About UNEP
United Nations Environment Programme
Division of Technology, Industry and Economics
top image
space space space

Newsletter and Technical Publications
<Planning and Management of Lakes and Reservoirs:
An Integrated Approach to Eutrophication>


5.3. Sources and Impacts of Eutrophication

5.3.1. Economic Sources of Eutrophication (Backward Linkages)

Nutrients are supplied to a lake by drainage from its catchment and direct rainfall. Focusing on the two nutrients important for eutrophication, nitrogen (N) and phosphorus (P), it is necessary to distinguish between natural sources of eutrophication and artificial sources or what is known as cultural eutrophication. Eutrophication that is induced by human activity, beyond natural levels, is known as cultural eutrophication. Table 5.2. presents some of the sources of cultural eutrophication.

Natural sources of nitrogen and phosphorus are derived from background nutrient cycles and biogeochemical processes, where the primary sources include nutrients in the soil and atmospheric input. Phosphorus and nitrogen deposition through rainfall includes natural particulate matter, such as pollen, dust, and soil particles etc., as well as chemical products from economic activities. Some of these can be from sources far away from the receiving watershed. Human activities have affected atmospheric input of nitrogen more than that of phosphorus, as many such activities emit nitrogen products into long-range atmospheric circulation patterns. Combustion of fossil fuels in industrial and energy production and in transportation is an important source of gaseous oxides of nitrogen. Average atmospheric phosphorus deposition in the United States and Europe in 1988 was estimated to be 430 g total of phosphorus per hectare per year (P/ha/year). The lowest levels were around 50 P/ha/year, which may reflect background levels, which are otherwise hard to measure directly. Nitrogen deposition is at least an order of magnitude higher, between 2 and 10 kg of nitrogen per hectare per annum (N/ha/year).

Table 5.2. Sources of cultural eutrophication.

Source Type of problem
Point Sources
Power plants Combustion of fossil fuels emit nitrogen products into the atmosphere, which are carried down by rainfall and other processes, causing eutrophication in water bodies
Sewage Treatment Plants Treatment process releases oxides of N and P in effluents, which drain into water bodies
Industrial Plants Industrial processes release N and P products in effluents, which drain into water bodies
Non-Point Sources
Agriculture Farming practices, including use of fertilisers rich in N and P, deposit increased amounts of these nutrients in the soil. Run-off from these farms cause eutrophication in water bodies
Sewage Direct discharge of sewage from domestic sources, not connected to treatment plants, will eventually make its way into water bodies

Both nitrogen and phosphorus are also found naturally in soils, predominantly as organic compounds and run-off into lakes contain both elements. Concentrations of these nutrients are, however, higher where, for economic reasons, agricultural activities have added nitrogen and phosphate containing compounds into the soil. This kind of cultural eutrophication is often the main cause of poor water quality in lakes.

When considering cultural eutrophication, it is useful to distinguish between point and non-point sources of nutrients. Point sources include discharges from industry and domestic wastewater treatment plants as well as agricultural point sources such as confined livestock units. Non-point or diffuse sources include excess run-off from development, silviculture and agriculture. Point sources of pollution are easier to identify and it is easier to design policies to reduce pollution from point sources than from non-point sources. Non-point sources, by the very nature of the problem, are harder to monitor and measure, as that involves dealing with a large number of agents. It is also costly to design programs to reduce pollution from non-point sources, where enforcement is very difficult.

Non-point sources of silt, organic matter and nutrients are often the largest cause of eutrophication around the world, primarily from agricultural activities. In the U.S. threatened or impaired uses of most lakes and reservoirs are associated with non-point sources. Intensive agricultural practices use nitrogen and phosphorus containing fertilisers and pesticides, which show up in the run-off. The quantities of nutrient loss depend on various factors including farming practice, nature and amount of fertiliser applied, density of domestic animals, soil type, soil drainage and rainfall. Some of these may vary with season and from year to year. Nitrogen losses increase with intensity of land use and with artificial additions more than those of phosphorus. This is due to the greater mobility of soluble nitrogen compounds. However, water logging makes phosphorus more soluble. Thus, unless there is heavy soil erosion or water logging, phosphorus losses from intensive farming are generally lower than nitrogen losses. Nitrate concentrations in waters have increased significantly in many countries since the 1960s, primarily due to the use of synthetic nitrogen fertilisers.

Sewage treatment plants are point sources of high concentrations of nitrogen and phosphorus. These facilities typically use bacterial oxidation of organic matter, which reduces pollution. However, this process oxidises all major elements in the waste, including nitrogen and phosphorus, which are then solubilized, and are found in high concentrations in the effluents from the plants. Household detergents, often with high phosphate content, also add to the nutrients in sewage effluents. Industrial effluents and storm water run-off from urban areas can also add to the increased nutrient concentration. Industries with high nutrient levels include those processing foodstuffs, such as breweries, canneries and sugar refineries, and those with metal finishing processes, which use phosphorus solutions.

In summary, efforts to slow or reverse eutrophication of freshwater systems generally confront the need to reduce inputs of nutrients. The easiest sources of nutrients to monitor and regulate are point sources, particularly municipal sewage treatment facilities and industrial discharges. However, non-point and airborne sources account for significant loading. These sources are much more difficult to contain. An economically efficient strategy for reducing nutrient input would first tackle the sources for which abatement plus oversight costs are least. This is likely to involve a combination of demanding expectations for abatement by point sources and more pragmatic, simple measures for non-point sources.

Previous page Table of ContentsTable of Contents Next page

  • Brochure
  • IETC Brochure

  • International Year of Forests
  • International Year of Forests

  • World Environment Day
  • ??????

  • UNEP Campaign
  • UNite to Combat Climate Change