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INTRODUCTION

Water is one of the most abundant substances on the earth. It covers approximately 71% of the globe surface with the majority held by oceans. It is estimated that 97% of the total quantity of water is in the oceans and 3% is fresh water. However, only a small portion of the fresh water is available to humans, animals, and plants. Most of the 3% of the fresh water supply is held frozen in polar ice caps. The fresh water in lakes, rivers and groundwater is only 0.3% of the total fresh water source of the earth. This fresh water forms the bulk of the water for drinking (10%), industry (21%), and agriculture (69%).

Water generates the hydrosphere, is a part of the atmosphere, and infiltrates deep into the ground. Water is the regulator of climatic conditions and is a necessary part of life of all living organisms, including human. In the absence of water, there will be no life on the earth. While fresh water is a renewable resource following the hydrological cycle, it is not evenly distributed on the Earth's surface. For example, in the Asian and Pacific regions, precipitation is exceptionally abundant on the southern slopes of the Himalayas, on the western slopes of the mountains of India and Indo-China, and on the islands of Indonesia. These areas receive annually from 1,500 mm to an excess of 3,000 mm of rain, and in some locations considerably more. On the other hand, almost all of the northwestern part of the region is extremely dry, with an annual precipitation of less than 200 mm. Moreover, not only a sharp difference exists in the amount of total annual precipitation, but the precipitation also varies considerably from one season to another during the year. The rainfall in a large part of the region is characterized by a monsoon climate pattern with very distinctive dry and rainy seasons. During the long dry season, temporary water shortage is experienced in many river watersheds, while during the rainy season severe floods may cause tremendous damage in the same watershed. In the Latin America and Caribbean regions, the distribution of precipitation is very uneven and this creates some very arid regions. Moreover, the population tends to concentrate in the less humid areas of the region. Over 90% of the population live in areas with less than 2,000 mm of annual precipitation. Further, three of the largest concentrations of the population - the metropolitan areas of Lima-Callao, Peru, Mexico City, Mexico, and Santiago, Chile, are in areas with less than 500 mm of annual precipitation. In these areas, and in other urban and industrial areas, there will be increasing competition among water uses, which will require change in water resources management.

The above examples indicate that an adequate quantity and quality of water is not available when and where it is needed. In addition, pollution and unsustainable abstraction of water resources have enormous environmental, socioeconomic and health implications. Although geographically the fresh water crisis appears to be a local problem, water scarcity is likely to bring about future international problems. Recently, it was suggested that national tension over water resources could create a dangerous situation. Further, it was predicted that both international and civil wars over water supply threaten to become a key part of the next century's landscape.

Lakes, water reservoirs and streams are the most valuable source of drinking water for the earth's population. The lives of many people depend on both natural lakes and artificial reservoirs, which provide drinking water and water for agricultural and industrial development. In addition, lakes and reservoirs provide unique recreational opportunities. However, the lakes and reservoirs have special characteristics, which make them vulnerable to pollution and degradation. Input of organic contaminants from different sources has resulted in the alteration of water quality. Eutrophication of lakes and reservoirs is a degradation process originating from the introduction of nutrients from agricultural run-off and untreated industrial and urban discharges. Accelerated eutrophication of lakes and reservoirs, experienced during this century in most parts of the world represents a serious degradation of water quality. Impairment of water quality due to the eutrophication can lead to heath-related problems and result in economic losses.

Many problems originate from the development of countries. Agricultural growth, including the construction of irrigation and drainage systems and the use of fertilizers and pesticides, cleaning of forests and the construction of factories in lake and reservoir watersheds, often cause eutrophication of the lakes and reservoirs. In developed countries, the management of many lakes and their watersheds has resulted in the prevention of eutrophication. However, nationally adapted and locally appropriate policies and guidelines for environmentally sound management of lakes and reservoirs are necessary tools for the preservation of precious water resources in all countries in the world.

Eutrophication of lakes and reservoirs has triggered several national and international cooperative studies over the past three decades. The objective of the studies was to correct the problem. Through these initiatives, eutrophication of specific water bodies is now under control. However, eutrophication at the global scale is still one of the most serious water quality problems, and continues to increase in many parts of the world.

Prevention of eutrophication or restoration of eutrophic lakes and reservoirs requires proper planning and management of the associated watershed. Generally, man-made accelerated eutrophication can be reversed through the elimination or reduction of nutrient supplies from different sources, such as municipal and industrial wastewater, agricultural wastes and fertilizers, etc. However, in most cases it is not possible to eliminate all sources of nutrients. Therefore it is important to understand the relationship which exists between the nutrient supplies and the degree of the eutrophication. This knowledge is necessary to develop sound management strategies to control the eutrophication of lakes or reservoirs at minimum costs. Recently, the importance of desirable public involvement in successful management of lakes and reservoirs and associated watersheds, has been recognized.

Many regional, national and international reports, and scientific publications have been prepared on the problem and remediation of eutrophication of inland and marine waters. In dealing with the problem of eutrophication of lakes and reservoirs, decision-makers have mainly relied on the use of different equipment or "hard" technologies without considering sound management practices, particularly planning and management of the entire watershed. Such practices consider understanding the problem, its origin and consequences, while at the same time providing appropriate solutions.

This publication deals with the problems of eutrophication of lakes and reservoirs by looking at its origins, consequences, solutions, and prognoses under an integrated approach. The objective of the preparation of the publication was to assist local authorities in their effort to prevent, reduce, and control the eutrophication of lakes and reservoirs through the application of sound management practices. Comprehensive information on the most important issues to be considered in the planning and management of prevention and/or remediation of the eutrophication of lakes and reservoirs is provided within the eight chapters of the publication. Numerous examples from different countries and ecosystems are included to help the reader to better understand the contents of each chapter. Therefore the publication should become a useful tool for decision-makers and managers dealing with water resources issues, particularly with eutrophication of lakes and reservoirs in different parts of the world. Below is a brief summary of issues discussed in individual chapters.

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Table of Contents

• Brochure
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• International Year of Biodiversity
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