The Tonle Sap : Characteristics and Threats By Gabriel Lamug‐Nañawa, SJ

Introduction

The Tonle Sap Lake is located at the heart of the Cambodian landscape. It is the largest freshwater lake and forest habitat in Southeast , and supports its largest water bird colony (Campbell et al., 2006). It is home to around 150 fish species and a host of invertebrates, reptiles, and birds (Bonheur and Lane, 2002). About 3 million people depend on this lake and its floodplain for their daily food and livelihood, including fishing and agriculture (Suong, 2006). Because of its rich and its socio‐economic value to so many people, the Tonle Sap Lake and its floodplain play a very important role in the lives of Cambodians. However, there is a growing number of serious threats, which could alter the characteristics and productivity of the lake. If significant changes occur, it would have a major impact on the lives of so many of the poor who depend directly on the lake.

A. Hydrological Aspects

The Tonle Sap Lake of today is the result of several thousand years of transformation. According to Tsukawaki (2002), the Tonle Sap Lake began to be formed in the early Holocene period as a chain of much smaller , presumably linked together by extensive wetlands, and that the connection to the was established about 5,500 – 5,000 years ago during the mid‐Holocene, creating a single large fluvial lake. However, Penny (2006) claims that Rhizophora pollen found in sediment cores taken from the lake bottom show that the Tonle Sap Lake has been linked to the South China Sea right from the early Holocene, and so was strongly influenced by saline tidal waters. Nevertheless, it is commonly agreed that by the late Holocene, the link between the Tonle Sap Lake and the Mekong River, providing fresh water from upstream, was well established (Rainboth, 1996).

The character of the Tonle Sap Lake is highly dependent on the Mekong River, to which it is connected by the Tonle Sap River. Originating from southeastern Tibet, together with the other major of Asia (i.e., Brahmaputra, Irrawaddy, Salween, and the Yangtze), the Mekong River runs an estimated 4,200 km to the in southern (Lamberts, 2001). The Lancang Jiang or “Turbulent River” as the Mekong River is called in China, begins at 5,200 m above sea level in the Tibetan Plateau, running south through the eastern end of Tibet, passing the western side of the Yunnan province of China, before leaving China at an elevation of around 500 m (Kummu and Sarkkula, 2008). It continues to descend into Myanmar, Thailand, Lao PDR and , before finally reaching the South China Sea in Vietnam (Figure 1). In Thailand and Lao PDR, the Mekong River is called Mae Nam Khong, literally meaning “mother water things”. The Mekong River has been regarded by generations as the mother who is the source of many things. It is the 12th longest river in the world and the 10th largest river by volume of outflow, discharging around 475 million m3 of fresh water annually (Kummu and Sarkkula, 2008).

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Figure 1. Mekong River Basin (Pech and Sunada, 2008)

Seasonal Flooding

One of the lake’s defining features is its annual flood regime. The Mekong River, which is 486 km long inside Cambodian borders, drains a total of 156,000 km2 or 86% of the country (Hak and Piseth, 1999). During the rainy season, as the water level of the Mekong River reaches about 9 m amsl, the water draining the Tonle Sap Lake into the Mekong River through the Tonle Sap River is overpowered and the direction of water flow in the Tonle Sap River is reversed. This begins the annual flooding of the Tonle Sap Lake and its surrounding floodplains, increasing its flooded surface area of 2,500 km2 during the peak of the dry season to around 12,000 km2 or up to 15,000 km2 during the peak of the wet season (Rainboth, 1996) (Figure 2). During the dry season, its largest dimensions can be around 160 km long, 35 km wide and 2 m deep, while during the wet season the flooded area can swell to about 300 km long, 100 km wide and 14 m deep (Rainboth, 1996, Hak and Piseth, 1999).

