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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.3. Impacts of Eutrophication

1.3.1. Characteristics of Eutrophication

Lakes and reservoirs can be broadly classed as ultra-oligotrophic, oligotrophic, mesotrophic, eutrophic or hypereutrophic depending on concentration of nutrients in the body of water and/or based on ecological manifestations of the nutrient loading. Strict boundaries for these groupings are often difficult to apply because of regional variations in ranges of limnological parameters and because of lakes falling in different categories depending on the criterion used. One solution to these ambiguities is to designate a range of values for a particular degree of eutrophication as a statistical distribution. Figure 1.3. illustrates a set of statistical distributions for three criteria for degree of eutrophication: total phosphorus concentration, mean chlorophyll concentration and mean Secchi disk visibility. As an example of the application of these distributions, the probabilities of classification of Lake McIlwaine (now called Lake Chivero), a reservoir in Zimbabwe, into three degrees of eutrophication is indicated in Figure 1.3.

Figure 1.3. Probability distribution of trophic categories based on total phosphorus concentration (top), mean chlorophyll concentration (middle) and Secchi disk visibility (bottom). Possible classification of Lake McIlwaine (now called Lake Chivero) is indicated (from Ryding and Rast, 1989).

In general terms, oligotrophic lakes and reservoirs are characterized by low nutrient inputs and primary productivity, high transparency and a diverse biota. In contrast, eutrophic waters have high nutrient inputs and primary productivity, low transparency, and high biomass of fewer species with a greater proportion of cyanobacteria than in oligotrophic waters.

Although the fundamental characteristics of eutrophication are similar in lakes and reservoirs, differences in basin shapes and flow patterns may lead to longitudinal variations in the degree of eutrophication in reservoirs (Figure 1.4.). In addition, water supply and power generation requirements often lead to large variations in water level in reservoirs. These changes in level usually expose or inundate littoral regions which may enhance nutrient supply.

Figure 1.4. Longitudinal zonation in environmental factors controlling trophic status in reservoirs (from Ryding and Rast, 1989).

Many, but not all, tropical lakes may be contrasted with temperate lakes in several characteristics of trophic status. Depletion of dissolved oxygen in the hypolimnion occurs in many tropical lakes regardless of trophic status. Higher year-round light and temperature can lead to less seasonality in development of algal blooms and to more efficient recycling of nutrients than in temperate lakes. Prolific growths of aquatic vascular plants may be a normal circumstance and not be indicative of human-caused eutrophication.

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