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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.1. Introduction

Eutrophication of lakes and reservoirs is enrichment with plant nutrients, mainly phosphorus and nitrogen, which enter as solutes and bound to organic and inorganic particles. Enhanced growth and increased abundance of aquatic plants often results in reductions in water quality. Augmented nutrient inputs to inland waters usually result from modifications of watersheds, such as deforestation, agricultural and industrial development and urbanization. Environmental conditions within the bodies of water and in the airshed and watershed all influence eutrophication. Factors within lakes that modulate impacts of nutrient enrichment include food web structure, exchanges between sediments and water, the shape of the basin and movements of water within the lake or reservoir. Climatic and hydrologic conditions in the watershed further alter impacts of eutrophication.

Eutrophication of inland waters ranks as one of the most widespread environmental problems. It has many significant and negative ecological, health and economic impacts on the use of a primary and finite resource, the water. Clean, fresh water is essential for many organisms and for human societies. Hence, impaired water quality threatens their existence. Symptoms of eutrophication include algal scums and toxins derived from algal blooms, massive infestations of certain aquatic plants, increased incidence of water-related diseases, turbid water, noxious odors and poor tasting water, depletion of dissolved oxygen, and tainted fish and fish kills. Some, positive impacts of eutrophication, such as enhanced plant production and improved fish yields, do occur and can be important. Indeed, in many countries, fish and other aquatic organisms are significant sources of food.

According to recent estimates from the United Nations, the world's human population is increasing by almost 100 million annually and large urban centers continue to grow rapidly. Half of the world's population is expected to live in urban areas by the 21st century. Each person will require clean water to sustain their existence and will generate phosphorus, nitrogen and organic carbon as waste each day. To meet everyone's need for food, fertilized agriculture and animal husbandry will generate additional organic and nutrient-rich wastes. We must find disposal solutions for the wastes or eutrophication will increase and lead to non-useable water supplies.

Everywhere in the world, with a few local exceptions, eutrophication is an acute problem. Sanitation programs often cannot cope with the magnitude of population growth, particularly in the new megacities (Figure 1.1.). All available venues, including legislation, urban planning, reuse and recycling policies, and development of effective and affordable wastewater treatment technologies, need to be part of the solution. While control of eutrophication is often technically possible, it may not be economically feasible.

Figure 1.1. Interrelations of population growth, gross national product (GNP) and wastewater treatment facilities in the world's largest cities (from Niemczynowicz, 1991).

Most strategies for management of eutrophication have been developed in temperate climates for highly developed economies and may require modification for application to other regions or economic conditions. For example, while phosphorus control is the primary focus in North America and Europe, nitrogen is frequently limiting in tropical waters and the relative importance of phosphorus and nitrogen can vary with time. Therefore, assessments of conditions in a particular lake or reservoir should be performed before a management strategy is employed. Variations in biological, chemical and physical conditions are characteristic of aquatic ecosystems whether or not perturbed by human activities. Therefore, discrimination of the influence of a development scheme from natural changes may be difficult. Hence, an understanding of the functioning of a lake or reservoir obtained from long-term descriptive data, experimental manipulations and integrative models are useful in the evaluation of management options.

The scientific basis for evaluating causes and impacts of eutrophication is derived primarily from limnology, the study of the physical, chemical, and biological processes in inland aquatic environments. Limnology has a long and successful tradition of applying scientific knowledge to the management of inland waters. Training in limnology should be an integral part of the education of anyone responsible for the management of lakes or reservoirs.

Much of our limnological understanding relies on investigations conducted in mid-latitudes. Therefore an assumption is that fundamental ecological processes, such as plant requirements for light and nutrients, operate in a similar fashion generally. Variations in rates and relative importance of ecological processes among lakes and reservoirs in the same or different regions are expected.

As an introduction to environmental aspects of eutrophication, this chapter begins with a brief review of limnological concepts followed by a characterization of eutrophication and a succinct summary of effects of eutrophication. Case studies are presented to illustrate the nature of these effects. The third section of the chapter examines causes of eutrophication and includes approaches to assess the degree of eutrophication, uses of models, and results of assessments of eutrophication throughout the world. Further discussion of external loading and internal recycling is also provided. Finally, management guidelines are suggested and brief conclusions offered.

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