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Figure 2. Tonle Sap Lake and its surrounding floodplain (Kummu et al., 2008)

As the water level of the Mekong River subsides towards the end of the wet season in October, the water flow in the 100 km‐long Tonle Sap River reverses once again and begins to drain the Tonle Sap Lake. This event is commemorated by the Cambodian Water Festival or Bon Om Touk, an annual celebration which takes place during the full moon of the Buddhist month of Kadeuk, which usually occurs in November. During this national festival, hundreds of thousands of Cambodians gather in for the traditional boat races and other festivities, which are meant to thank the divinities of the river for a rich harvest of rice and fish and to pray for the same in the year ahead.

Based on records for the period 1997 – 2004, Kummu et al (2008) computed the annual water balance of the Tonle Sap Lake. They concluded that 57% of the water flowing into the lake during the wet season comes from the Mekong River, either by discharge through the Tonle Sap River (52%) or by overland flooding (5%). The remaining inflow (43%) comes from tributaries to the Tonle Sap Lake (30%) and direct precipitation (13%). Then during the dry season, the water returns to the Mekong River (88%) through the Tonle Sap River (87%) and overland flooding (1%), while the rest evaporates directly (12%).

Sediments and Productivity

An important aspect of the Tonle Sap Lake is the influx of sediments and their effects on the productivity of the lake and the surrounding floodplains. It is commonly understood that much sediment accompanies the flood waters entering the Tonle Sap Lake from the Mekong River during the wet season, and that this sediment and associated nutrients are the reason for the rich productivity of the lake and the floodplains that surround it (Kummu et al., 2008). Kummu et al (2008) claim that around 28% of the total suspended sediments come from the lake’s tributaries, while the remaining 72% come directly from the Mekong River (Kummu et al., 2008), with more than half the total sediment flux originating from China (Kummu and Varis, 2007).

However, Campbell (2009) argues that even if all the sediments and nutrients coming from external sources were to cease, internal loading in the Tonle Sap Lake would be sufficient to maintain the present trophic state for years and perhaps decades to come. The predominant energy source in the Tonle Sap Lake is algal carbon from phytoplankton and periphyton, while carbon coming from terrestrial sources such as flooded trees, grasses, and macrophytes contributes very little to fish

3 populations (Campbell, 2009). Furthermore, computer models by Sarkkula and Kopponen (2003) suggest that sediments coming from the Mekong River are trapped by the flood forest vegetation around the lake so that agricultural fields nearby do not receive natural fertilization from the Mekong River sediments.

In conjunction with the influx of sediments, the lake’s high productivity is also usually attributed to the annual flood cycle. The Tonle Sap Lake’s floodpulse, as its seasonal flood regime is called, is commonly regarded as one of the key contributing factors to the lake’s high productivity, particularly because of the terrestrial primary production being transferred to the aquatic phase (Lamberts and Koponen, 2008). However, Campbell (2009) claims that this is not the case. Instead, he says, productivity is high because the flooding produces a large and brightly‐lit shallow water body that is organically rich due to rapid algal growth.

The recession of water during the lake’s annual flood regime also contributes to high productivity. The wind and the shallow depths during the dry season keep the sediments in suspension, making them available for the next season’s floodpulse. If the high water level was maintained, the sediments would sink to the bottom and would be unavailable to support the production of algae (Campbell, 2009).

The seasonal fluctuations of primary productivity of the Tonle Sap Lake have not yet been sufficiently studied and documented. Studies measuring the productivity of the lake cite fish catch as a reliable indicator, but Campbell (2009) argues that there is no obligate relationship between these two.

A number of studies have suggested that the influx of sediments from the Mekong River would eventually fill up the Tonle Sap Lake. However, Carbonnel (1963), Sarkkula and Koponen (2003) and Kummu et al (2008), have claimed that the material brought by the Mekong River during the flood period is negligible, and does not significantly modify the sediments’ accumulation rate. In fact, the great majority of alluvial matter from the Mekong River is deposited in flood forests along the Tonle Sap River or in the area between Chnuk Tru and Kompong Chhnang.

