|Central Eurasian Water Crisis: Caspian, Aral, and Dead Seas (UNU, 1998, 203 pages)|
|Part I: introduction|
|3. Major environmental problems in world lakes|
Everywhere in the world, lakes and reservoirs are becoming more and more important as the most dependable sources of water in large amounts. Lakes and reservoirs are more attractive water sources than rivers and underground water for big water consumers such as cities and industrial centres. In terms of size, number, and distribution, man-made lakes today are quite comparable with freshwater natural lakes.
On all the continents, however, the environments in those lakes and reservoirs are deteriorating rapidly and extensively due to various human impacts. Both the quantity and the quality of lakes' water resources are being threatened in that an increasing number of lakes have already lost not only their value as sources of water but also such traditional roles as the space for fisheries and waterborne transportation.
For this very reason, the International Lake Environment Committee (ILEC) was established soon after the First World Lake Conference held in Japan in 1984. Since 1986, ILEC has made efforts to formulate guiding principles and programmes on the environmentally sound management of lakes and their watersheds along the line of sustainable development policies. Its activities have so far included the publication of lake management guideline books (Jørgensen and Vollenweider, 1989; Jørgensen Löffler, 1990; Hashimoto, 1991; Matsui, 1993; Jørgensen, 1993), convening training courses in lake management, symposia, and workshops, promoting environmental education in schools of several countries, supporting a series of World Lake Conferences, and, above all, compiling environmental data on world lakes. Detailed limnological and socio-economic data on some 220 lakes have already been accumulated and are being published by a joint project of ILEC, the United Nations Environment Programme (UNEP), and Lake Biwa Research Institute (LBRI and ILEC, 1988, 1989, 1990, 1991). Descriptive information on more than 500 lakes is also available for reference.
Figure 3.1 shows the range of water volume in relation to mean transparency for 145 lakes whose data had been published before 1990. The range covered is very wide, amounting to the order of 107, though most of the lakes are in a narrower range of 102-104 km3. Transparency appears to depend on lake size, but certain lakes, e.g. shallow lakes on windy plains such as Lake Winnipeg (Canada) and Lake Balaton (Hungary), tend to have exceptionally small Secchi depth values.
A preliminary synthesis of the accumulated data on world lakes and reservoirs revealed the global prevalence of six types of environmental disruption as major problems of urgent concern (fig. 3.2): decline of water level, accelerated siltation, acidification, eutrophication, toxic contamination, and extermination of ecosystems and biota.
Falling water levels and the resulting shrinkage of lake areas are due to the overuse of water drawn from the lake itself or from inflowing or outflowing rivers.
The extreme case of the Aral Sea is now widely known. Similar situations are also reported for other Central Asian lakes such as the Caspian Sea in the middle decades of the twentieth century (Golubev, 1992), Lake Balkhash in Kazakhstan, Lake Qinghai in China, and some lakes in Iran. Lake Mono in California also lost about 30 per cent of its former area owing to a fall in the water level of 11 m, and suffers from raised salt concentration in the lake water (as does the Aral Sea), owing to the diversion of 85 per cent of its tributary river water to the city of Los Angeles.
Aside from such lakes in the arid zone, the water level in other lakes has often been significantly lowered by dredging their outlets in order to increase the capacity of hydroelectric power generation in outflowing rivers. This has often caused the temporary advance of eutrophication, as, for instance, was the case in Lake Sevan Armenia (Oganesian, 1991).
Rapid siltation of lakes is the result of soil erosion accelerated by the overuse of farming and grazing lands, deforestation, the over-harvesting of fuelwood, and other imprudent land uses in lake catchment areas.
This is a very serious problem facing many lakes and reservoirs in China, India, Africa, and other less developed countries. Particularly alarming is the fact that cultivated fields and pastures, which have been sustainably used as more or less stationary semi-natural systems for centuries, are being devastated almost irreversibly by overuse. Overpopulation, as well as the residents' strong desire for more cash income to buy imported industrial products, has encouraged the abandonment of traditional sustainable methods of land use. The same situation is causing desertification in arid regions and forest destruction in the humid tropics.
The silt load of lake water, indicated by the concentration of suspended solids, is significantly correlated with the area of cultivated land per lake water volume in lakes in humid/subhumid climates, as illustrated in figure 3.3. A correlation in the arid zone could not be detected owing to the scarcity of available data.
Acidification of lake water is caused by the input of acid air pollutants such as acid rain and dry fallout, either directly on the lake surface or indirectly via inflowing rivers.
Tens of thousands of small lakes in northern and central Europe and in North America have already become too acid to allow fish to breed there and, in extreme cases, almost any kinds of animals to survive. At present, the acidification of lake water is largely confined to rather limited regions such as Scandinavia, parts of central Europe, the north-eastern United States, and neighbouring parts of Canada, most probably owing to particular geological conditions, though acid precipitation itself occurs more widely over the northern hemisphere.
