Freshwater never consists solely of water but contains, dissolved within it, other substances from the atmosphere or the soil. Lakes which are fed by water flowing over soils and rocks that contain many easily dissolved minerals can become quite salty. Where the lake is fed by streams and rivers but there is no outlet, evaporation may exceed the water flowing in and the minerals become trapped and concentrated. If the minerals are predominantly chlorides, mainly sodium chloride (common salt) and magnesium chloride, salt lakes are formed with the water being distinctly briny. Lakes containing more than 5 grams of minerals per litre of water are considered to be salt lakes. Seawater contains 35 grams per litre, but some salt lakes contain minerals at concentrations many times that of sea water. Where the minerals are predominantly carbonates and bicarbonates, particularly in volcanic areas, the water becomes alkaline and soapy, forming soda lakes.
The largest salt lake in the world is the Caspian Sea. This is fed by freshwater from the Volga River, but has no outlet and hence salts accumulate. Although the Caspian Sea is quite salty, it is dilute enough to support a fairly normal range of organisms. Some salt lakes are so salty that only very specialised organisms can live in them. 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. 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. 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. This forms a sea of salt 100 miles long and 85 miles wide. 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.
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. 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 severe osmotic stress such high salt concentrations cause means that few organisms can live under these conditions. High levels of magnesium and calcium found in the Dead Sea appear to be a particular problem. The lake's food chain consists mainly of just two groups which can tol erate these conditions: an alga and several species of halobacteria (salttolerant archaea). At times, the lake develops a reddish tinge. This is due to blooms of an alga, Dunaliella parva, which, together with related species, is a common inhabitant of saline lakes and ponds. The halobacteria live off compounds produced by the algae. Several species of halobacteria, including Halobacterium halobium, have been isolated from the Dead Sea and are unique to it. How the algae and bacteria can tolerate such high salt concentrations will be considered in Chapter 6. Much of the water from the Jordan River, which is the main supply of freshwater to the lake, has been diverted and this often raises the salt concentration beyond that in which even these organisms can grow: the Dead Sea is dying.
The salt concentrations in some other salt lakes are low enough to permit the growth of some multicellular organisms. The most characteristic animal of salt lakes is the brine shrimp, Artemia. This is a crustacean belonging to the group known as fairy shrimps (Anostraca), which are common inhabitants of temporary ponds and saline waters. These feed on the halobacteria and algae and, since they have few competitors, can grow in enormous numbers under the right conditions. They are restricted to lakes and ponds which are too saline to support fish and other predators which would rapidly eat the brine shrimps. The fact that they are so palatable to fish was discovered in the 1950s by CC Sanders, a keen keeper of tropical fish. This led to an industry harvesting brine shrimps from the Great Salt Lake for sale as fish food. At its peak, in 1965, this industry harvested 77.2 tons each year of brine shrimp cysts, an indication of the enormous numbers growing in the lake. The numbers, and the industry, have since declined. Increased rainfall and snow in the mountains have caused the lake level to rise. The more dilute water has allowed the growth of waterboatmen, aquatic insects that feed on, and have reduced, the brine shrimp population. Brine shrimps have remarkable abilities to survive both high salt concentrations and desiccation, phenomena which will be explored in Chapters 3 and 6. Their survival abilities have led to the development of another, rather unusual, industry. Packets of dried cysts are marketed as instant pets called 'sea-monkeys' - just add water and nutrients and you have 'instant life'. Sea-monkeys have developed quite a following, as you will discover if you check them out on the Internet (try, for example, http://www.sea-monkeys.com/).
One of the best-known soda lakes is Lake Nakuru in the East African Rift Valley. This is famous because it supports large numbers of flamingos, at times numbering as many as 1.5 million. The minerals in the lake are mainly carbonates and bicarbonates which make the water alkaline. Organisms living in the lake have to survive the alkaline conditions and also wide variations in mineral concentrations as the lake level falls and rises due to evaporation or rain. The main photosyn-thetic organism is a cyanobacterium, Spirulina platensis. This is eaten by one species of copepod crustacean, one fish and by the lesser flamingo. Rotifers, waterboatmen and midge larvae are also found in the lake. Spirulina contains an unusually high proportion of protein. It forms a thick scum around the edge of soda lakes which is harvested in Mexico and in Chad by local people to make nutritious biscuits. It may be useful as both a source of protein and of animal feed. The lesser flamingo has filters in its beak which enable it to extract Spirulina from the water. The spacing of the filter in the beak of the greater flamingo is larger and they feed on the small invertebrates; thus, where the two flamingos occur together, they do not compete for food.
Was this article helpful?