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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.5. Management Guidelines

Wise management policies for economic development with an aim to moderate eutrophication should encompass the whole watershed of a lake or reservoir. Two major elements that should be included in management of eutrophication are information about the loading of nutrients and an assessment of their impacts. Planning decisions should accept the uncertainty inherent in the understanding of causal relations and in the natural variability of aquatic ecosystems as they are influenced by fluctuations in climate, hydrology and biological populations. Hence, probabilistic forecasts are more appropriate than specific predictions.

Following four strategies, with different time frames and purposes, can be applied to the management of eutrophication:

• Waste water treatment to meet a standard acceptable for a specific use.
• Manipulation of conditions within the lake or reservoir to ameliorate the symptoms of the problem.
• Control of an input at its source, such as the removal of phosphates from detergents.
• Actions within the watershed and/or body of water that address the causes of the problem.

Approaches to the first two strategies are dealt with in Chapter 6 of this publication. Implementation of the third type of option involves economic and cultural aspects considered in Chapters 5 and 2, respectively, of this publication. General aspects of the fourth strategy are discussed here.

In situations, which commonly exist in developing countries, eutrophication may have positive impacts that may override negative aspects. Greater yields from aquaculture or fish harvest from lakes or reservoirs may well exceed perceptions that green waters are not aesthetically pleasing to some. Furthermore, eutrophication control measures that are linked to biomass harvesting of fish and plants may be more economically and socially acceptable in some developing countries.

Shallow lakes present different challenges to eutrophication assessment and control than deep lakes, especially in tropical settings. Reduction in external nutrient loading is often not effective because internal loading from nutrient-enriched sediments is sufficient to maintain eutrophic conditions. Resuspension of surficial sediments can lead to high concentrations of suspended matter, which hampers operation of water purification facilities and can further enrich the water with nitrogen and phosphorus. Pantropical introductions of water hyacinth (Eichhornia crassipes ) and the aquatic fern (Salvinia molesta ) have led to prolific infestations of these floating aquatic plants that thrive in shallow, nutrient-rich waters. While harvesting of these plants can reduce nutrients and provide organic matter for other purposes, removal is often not possible on a sufficient scale to maintain open water as a habitat or for navigation. Problems with the introduction of water hyacinth are discussed in Chapter 7 of this publication.

Point sources of nitrogen or phosphorus loading can be effectively controlled, if sufficient financial resources are available. Well-developed technologies exist on several scales to remove nitrogen and phosphorus from effluents of sewage and industrial plants. Reclamation and recycling of phosphorus contained in these wastes is possible, if heavy metal contamination is not a problem. In contrast, reducing inputs to lakes and reservoirs from non-point sources usually requires modifications to land uses. In particular, establishment of riparian vegetation can reduce nitrogen and phosphorus transfers from land to water. If the watershed is forested, policies should encourage retaining half the forest and should ensure that forest is left along stream and river corridors. Grazing pressure and extent of mechanized agriculture should be regulated.

In hypereutrophic lake application of conventional remedial measures to reduce its nutrient input are unrealistic. There is sufficient nutrients recycling in the system. In some extreme cases, bacteria dominate production. Corrective measures, such as algicides, sediment sealing or aeration have to be employed. The selection of the corrective measure depends on the expected use of the lake, and for hypereutrophic lakes the uses will most likely be fish production, irrigation and perhaps recreation that does not entail contact with the water.

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