B. Biodiversity

Tonle Sap Lake is a major habitat for wildlife (Campbell, 2009), including fish and birds. There have been 149 fish species from 35 families recorded in the lake (Campbell et al., 2006), but only 4 families account for 61% of the fish species: Cyprinidae (39%), Bagridae (8%), Siluridae (7%) and Pangasiidae (7%) (Campbell et al., 2006). There are more fish species found in the Mekong River (923 fish species from 91 families) than in the lake (MRC, 2003), presumably because of a greater diversity of habitats in the river. None of the fish found in the Tonle Sap Lake are endemic to that water body alone.

The breeding cycles of many fish involve migration between the Mekong River and the Tonle Sap Lake. For example, at the beginning of the wet season many white fish species spawn in the tributaries and mainstream of the Mekong River near the border of Cambodia and Lao. The flowing waters carry the eggs south and into the floodplains along the Mekong River. Some eggs reach the junction of the Mekong and Tonle Sap Rivers and are carried into the Tonle Sap Lake (Campbell et al., 2006). Then, as the dry season approaches and the waters begin to recede, the white fish species follow the water flow back into the Mekong River. They swim north up the Mekong River and find shelter in the deep pools and canyons on the mainstream between Kompong Cham in Cambodia and Siphandone in southern Lao (Poulsen, 2000).

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There are five species of turtles that occur in and around the Tonle Sap Lake (Long, 2003), 16 species of snakes from 7 families (Campbell et al., 2006), and Siamese Crocodiles.

The best studied faunal group on the Tonle Sap is the birds. There are 220 species of birds found in different parts of the lake, including such gregarious large waterbirds as storks, pelicans, ibises, and cormorants. Seventeen species are identified as ‘Globally Threatened’ or ‘Near Threatened’, while the Houbaropsis bengalensis and Leptoptilos dubius (Figure 3) share IUCN ‘Endangered’ status (Campbell et al., 2006).

Figure 3. Greater Adjunct (Bird Quest)

These birds frequent the swamp forest and grassland habitats around the Tonle Sap Lake, establishing large breeding colonies. The importance of these breeding sites was firmly established when, in 1997, the Tonle Sap Lake and its floodplains were declared a Biosphere Reserve under the UNESCO Man and Biosphere Reserve Programme (Campbell, 2009).

Surrounding the permanent lake are floodplains composed of 360,000 ha of flood forest and 157,000 ha of degraded forest and other types of vegetation. Based on remote sensing data, it is estimated that there used to be about 1 million ha of flood forest around the lake (MRC, 1997).

The vegetation on the Tonle Sap floodplain can be divided into two forest associations: (1) the short tree shrubland, which covers the majority of the floodplain, and (2) the stunted swamp forest, which is generally found around the periphery of the lake itself.

The dominant woody species of the short tree shrubland cover around 80% of the Tonle Sap floodplain (Campbell et al., 2006) and generally consist of deciduous trees reaching a maximum height of 4 m, with taller individuals nearer the permanent lake and shorter ones nearer the outer boundary of the inundated areas. This shrubland is dominated by plants from the Euphorbiaceae, Fabaceae, and Combretaceae families (Wikramanayake, 2001).

The stunted swamp forest, consisting of deciduous trees with heights around 7‐15 m used to dominate the immediate periphery of the Tonle Sap Lake and covered around 10% of the entire floodplain. Today, this forest association is broken into patches of tall trees and open areas with floating aquatic vegetation. The dominant species of this type of forest are Barringtonia acutangula and Diospyros cambodiana (Wikramanayake, 2001).

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The majority of the woody species of both forest associations are deciduous, shedding their submerged leaves during the period of flooding. These trees and shrubs bear fruits and seeds during the flood season, which are then dispersed by floodwaters and fish (Campbell et al., 2006).

C. Socio‐Cultural Aspects

The Tonle Sap Lake played an important part during the Angkor Empire, which flourished between the 9th and 15th centuries and was one of the most powerful empires in . At its peak during the reign of King Jayavarman VII (1181‐1219), the Angkor Empire encompassed parts of Lao PDR, Thailand, Vietnam, Myanmar, and Malaysia. Over 1 million people lived in the capital of the empire, in what is now the province of , and the Tonle Sap Lake was a vital resource which helped sustain the needs of the people.

