Water-poor countries located near oceans have developed ways to remove the salt from the ocean's water, a process called desalinization. How? The process copies nature. Huge quantities of salt water are heated so that the water evaporates and leaves the salt behind; the water vapour is cooled so it condenses to form fresh water. Removing the salt from sea water is extremely expensive, however, and demands a great deal of energy. The world's largest desalinization plants are in Saudi Arabia.

Advanced knowledge

Eutrophication

The phenomenon of eutrophication is particularly associated with lakes and slow-flowing waters. It is widely, and erroneously, believed that pollution by plant nutrients and organic matter actually causes eutrophication. It is more accurate to say that pollution accelerates what is probably a natural process. To understand the causes and consequences of eutrophication requires some knowledge of the special characteristics of lakes.

In temperate latitudes, most lakes were formed by glaciation. Moving glaciers gouged out hollows in the earth, and when the ice retreated these hollows became filled with water from the melting ice. Such lakes are not. Therefore, geologically ancient phenomena. In modern times, substantial man-made lakes have become common in many parts of the world. A lake is a body of water which is very slow-moving. Some lakes have rivers flowing into or out of them. Even those which do not, however, are not static; water moves slowly into or out of the lake via the ground. Because the water moves only very slowly; some physical and chemical processes occur in lakes which do not occur in moving waters. Of particular importance are stratification, and temporal variations in chemical quality of the water.

Stratification occurs because the lake water is heated by the sun at the surface. Because warm water is less dense than cooler water, and water is a poor conductor of heat, during the warmer months of the year an upper layer of warm water, the epilimnion, becomes established and sharply delineated from a lower layer of cooler, denser water, the hypolimnion. Between them is a very narrow zone, the thermocline, within which the water temperature drops very sharply with only a slight increase in water depth. Little or no vertical mixing can take place, the like being effectively divided horizontally into two distinct layers separated by tile thermocline. Obviously, stratification cannot occur in very shallow lakes.

Photosynthesis can only occur in shallow water, where light can penetrate. At the lake margins, emergent plants and rooted aquatic macrophytes occur, but as tile depth of the water increases, primary production is possible only by phyoplankton in the surface waters, within the epilimnion. During the winter, phyoplankton growth is restricted by low temperatures and low light intensity. In spring and summer Increasing temperatures and light intensity stimulate phyoplankton growth, leading to an increase in population density and the depiction of nutrients In the water of the epilimnion. Plant growth and reproduction slow down, and as the plant cells senesce and die, they 1 sink into the hypolimnion and eventually to the bottom of the lake, where they begin to decompose. The inorganic nutrients which are the products of decomposition remain in the hypolimnion, however, as the stratification prevents vertical mixing of the water and upwards diffusion is slow. As the autumn approaches, reduced temperatures, light intensity and limited nutrients accelerate the decline of the phytoplankton population. In the winter, the epilimnion cools and becomes more dense. Its water sinks, displacing the hypolimnion which is now warmer and lighter than the epilimnion. The lake waters become thoroughly mixed, and nutrients from the hypolimnion are brought to the surface, bringing about conditions suitable for the start of the next annual cycle.

Underlying these annual cycles is a progressive change in the physical and chemical characteristics of the lake. At its formation, the lake contains few plant nutrients or dissolved minerals of any kind, and a negligible quantity of organic matter. With the passage of time, dissolved minerals including plant nutrients enter the lake from surface runoff and groundwater infiltration, at a rate which depends largely upon the climate and the geology of the surrounding area. As the nutrient levels rise, a flora and fauna becomes established and develops, contributing an increased content of organic matter in the lake. Organic matter is also gradually accumulated from outside the lake, progressively building up a layer of sediment on the lake bottom. Airborne dust also fills into the take, and the take begins slowly to fill up, 'the rate at which this happens varies from the barely detectable up to a few millimetres per year. The gradual deposition of material oil the floor of the lake basin causes the like to shrink, new land being formed at its edges. This new land is colonised by terrestrial plants, and in some lakes it is possible, by walking away from tile lake's edge, to see clearly the various stages of development of the terrestrial flora, a classic example of ecological succession. In areas where these processes have occurred, for various reasons, at different rates in different lakes, it is possible to see contemporaneously all the stages of a lake's development from nutrient-poor, sparsely-populated lakes of low productivity, through various stages of nutrient enrichment to swamp or marsh and eventually dry land.

The term eutrophication is applied to the process whereby the nutrient levels of lakes increase from oligotrophic (nutrient poor) to eutrophic (nutrient rich). It appears to be a natural process, although some authors have argued that it is not inevitable or intrinsically unidirectional. Since its basic cause is, however, the accumulation of plant nutrients and organic matter in the lake basin, clearly anthropogenic influences will accelerate it. The transition from oligotrophic to eutrophic is accompanied by qualitative and quantitative changes in the biota. Since plant growth is commonly limited by nutrient levels, a gradual increase in nutrient levels would be expected to lead to successional changes in the plant community.