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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.6. Conclusions and Recommendations

Eutrophication of lakes and reservoirs is a serious environmental problem caused by enrichment with plant nutrients, primarily nitrogen and phosphorus, that leads to reductions in water quality. Increases in human populations and in agricultural, industrial and urban development often contribute to eutrophication. The scientific basis for evaluating the causes and impacts of eutrophication is encompassed by limnology, and training in limnology should be an integral part of the education of everyone responsible for managing lakes or reservoirs. The introductory sections in this chapter are designed to provide an overview of key limnological concepts relevant to the understanding of eutrophication. Physical, chemical and biological aspects are considered.

Environmental conditions within lakes and reservoirs and in their watersheds all influence eutrophication. Rivers and streams are major routes of transfer of nitrogen and phosphorus, and they integrate point and non-point sources of nutrients. Atmospheric deposition can also contribute nutrients, and human activities have augmented this route of transfer. Recycling of nutrients from sediments to overlying waters can sustain eutrophic conditions for long periods after external loading is reduced.

Impacts of eutrophication that degrade the quality of water include large increases in algae and associated production of toxins, proliferation of aquatic plants, development of deoxygenated conditions, shifts in species composition, enhanced internal recycling of nutrients, elevated nitrate concentrations, and augmented incidence of water-related diseases. Other impacts, such as increased fish yields, greater nutrient reuse and interactions with acidification and contaminants, may be positive. To further illustrate the effects of eutrophication a series of case studies are described. Examples are provided for tropical South American and Ukrainian reservoirs, subtropical Lake Okeechobee, the U.S.A., Laguna de Bay in the Philippines, the North American Great Lakes, and prairie lakes in Canada and the U.S.A.

Operational assessment of eutrophication is derived from the underlying ecological processes, and a suite of alternatives is explained. In addition, results from such assessments for several parts of the world are summarized. Finally, management guidelines are proposed.

The complexity of the causes and impacts of eutrophication on inland waters requires coordinated planning and management to reduce its occurrence. Multiple government agencies working together with private businesses and the public must integrate their efforts.

The primary step in the reduction of eutrophication of a lake or reservoir is to limit, divert or treat inputs of nutrients and associated particles. In many north temperate lakes and reservoirs, controlling external supplies of phosphorus has improved water quality. However, in some north temperate waters and in other regions, nitrogen may be the primary nutrient limiting algal growth, and controlling supplies of nitrogen is difficult.

Because lakes and reservoirs can trap and recycle nutrients and organic matter, reduction in loading from the watershed may not reverse the impacts of eutrophication. Hence, it may be necessary to modify internal chemical, biological or physical processes. Food web interactions can modulate nutrient utilization. In shallow lakes and reservoirs sediments and fringing wetlands and riparian zones often are components important for nutrient recycling.

Evaluation of eutrophication of lake or reservoir, particularly, in developing countries, must balance benefits gained from its increased fertility against problems associated with degraded quality of water used for domestic and industrial purposes.

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