Recognition of the lake’s role in the life of the people is seen on the stone walls of the 13th century Temple in , Siem Reap. Carved on the walls are images of fish, waterbirds, reptiles, and mammals. Together with scenes of fishing and everyday rural life, these stone carvings communicate and celebrate the important role that the Tonle Sap Lake played in the life of the people of the Angkor Empire (Bonheur and Lane, 2002).

A study by the in 1998 estimates that about 1.2 million people live in and around the Tonle Sap Lake and its floodplain, enclosed by the roads circumnavigating the lake. A fraction of this number lives on floating villages which are situated along major tributaries of the lake. Another 1.4 million people live in the area extending 15km outward from the roads (MRC/UNDP, 1998). The livelihood of the majority of these people would involve the lake and its floodplain.

Fishing in the Tonle Sap Lake takes between 263,000 and 391,000 tonnes annually, making it the 4th largest captive fishery in the world (van Zalinge, 2000). When other aquatic animals are included, the total catch rises to 720,000 tonnes for all the inland fisheries of Cambodia (Hortle, 2004). Fishing is an important way of life since it is believed that the fisheries provide 70% to 80% of the total animal protein intake of the Cambodian population (Bonheur and Lane, 2002).

However, the great majority of fish catch does not occur in the open lake but inside predetermined fishing lots. A central aspect of fisheries management in the Tonle Sap Lake deals with fishing lots. The lake and the surrounding floodplains have been divided into common areas which are open to the general public, and concession areas or fishing lots which are privately owned. These fishing lots are auctioned by the state for a two‐year period to private business entities. These fishing lots, most of which occupy the floodplains, are where most of the fish is caught. In the year 2000, the most productive areas of the Tonle Sap Lake were covered by 54 fishing lots (Bonheur and Lane, 2002).

Management of the Tonle Sap Lake’s resources has not been easy, partly because there are several government agencies with overlapping jurisdictions, namely the Ministry of Agriculture Forests and Fisheries (MAFF), the Ministry of Environment (MoE), and the Ministry of Water Resources and Meteorology (MOWRAM). There is also the Cambodian National Mekong Committee (CNMC) and other tourism and mineral agencies that stake a claim on the lake (Campbell, 2009).

D. Threats

Like many other lakes in populated areas, there are growing pressures on the Tonle Sap Lake ecosystem and its surrounding floodplains. These threats arise from within the lake itself and its surrounding floodplains, within the lake catchment, and from external sources inside or outside the

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Mekong Basin. These threats may be due to rapid human population growth, the over‐harvesting of aquatic resources, and the operation of built structures on the tributaries or mainstream of the Mekong River (Campbell et al., 2006).

Population Increase and Over‐Harvesting

Since the 1990s when access to the Tonle Sap Lake became more secure, the population around the lake has rapidly grown, both from people migrating from other parts of Cambodia and from overall population increase. The estimated population living in the Tonle Sap Basin is 4.5 million, growing at a rate of 4.8% (Leang, 2003). Thus, the harvest of fish, birds, wood and other resources which was previously sustainable given a lower population size, has now begun to threaten the health of the entire ecosystem (Bonheur and Lane, 2002).

The increasing market demand for fish and other aquatic animals, in Cambodia and in neighboring countries, has also driven an increase in fishing effort by subsistence and commercial fishers. Thus, van Zalinge (2000) reports that while the total catch has not decreased significantly, catch per unit effort has. The composition of the catch has also changed, with average fish size becoming smaller (Hortle, 2004), while larger fish species are no longer encountered (Bonheur and Lane, 2002).

Other animals such as water birds, and aquatic animals such as turtles and snakes, are all in decline (Campbell et al., 2006). All the turtles that can be caught are harvested, most of which are sold to China (Holloway, 2003). Watersnakes are also harvested and sold at a rate of at least 8,500 per day (measured during the peak of the wet season in 1999), primarily for crocodile and human food (Stuart, 2000). Stuart el al (2000) claim that this harvest rate represents the greatest exploitation of any single snake assemblage in the world.

