involves arduous hours of walking and carrying, mainly by women and children, and where debilitating water-borne diseases are common.
The resource is renewable, but distributed unevenly, and its efficient management requires an elaborate infrastructure of reservoirs, treatment plant, pipelines, and sewerage, coordinated within an overall strategy by an authority with the power to prevent abuses. For people in those regions, improvements in living standards depend crucially on the establishment of such strategies for water management, and once living standards begin to rise it is inevitable that the demand for water will increase substantially. As rising demand encounters limits in the supply available, conflicts may ensue, as they have already between Israel and Jordan over abstraction from the river Jordan. This is one of the most formidable challenges facing us. It is encouraging to note, however, that throughout history, competition between nations for scarce water resources has almost invariably been settled peacefully.
grass and a corresponding increase in the area growing cereals. In 1938, less than 1.2 million ha was sown to barley and wheat; in 1966 those crops occupied 3.3 million ha. During the same period, the area devoted to permanent and temporary grassland fell from 8.4 million ha to 6.8 million ha. The 2.1 million ha increase in the cereal area was achieved by reducing the grassland area. (MAFF, 1968, p. 34)
Thus the change from grassland to cereal cropping led inevitably to an increase in the movement of nitrate from the soil and into surface and ground water. The widespread introduction of soluble, nitrogen-based fertilizers exacerbated the problem, especially when heavy applications were followed by very wet weather, but the fertilizer contribution should not be exaggerated. In 1964, for example, nitrogen runoff was measured following 114 mm of rain in two falls in Missouri (SMITH, 1967). Bare soil, which had received no fertilizer, lost 0.9 kg N ha-1; unfertilized maize and oats lost 0.3 kg N ha-1; and continuously grown maize, fertilized with 195 kg N ha-1, lost 0.1 kg N ha-1.
This is not the only source of nitrogen reaching both land and water. Substantial and increasing amounts also arrive from the air. Elemental nitrogen is oxidized by lightning, in the course of burning plant materials, and in high-compression internal combustion engines, and biologically by the action of nitrogen-fixing soil bacteria. Urine from farm livestock releases ammonia, also a soluble compound.
It has been found that in the mid-1970s much of Europe received 2–6 kg N ha-1 yr-1 and that some areas now receive 60 or more kg N ha-1 yr-1. This level of fertilization may be altering the composition of certain ecosystems, especially those established on nitrogen-poor soils (MOORE, 1995).
Plants have similar physiological requirements whether they grow on dry land or in water. If plant nutrients enter water, therefore, they will stimulate the growth of aquatic plants. Nitrate alone is not enough, of course. The full range of nutrients must be supplied and plant growth is limited by the availability of the nutrient in shortest supply (in water this is usually phosphorus);
this is the ‘law of the minimum’ first stated in 1840 by the German chemist Justus von Liebig (1803–73). Other nutrients are less mobile than nitrate, so nitrate leaching has less effect on plant life than might be supposed.
Agricultural change apart, the movement of nutrients from the land and into water is an entirely natural process, an inevitable consequence of the drainage of rain water. As water moves downward through the soil to join the ground water, soluble soil compounds dissolve into and are carried by it. Were this not so, freshwater aquatic plant life would be severely restricted.
Water draining into surface waters, such as rivers and lakes, also carries fine particulate matter that is deposited as sediment when the power of the stream falls below a certain threshold. Fast-flowing streams rapidly remove material that enters them and accumulations occur only in slow-moving rivers and still water. It is there, and only there, that sedimentation and eutrophication may cause difficulties.
Eutrophication leads to the proliferation of aquatic plants, especially algae, and cyanobacteria, organisms that derive nutrients directly from the water, rather than through roots attached to a substrate.
A eutrophic lake or pond can usually be recognized by its surface covering of green algae. The life cycles of such organisms are short and as they die their remains sink and are decomposed by aerobic bacteria, whose populations increase in proportion to the food supply available to them. The bacteria obtain the oxygen they need from that dissolved in the water, and under eutrophic conditions the amount they remove exceeds the amount being introduced, so the water is depleted of dissolved oxygen. A common measure of water pollution is its ‘biochemical oxygen demand’ (BOD), calculated from the reduction in the amount of dissolved oxygen in a water sample incubated in darkness for 5 days at a constant 20°C; it is also a measure of bacterial activity.
If the water body is used for water abstraction, angling, or navigation, eutrophication is likely to reduce its value. The cost of treating water to bring it to potable standard will increase, navigation may be impeded by plants, and preferred species of fish may disappear. At high densities, some algae and cyanobacteria produce potent toxins. The alga Prymnesium parvum is highly toxic to fish, and toxins produced by such cyanobacteria as Microcystis, Aphanizomenon, and Anabaena attack the liver and may be neurotoxic. In 1989 there were outbreaks of toxic cyanobacteria in some British lakes and a number of dogs died after swimming in them and ingesting their water. Not surprisingly, eutrophication also brings about marked changes in the populations of aquatic organisms. The water supports fewer plant and animal species, but more individuals, the water becomes more turbid because of the large amount of organic matter suspended in it, the water becomes increasingly anoxic, and the rate of sedimentation increases.
