Lake Natron

Description
Location and General Description

The East African Halophytics encompass two saline (soda) lakes, Lake Natron and Lake Bahi, both situated in the eastern arm of the Rift Valley in Tanzania. Lake Natron is the larger of the two lakes, measuring a maximum of 57 km long and 22 km wide. It is situated in Arusha District of northern Tanzania (on the Kenyan border) at 36 °E longitude and 2° 30’ S latitude. Lake Bahi is about half the size of Lake Natron and is located in Singida District of central Tanzania, near the major town of Dodoma, at about 35 °E longitude and 6 °S latitude.

Both of these saline lakes are situated in Tanzania’s low elevation semi-arid region. This climatic zone receives erratic rainfall below 800 mm per year, with substantial annual variation. According to Prins (1987) there is a significant relationship between elevation and rainfall in this area, with every 100 m rise in elevation corresponding to an increase of 35 mm in annual rainfall. Both of the lakes are situated at low elevation. Lake Natron is situated at 610 m on the Rift Valley escarpment, to the east of the Saleh Depression. Most of the rain in this area falls between December and May, followed by a long dry season (Zimmerman et al. 1996). Daily temperatures at the lakes are frequently above 40 °C. These hot, dry conditions expose the lakes to high evaporation rates, causing water loss that exceeds input from rainfall.

ong term climatic desiccation in the area has caused these formerly large freshwater lakes to shrink to their present remnants, which are highly saline. The continuation of this process threatens their long-term future, and there are already a number of other generally dry saline lake beds in the area which used to contain water. The lakes of this ecoregion are significantly smaller than they were during the pluvial period some 5000 to 6000 years ago, and unless the water supply exceeds evaporation they will continue to shrink until they become pans and eventually saline grasslands.

he Great Rift Valley was formed at a time when volcanic activity was more prevalent than today, spreading wide sheets of lava and volcanic ash over the ancient land surface (Greenway and Vesey-Fitzgerald 1969). In more recent times, rapid weathering of the many volcanic hills and mountains along the Rift Valley has continued to deposit volcanic ash in depressions, resulting in deep sodium-rich soils. These highly sodic soils form the poorly drained beds of the lakes.

Lake Natron is principally fed by the Ewaso Ngiro River, which has its catchment in the central Kenyan highlands. The lake is also fed by hot, mineral laden springs that bubble up on the periphery of the lake, bringing soluble salts to the surface. Lake Bahi is fed by the Bubu River, which originates in the Mbulu Highlands of central Tanzania. Water loss from both lakes occurs solely through evaporation, as neither lake has an outlet (surface or subterranean). The high evaporation rates and the constant input of salts from the saline substrate result in extremely saline water. Lake Natron is the ultimate alkaline lake. The water in this lake is so rich in dissolved sodium carbonate that it is frequently viscous to touch (Finlayson and Moser 1991).

The lakes are devoid of macrophytic vegetation, but are very productive in terms of algae, with blue-green algae (Cyanophyta) such as Spirulina spp. dominating the saline waters (Finlayson and Moser 1991). The large saline mud flats surrounding the waters are generally considered to be saline deserts and are devoid of macrophytic vegetation. These salt-encrusted flats are covered in a layer of blue-green algae during the rainy season. A few halophytic plant species are able to grow on the saline soils fringing the lakes. The dominant halophytes surrounding the lakes are Cyperus laevigatus, Dactyloctenium spp., Juncus maritimus, Salvadora persica, Sporobolus spicatus, Sporobolus robustus, Suaeda monoica and Triplocephalum holstii. The grass Sporobolus spicatus can tolerate much higher salinities than S. robustus (Finlayson and Moser 1991). The slightly less alkaline plains surrounding the lakes are dominated by grasses and by Sesbania sesban, with scattered Acacia xanthophloea trees (White 1983). The saline vegetation surrounding Lake Natron grades into the Serengeti Volcanic Grassland to the south and elsewhere into Acacia-Commiphora Bushland and Thicket. Lake Bahi is completely surrounded by Acacia-Commiphora Bushland and Thicket.

Biodiversity Features

The waters of these saline lakes are extremely inhospitable, with a saturated salt solution of pH 9 to 10, and temperatures that reach up to 41°C near the mineral springs. In addition, these conditions are not constant, but change abruptly during the rains. Heavy rains temporarily flood the lakes with cold water and lower the pH dramatically because the rain is slightly acidic. The mud flats surrounding the permanent water are just as inhospitable. Temperatures are extreme, winds are strong, vegetation is scarce and movement across this thick muddy environment is impossible for most mammals. As a result of these extreme and highly variable environments, species richness is low. However, the population sizes of the few species adapted to these environments are large.

A single species of fish known as the white-lipped or alkaline tilapia (Oreochromis alcalica) is found in abundance in the waters of these lakes. This small fish (about 10 cm in length) lives on the edges of the hot spring inlets, where the water is between 36 and 40 °C. It is endemic to the saline lakes of the Rift Valley (Lott 1992) and is adapted to withstand the high temperatures and salinity of the lakes as well as the changes in conditions brought about with the rains.

