the Tonle Sap Lake, the Tonle Sap River and surrounding wetlands, waters of the lake's basin drains via the Tonle Sap River into the Mekong. As a result, the Tonle Sap River flows six months a year from southeast (Mekong) to northwest (lake) and six months a year in the opposite direction. The mean annual reverse flow volume in the Tonlé Sap is , or about half of the maximum lake volume. A further 10 percent is estimated to enter the system by overland flow from the Mekong. The Mekong branches off into several arms near Phnom Penh and reaches Vietnamese territory south of Koh Thom and Loek Daek Districts of Kandal Province.
As a natural flood reservoir for the entire Mekong river system, Tonle Sap Lake regulates floods in the lower reaches of Phnom Penh during the rainy season, and is also an important supplement to the dry season flow of the Mekong Delta.
Sedimentation
Although the large amount of sediment in the Tonle Sap basin is a natural phenomenon, rapid rates of development and resource exploitation has focused the attention of observers who fear the basin is in danger of filling with sediment. These fears were first reported by local people who noticed some areas becoming shallower. With increased sedimentation, already vague transit routes between capital and regional centers would likely be shut down altogether and could restrict the migration of fish into the lake.
Because sediment contains nutrients that fuel food webs, the Tonle Sap benefits from the influx. Sediment-bound phosphorus serves as food for phytoplankton through higher plants, and research has shown that the metabolizing of the chemical contributes to food abundance and quality. Internal nutrient cycling, therefore, plays an essential role in productivity of a floodplain. The nutrients bound to suspended sediments are important for the Tonle Sap system, particularly its long-term sustainability.
Ecology Wetlands, flooded forests, and deciduous forests
The land cover of the Tonle Sap basin is 55% forest and 45% agricultural land. The lake is surrounded by freshwater mangroves known as "flood forests", accounting for 3% of the basin area, and the floodplain is surrounded by low hills and covered with evergreen or deciduous seasonal tropical forest dominated by species of Dipterocarpaceae, Leguminosae, Lythraceae, Fagaceae and in some places Pinaceae, Podocarpaceae, or bamboo. Farther from the lake, the forest gradually turns into a thicket, and finally into a meadow. In areas with higher quality soils or higher altitudes, deciduous mixed forests and semi-evergreen forests occur. This variety of vegetation types underlies the species diversity of the Tonle Sap ecosystem, with interlocking forests, grasslands and marshlands providing refuge for local wildlife.
Species diversity
The lake is home to at least 149 species of fish, 11 of which are globally endangered, and the lake area is also home to 6 near-threatened species. These include spot-billed pelican, greater adjutant, Bengal florican, Oriental darter, grey-headed fish eagle, and the Manchurian reed warbler. Specifically, the large colonies of unique birds constitute the Preak Toal Bird sanctuary. In addition, the Tonle Sap also supports significant reptile populations including nearly extinct Siamese crocodiles and a large number of freshwater snakes. Although the area around the lake has been modified for settlement and farming, about 200 species of plants have been catalogued. Although much of the Lake District has been turned into farmland, 200 species of higher plants are still recorded, including Far Eastern reed.
The Mekong giant catfish, which lives in Tonle Sap, is one of the largest freshwater fish in the world. The fish can grow to long and can weigh anywhere between . The largest of these catfish ever caught weighed . Its population has been declining since the mid-1970s. It is currently illegal for fishermen to catch and retain Mekong giant catfish, and only a few are used for scientific research.
Tonlé Sap Biosphere Reserve
In 1997, UNESCO named Tonle Sap an ecological hotspot. In 2001, by royal decree, the lake and its surrounding provinces became the Tonle Sap Biosphere Reserve. Nine provinces are part of the reserve: Banteay Meanchey, Battambang, Kampong Chhnang, Kampong Thom, Preah Vihear, Pursat, Siem Reap, Oddar Meanchey, and Pailin. Additionally, three zones were established: a core zone, a buffer zone, and a transition zone. The core area of a Biosphere Reserve is defined as an area devoted to biological resources, landscapes, and ecosystems, allowing practices that for conserving biodiversity, monitoring minimally disturbed ecosystems and undertaking non-destructive research and related activities. As of today, the three zones are Prek Toal, Boeng Chhmar, and Stung Sen.
Despite this government protection, illegal fishing, poaching, and cutting of the forest for farmland are all still major problems. Because people living around the lake are extremely poor and depend on the lake for their survival, it is likely that this unsustainable living will continue. During recent years, the amount of fish caught has been steadily declining, which means peasants must also work harder to provide for their families. The government is working on supporting and educating these people to break this cycle of poverty and non-sustainability. Finding a balance between survival and conservation seems to be the major question for the future.
Scientists have also been concerned that high dams built in southern China and Laos will affect the strength and flow of countercurrents into the Tonle Sap, reducing the number of fish in the lake. Tonle Sap habitat for nesting, breeding, spawning, and foraging in the floodplain, which will adversely affect fish productivity and overall biodiversity in the Tonle Sap.
Forest loss hotspots are located in low floodplain areas where protected areas are located. Significant farmland expansion is mainly in the intersection between the lower and upper floodplains. Population growth, fuelwood gathering and logging are the main causes of forest loss. Intensification of agricultural activities and upstream hydropower development reduces buffers to natural habitats and increases the risk of forest loss. By the 2030s, hydropower development may lead to large-scale changes in habitat, with the area of coastal forests likely to decrease by 82%, while the area of rain-fed habitats may increase by 10–13%. In July 2020, under the influence of the El Niño phenomenon and the impoundment of dams in the tributaries of the Mekong River, the water level of lake hit a record low for the same period in the past 60 years.