A eutrophic lake is an old lake, and eutrophication is an ageing process. When it first forms, a lake typically supports little plant life, but fish such as trout, which feed on insects caught at the surface, may thrive. Its water is clear and well oxygenated, but very deficient in nutrients. There is little or no sediment at the bottom and plants grow beside it, but well clear of the water. A lake in this condition is said to be ‘oligotrophic’ (the Greek oligos means ‘small’ and trophe ‘nourishment).
Rivers flowing into the lake bring nutrient and particulate matter, and in time the lake becomes
‘mesotrophic’ (Greek mesos, ‘middle’). Its water is still clear enough for light to penetrate deeply, so algae flourish, but without proliferating uncontrollably because they are grazed by a diverse population of invertebrate and vertebrate animals, including fish. Sediment is accumulating on the bottom. This provides anchorage and nutrient for rooted plants, which now extend from the banks and into the lake margins, colonization by plants that have to reach the air being limited only by the depth of water. The accumulation of sediment also raises the bottom, so the lake has become shallower. In a eutrophic lake (Greek eu-, ‘well’) the sediment is deep and the lake shallow. Plants rooted in the sediment extend far from the banks. The three drawings in Figure 3.4 illustrate this life cycle.
Life cycles, which paradoxically are linear so far as individuals are concerned, end in death, and the life cycle of a lake is no exception. It is the fate of all lakes and ponds eventually to become dry land or, if they occupy low-lying ground where the water table is at or very close to the surface, a bog, marsh, or fen. Accumulating sediment makes the water shallower, but its colonization by plants also removes water, by transpiration. Once plants are established across the whole area of a lake, its demise is fairly rapid. Aquatic plants give way step by step to land plants that can tolerate waterlogging around their roots, and then these are replaced by true dryland or wetland plants. As the sediment dries and becomes soil, it is the acidity of the soil that determines whether the lake evolves into lime-loving grassland and, over much of north-western Europe, from there to scrub followed by woodland and forest, or to acid-loving heath. Figure 3.5 illustrates this development.
Such eutrophication is natural, but the life span of a lake should be measured in thousands of years.
Artificial eutrophication, caused by discharging sewage and other wastes into lakes, short-ens it greatly. Untreated human sewage may have a BOD of 300 mg litre-1, paper-pulp effluent 25000 mg litre-1, and silage effluent 50000 mg litre-1. Deoxygenation is by far the commonest type of freshwater pollution. Bacteria decomposing the faeces from one human use 115 g of oxygen a day; this is enough oxygen to saturate 10000 litres of water (MELLANBY, 1992, p. 88). Halting natural eutrophication may be undesirable, even if it is practicable, but artificial eutrophication should be prevented or, if it is too late for prevention, cured.
It can best be remedied, of course, by finding alternative means of waste disposal or at least by reducing the nutrient content of the discharges, especially of phosphates, which are the limiting nutrient in most waters. This can be done by reducing the phosphate content of detergents, which
are the principal source, or by stripping the phosphate from sewage before it is discharged. This is possible, with 90–95 per cent efficiency (MASON, 1991, p. 131). but there have been cases of a reduction in phosphate input being followed by the release of phosphate from sediment by mechanisms which are not well understood. In extreme cases it may be feasible to remove the sediment itself by dredging. Where land drainage is the main source of sediment and nutrient, reducing soil erosion may be effective. If oligotrophic water is available, using it to recharge a eutrophic lake may bring benefits. Beyond such measures as these, remediation usually involves manipulating the plant and animal populations. Obviously, no two water bodies are precisely similar and remedial measures must be appropriate to the particular conditions encountered.
Figure 3.4 The life cycle of a lake. A, Oligotrophic. Little bottom sediment;
water nutrient-poor; plants grow on banks only. B, Mesotrophic. Mud accumulating on the bottom; plants rooted in mud extending into the lake;
moderate nutrient supply. C, Eutrophic. Deep bottom sediment; plants rooted in mud far into the lake; water very rich in nutrients; depth of lake decreasing owing to accumulation of sediment and evapotranspiration
It is easy to over-dramatize the problems of eutrophication. They are confined to still or slow-moving waters, which limits their extent. Nevertheless, remediation is often necessary, because the affected water body represents a valuable resource, and it is always complicated and expensive. Prevention being better than cure, control of discharges into surface waters, introduced primarily to improve the quality of river water that is not liable to eutrophication, will nevertheless reduce eutrophication in lakes fed by the improved rivers. The principal cause of river pollution is identical to that which produces artificial eutrophication.