Distribution and Transformation of Nutrients and Eutrophication in Large-scale Lakes and Reservoirs

We present readers with an overview of lake management problems and the tools that can be applied to solve problems. Lake management tools are presented in detail, including environmental technological methods, ecotechnological methods and the application of models to assess the best management strategy.

Eutrophication is the phenomenon of phytoplankton bloom caused by excessive nutrients (N, P and other inorganic salt) in water environment. The most widely accepted definition of eutrophication is that which describes the nutrient enrichment of waters which results in the stimulation of an array of symptomatic changes, among which increased production of macrophytes and algae, deterioration of water quality and other symptomatic changes are found to be undesirable and interfere with water uses. Some of problems were brought about by eutrophication including turbid waters, bad smell, anoxic conditions and chironomid and Culex midge plagues, even more intoxicating phenomena for animals and people (Beat et al., 2008; Cecilia et al., 2005; Datta et al., 1999).

Eutrophication of the water body is a topic of widespread interest and the factors that cause it are mainly to do with nutrients, dissolved oxygen, water temperature, illumination and sediment (Wetzel and Limnology, 2001). Furthermore, hydrodynamics also play a crucial role in eutrophication. This consists of flux, flow velocity and water level. Hydrodynamics affect the water body mainly by wind drift and waves, which drive the sediment in the water body to move. Some kinds of substances in the water body then began to mix, dissolve, deposite, suspend, adsorb, entrain and cohere (Xu et al., 2009). In this condition, substances and sediments in the water body will change, which ultimately leads to water quality variety (Liebhold et al., 2004). When the flow velocity is fast, it is hard for eutrophication to occur even if the level of nutrients are high enough to trigger it, because some algae could be washed downstream by the flow before their growth has reached its peak (Zeng et al., 2006). Then the conditions for growth are destroyed and will not result in eutrophication. However, in other slow-flowing water bodies like lakes, reservoirs, estuaries, bays, inland seas, the flow velocity is slow and the water body is changing slowly (Smith, 1935). This condition slows down the spread of the nutrients and aggravates accumulation of the nutrients especially nitrogen and phosphorus, which offer fundamental nutrients for the growth and reproduction of algae.

Water eutrophication admits superfluous nutrient substances (nitrogen and phosphorus), which lead to the abnormal growth of algae and other aquatic life, changes in water diaphaneity and dissolved oxygen. All the factors above accelerate water substance ageing and affect the aquatic ecosystems and water body functions.

In recent years a worldwide increase in inputs of nitrogen (N) and phosphorus (P) into the lakes and reservoirs has led to considerable eutrophication and an increase in the frequency of toxic algae blooms (Carpenter et al., 1998; Cloern, 2001; Zeng et al., 2006; Fedro et al., 2007). The considerable changes in trophic composition and ecosystem structure (e.g. the plankton community) of the water environments adjacent to the lakes and reservoirs plumes have been attributed to the changes in Si:N:P ratios caused by excess of N and P, meaning cultural eutrophication (von Sperling et al., 2008; Burkholder et al., 2007; Yunev et al., 2007). For example, there are more than 30 lakes in southwest China. Although some of these lakes are seldom polluted, most of them have been heavily contaminated by recent human activities and they are experiencing a major deterioration of the water quality and a decrease in aquatic species. The increase in nutrient levels is believed to be an important factor in these problems.

Eutrophication has become a major environmental water issue in many parts of the world. Emission of excessive nutrient loads generated from point and non-point pollution sources into water bodies such as lakes, reservoirs and rivers leads to high phytoplankton biomass growth. An overabundance of algal biomass causes water quality problems, like variation in the diurnal oxygen level, oxygen depletion in deep water, unpleasant tastes and odors in the water supply, clogging of filters in water treatment plants, and adverse effects on water-contact sports and recreation. With the rapid development of agriculture and industry, eutrophication has recently been observed in a variety of water bodies in China, including Chaohu Lake (Yin and Hang, 2003), Dianchi Lake (Sun and Guo, 2002), Taihu Lake (Liu and Chen, 2006) and the Miyun Reservoir (Li et al., 2007).

Eutrophication is commonly considered as one major aspect of global environmental degradation (Nixon, 1995). To protect and manage the water quality, indices are useful tools to communicate with managers because they reduce complex scientific data, integrate different types of information and produce results that can be easily interpreted in the perspective of water quality management (Potapova and Charles, 2007). Many researchers consider that the main reason for the dramatic propagation of algae is the increase in concentration of nutrient materials, such as nitrogen, phosphate, etc. Nonetheless, the relationships between physiochemical and biological factors that impact the growth of algae, such as sunlight, nutrient salts, changes of seasons, water temperature, pH value, and the algae itself, are very complex. It is very hard to forecast the trend in algal growth and to set the eutrophication indicator. A common method was to indicate the water nutrient level using the main representative parameters. The parameters mainly include the concentrations of total phosphate (TP), and total nitrogen (TN), transparency, concentration of chlorophyll a, dissolved oxygen (DO), and so on. According to the parameters, the lakes are divided into different trophic levels, such as poor, middle, eutrophic and so on. The trophic level criterion is different for different countries and different reseachers. Tables 7.1, 7.2 and 7.3 show the trophic level proposed by the United States Environmental Protection Agency and used by Dianchi Lake and East Lake in Wuhan.