![]() ![]() How the algae and bacteria can tolerate such high salt concentrations will be considered in Chapter 6. Several species of halobacteria, including Halobacterium halobium, have been isolated from the Dead Sea and are unique to it. The halobacteria live off compounds produced by the algae. This is due to blooms of an alga, Dunaliella parva, which, together with related species, is a common inhabitant of saline lakes and ponds. ![]() At times, the lake develops a reddish tinge. The lake's food chain consists mainly of just two groups which can tolĮrate these conditions: an alga and several species of halobacteria (salttolerant archaea). High levels of magnesium and calcium found in the Dead Sea appear to be a particular problem. The severe osmotic stress such high salt concentrations cause means that few organisms can live under these conditions. About one-third of its volume consists of minerals in solution, making it so dense that bathers find it almost impossible to sink or dive in it. The Dead Sea is the most concentrated natural salt lake in the world and lies at the lowest point on the Earth's land surface. The salt flats may fill with water after heavy rain, but strong winds or high temperatures make them such efficient evaporation pans that they rapidly dry out again. This forms a sea of salt 100 miles long and 85 miles wide. This has happened to many of the salt lakes of the Australian desert, but the largest salt flat is Salar de Uyuni high in the Andes of Bolivia. If evaporation continues to exceed the flow of water into a salt lake, it will eventually dry up completely, leaving behind a salt flat consisting of a large expanse of brilliant white crystals. The best known are the Dead Sea in the Middle East and the Great Salt Lake in the USA, but they are found in many parts of the world. Some salt lakes are so salty that only very specialised organisms can live in them. Although the Caspian Sea is quite salty, it is dilute enough to support a fairly normal range of organisms. This is fed by freshwater from the Volga River, but has no outlet and hence salts accumulate. The largest salt lake in the world is the Caspian Sea. Where the minerals are predominantly carbonates and bicarbonates, particularly in volcanic areas, the water becomes alkaline and soapy, forming soda lakes. Seawater contains 35 grams per litre, but some salt lakes contain minerals at concentrations many times that of sea water. Lakes containing more than 5 grams of minerals per litre of water are considered to be salt lakes. If the minerals are predominantly chlorides, mainly sodium chloride (common salt) and magnesium chloride, salt lakes are formed with the water being distinctly briny. ![]() Where the lake is fedīy streams and rivers but there is no outlet, evaporation may exceed the water flowing in and the minerals become trapped and concentrated. Lakes which are fed by water flowing over soils and rocks that contain many easily dissolved minerals can become quite salty. Freshwater never consists solely of water but contains, dissolved within it, other substances from the atmosphere or the soil. ![]()
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