However, soil scientists have recently tried to predict how long it will take for the buffer capacity of the soil to be exhausted by continued acid precipitation. The predicted value of course depends on soil type, but may range, e.g. in Japanese soils, from several years to several decades. This is an alarming prediction, which suggests that the acidification of lake water may sooner or later become a global issue.
Eutrophication is spreading, owing to the combined effects of such factors as industrialization, the urban concentration of populations, changing ways of life toward mass consumption and mass dumping, the increased application of fertilizers on cropfields, deforestation and other types of ecosystem disturbance in catchment areas, the submergence of terrestrial biomass caused by reservoir construction, aquaculture within lakes, and so on.
Eutrophication has been, and perhaps will continue to be, the most widespread type of environmental pollution of water bodies. It is expanding rapidly from industrialized or urban areas to rural areas, from small streams to larger rivers, from small lakes to larger lakes, and from inland waters to coastal marine waters. The current situation in some lakes of overpopulated areas is critical, because newly growing large cities have to depend for their water supply on hyper-trophic lake water filled with blooms of blue-green algae. Eco-technological measures are not effective enough to overcome the crisis in the short term, and large-scale environmental technology such as sewage treatment systems is too expensive to be easily adopted.
The post-war epoch of environmental pollution started almost 40 years ago with a series of heavy metal contamination events in coastal and inland waters of Japan and some other countries. The situation has not yet been improved very much. The development of monitoring and warning systems for toxic contamination may prevent the recurrence of such tragedies as Minamata disease (caused by mercury poisoning) in industrialized countries, but many water bodies, including lakes and reservoirs, still remain dangerously contaminated. The kinds of identified toxins have significantly increased, including various kinds of mutagens and carcinogens whose risks have to be managed in a different way from those of heavy metals, agro-chemicals, etc.
There are also indications that contamination is spreading to less developed parts of the world, keeping pace with the advance of industrialization and urbanization. During ILEC's survey of world lake environments, however, it was recognized that information on toxic contamination of lakes in less developed countries, especially reliable numerical data, was disappointingly scarce. Here is one of the most urgent needs for international cooperation.
The collapse of aquatic ecosystems and loss of biodiversity in natural lakes is the ultimate result of all the above-mentioned five processes.
Freshwater ecosystems are by no means as rich in plant and animal species as terrestrial ecosystems, but they are highly specialized with a high percentage of endemic species. For instance, the current number of fish species is said to amount to 22,000, of which one-third live in freshwater environments. In relation to the very limited area of inland waters, this percentage is surprisingly high and indicates the diversity of freshwater environments and the profound effect of geographical isolation.
Lake ecosystems, with their unique environment and biota, very vulnerable to or intolerant of disturbance from outside. This is shown by the frequent invasion of alien species and their explosive prolification in many lakes. The opportunities for such immigration are ever increasing owing to the development of intercontinental travel and transportation. Waterweeds of New World origin are flourishing and suppressing native species in Old World lakes (e.g. Elodea canadensis, E. nuttallii, Egeria densa) and vice versa (e.g. Myriophyllum spicatum). It is said that Nile perch, a big carnivorous fish introduced into Lake Victoria, have already exterminated some 200 native fish species, a greater part of which are endemic to the lake (Chamberlain, 1993). The recent invasion by zebra mussel (Dreissena polymorpha) from Europe of the Great Lakes of North America offers another very remarkable example (Nalepa and Schlosser, 1993). Similar cases are known from many other lakes.
Lake ecosystems are also vulnerable to environmental changes. The excessive advance of such processes as siltation, water level decline (and resultant salinization), acidification, toxic contamination, and eutrophication may lead to the eradication of high plants and animals in the lake. We are thus losing highly characteristic gene pools in lake ecosystems from time to time.
The exploding world population and developing industries are always thirsty for fresh water. A critical shortage of freshwater resources may possibly arrive sooner than that of food in the near future. As this brief review shows, the critical situation of world lake environments should properly be placed among the important environmental problems of global scale because of its worldwide occurrence and profound influence on human life. In order to halt its further advance, concerted international efforts are indispensable. The International Lake Environment Committee sincerely hopes that the Fifth World Lake Conference (Stresa, 1993), together with the preceding and subsequent conferences, will take significant steps forward in responding to the challenge of this problem.
This paper originally appeared in Memorie dell'lstituto Italiano di Idrobiologia, 52: 1-7, 1993, and is reprinted with permission. The original text has been partially modified and/or abridged by the editors.
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Golubev, G. N. 1992. "Environmental problems of large Central Asian lakes." In: Proc. Symp., Water Resources Management - With Views of Global and Regional Scales. Otsu: UNEP and ILEC, pp. 55-63.
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