Lakes Natron and Bahi have distinctively large populations of wetland birds, flamingos in particular. According to an aerial survey by Woodworth et al. (1997) Lake Natron has the highest concentration of flamingos in East Africa. Both the greater and the lesser flamingo (Phoenicopterus ruber and Phoeniconaias minor) are found at these lakes, with the lesser flamingo outnumbering the greater by 100 to one.

Lake Natron is the major breeding ground for flamingos in East Africa and is the only regular breeding site for the lesser flamingo in Africa (Simmons 1995). Lesser flamingos bred at Lake Natron in 9 out of 14 years from 1954 to 1967. Only one other breeding site was recorded during those years, Lake Magadi (Kenya) in 1962 (Brown and Root 1971). The Magadikgadi Pan in Botswana is the main breeding site for the greater flamingo in Africa (see the Magadikgadi Pans Halophytics ecoregion), but Lake Natron is also important. Between 1951 and 1971 the greater flamingo bred at Lake Natron in 5 out of 12 years (Brown et al. 1973). While Lake Natron is an essential breeding site, it is not a focal feeding site for flamingos. The major feeding sites are Lakes Nakuru and Bogoria in Kenya, which are slightly less saline and have a greater abundance of small crustaceans and blue-green algae.

Africa’s flamingo populations are not isolated, and flamingos migrate between the soda lakes of East Africa and the Etosha and Magadikgadi Pans in southern Africa (Simmons 1995). Therefore, flamingo conservation stretches across many political boundaries, and threats to all key habitat need to be considered in an attempt to conserve the African flamingos. An Africa-wide assessment of greater flamingo populations in 1975 estimated 165,000 greater flamingos in Africa (75,000 in southern Africa, 50,000 in East Africa and 40,000 in northwest Africa). Present estimates indicate a decrease of 48 percent, to only 85,000 birds. The decrease has been most significant in southern Africa where numbers have decreased from about 75,000 to 47,500 birds. This dramatic decline may be a result of poor breeding success in the Etosha Pan over the past 40 years (Simmons 1995).

The area surrounding Lake Natron has few motorable tracks, and as a result the fauna has not been well studied. Prins (1987) includes this area in the Masai ecosystem, the western boundary of which is formed by the Rift Valley Escarpment. He notes a number of unusual features in the fauna in this ecosystem. The blue wildebeest (Connochaetus taurinus hecki), for example, is a light tawny color in this area, while that of the Serengeti blue wildebeest to the west (Connochaetus taurinus albojubatus) is darker. According to local information from hunters and nomads, zebra (Equus burchelli) and gazelles do not cross the escarpment, and the Thomson’s gazelle (Gazella thomsoni) is divided into an eastern lighter race and a western darker race. Animals are known to move up and down the escarpment within the Masai ecosystem. Limited studies show that the blue wildebeest migrate between Lake Natron and Lake Manyara National Park (Prins 1987). Other large mammals found in this ecosystem include the endangered African elephant (Loxodonta africana), the critically endangered black rhino (Diceros bicornis), giraffe (Giraffa camelopardalis), buffalo (Syncerus caffer), impala (Aepyceros melampus) and Grant’s gazelle (Gazella granti) (Mwalyosi 1977, Prins 1987). Tomier’s Leaf-toed Gecko (Hemidactylus squamulatus) is a specialist to the vegetation surrounding the soda lakes of the Rift Valley, which include the lakes of this ecoregion. There are no predators in the lakes or on the mud flats surrounding the lakes as it is not possible to move across the thick mud. The usual savanna predators, such as lion (Panthera leo), and cheetah (Acinonyx jubatus), are present on the plains and surrounding the lakes.

Current Status

These two lakes are naturally fragmented as they are widely separated along the Great Rift Valley of East Africa. The environment of these saline lakes is inhospitable and the lakes are largely inaccessible, surrounded by expansive mud flats. As a result, these habitats are sparsely populated and have not been subject to human fragmentation in the past. It is only recently that these lakes have been seen as useful, largely for soda ash extraction, and have become a site of active development.

Both Lake Natron and Lake Bahi fall outside of the protected areas network and are offered no formal protection. The area surrounding Lake Natron falls within a large game controlled area, an area in which hunting is allowed but is regulated. This offers some protection to wild animals in the area, but no protection to the ecosystem as a whole.

Types and Severity of Threats

The major threat to this ecoregion is the hydroelectric power scheme proposed by the Kenyan government for the Ewaso Ngiro River (Anonymous 1993, Johnson and Bennun 1994), which is the main source of water into Lake Natron. Altering its flow in any way would potentially change the hydrology and ecology of the lake, threatening the world’s largest and most secure breeding site of the lesser flamingo.

Lake Natron is also the site of small-scale sodium bicarbonate (baking soda) extraction plant (Stuart and Stuart 1998), which directly impacts a small area of the lake. These impacts are largely related to the infrastructure associated with the plant.

Lastly, uncontrolled tourism is a potential threat to the ecoregion. At present, the area is not on the popular tourist route, as it is difficult to access and has no facilities. Lake Natron is in fact so inaccessible that it was only discovered in 1954 (Brown 1957). Because of this, however, more adventurous 4x4 motorists may visit the area without permission, and there is the threat that they may not behave in an ecologically sensitive way (Stuart and Stuart 1998). Driving onto the mud flats may, for example, disturb breeding flamingos, causing them to desert their eggs and young. Such disturbances have been recorded at Etosha Pan (Berry 1971).