In the floodplains, both the shrubland and swamp forests around the Tonle Sap Lake have been highly disturbed by the growing population. Wood is taken unabatedly for fuel and land is converted for agriculture and settlement (Bonheur and Lane, 2002). Today, only a small area of the flood forest remains pristine.

Infrastructure

There are two types of infrastructure along the Mekong River system that can alter the physical and chemical characteristics of water flowing into the Tonle Sap Lake. One is the irrigation projects which would divert water from the river to crop fields or communities in need of fresh water, while the other is the reservoirs or dams whose purpose is to generate electricity. There have been no reports of major constructions or plans to divert water from the Mekong River for irrigation, but many regarding dams. The effects of hydroelectric dams are twofold: changes in flow levels and in the transportation of sediments downstream.

In order to regulate the amount of electricity being produced throughout the year, hydroelectric dams are expected to store water during the wet season and release water during the dry. Thus, compared to its natural cycle, Mekong River water levels downstream of the dam are expected to be lower during the wet season and higher during the dry.

In view of the effect the Chinese dams (Figure 4) will have on the Tonle Sap Lake, Kummu and Sarkkula (2008) have pointed out that flow alterations which may be directly attributable to these dams will only be significant for areas that are near the dams. As the distance increases from the dams, its effect on water levels would decrease. Furthermore, while almost 60% of the Tonle Sap

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Lake inflow comes from the Mekong River (Kummu et al., 2008), the Mekong River Commission reports that only 16% of the flow of the Mekong River originate from China (MRC, 2009).

Figure 4. Mainstream dams of the Mekong River (International Rivers)

However, Kummu and Sarkkula (2008) also claim that relatively small rises in the dry season lake water level would permanently inundate disproportionately large areas of Tonle Sap Lake floodplain, rendering it inaccessible to floodplain vegetation. Also, higher dry season water levels would result in less suspended sediments available for the next season’s nutrient pulse, which is one of the significant drivers of the lake’s primary production (Campbell, 2009).

One modeling study at the Mekong River Commission Secretariat (Halcrow, 2004) shows that the operation of two large dams in China would affect the Tonle Sap Lake by decreasing wet season maximum water levels by about 0.4m and increasing dry season minimum water levels by about 0.3 m. This change in water levels would result in a reduction of the seasonally inundated area from 10,620 km2 to 9,164 km2 or a decrease of about 11% (Campbell et al., 2006).

The water level records of the Tonle Sap Lake show that there has already been a decrease in maximum and minimum annual water levels during the past century. The mean maximum water level for the period 1925 to 1935 decreased by 0.52 m when compared with the period 1996 to 2002 (Campbell et al., 2006). Campbell (2006) disagrees with Bonheur and Lane (2002) who claim that the decrease in water levels was caused by dams built within the Mekong Basin. Rather, he argues that the most likely explanation for this decrease is a change in rainfall quantities on the Tonle Sap Lake catchment, while also compounded by small decreases in the flow of the Tonle Sap River.

Kummu and Varis (2007) computed a theoretical trapping efficiency of 94% for the combined effect of the whole cascade of 8 dams in China, potentially making a very significant impact on the whole Mekong sediment budget. However, Campbell et al (2009) argue that since the Chinese dams are located more than 1000 km away from the junction of the Mekong and Tonle Sap Rivers, the trapping of the sediments in the dams would not cause any noticeable impact on the Tonle Sap Lake.

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Conclusion

The Tonle Sap Lake is a very important natural resource for the people of Cambodia. Many people depend on the lake for their livelihood and for their very lives. However, there are many threats which can affect the health and productivity of the Tonle Sap Lake. These identified threats and its impacts should be understood by the different sectors which rely on the lake so that concrete steps can be taken to safeguard this valuable resource with is at the heart of Kingdom of Cambodia.

References

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