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Newsletter and Technical Publications

Lakes and Reservoirs vol. 3

Water Quality: The Impact of Eutrophication

How Bad Is Eutrophication at Present?

Background comments on lake pollution

The demand for surface water for many purposes is increasing globally, mainly due to population growth and irrigation, particularly in arid and semi-arid regions. Eutrophication often becomes apparent to the public as populations increase in density. The total impact of humans on nature is probably about eight times higher today than 40-50 years ago, given the growth in population, in industrial and agricultural production, and in technological development (we use more chemicals, traffic density has increased, etc.).

The International Lake Environment Committee (ILEC), in cooperation with the United Nations Environment Programme (UNEP), undertook a project entitled “Survey of the State of the World Lakes”. This aimed to collect and compile environmental data on many important lakes of the world. Sets of detailed data from 217 lakes worldwide were gathered as a result of this project. Through this project it was possible to identify six major environmental problems, all having a significant impact on water quality, eutrophication being one of them (Table 2). In addition, all six environmental problems are interrelated and to a certain extent compound the problems. All are caused by the same three basic factors (Fig. 3).

Fig. 3: Urbanization, population growth and industrialization are amongst the basic factors which cause environmental problems in lakes and reservoirs.


All 217 lakes included in the ILEC survey showed an increase in the level of eutrophication over the past 50 years. For a number of lakes in industrialized countries, wastewater treatment to remove nitrogen and/or phosphorus has stopped water-quality degradation. By 2000 nutrient inputs to 66 world lakes were reduced. Even so, most are still more eutrophied (nutrients concentration is higher) today than they were 50-60 years ago. This is so for Lake Biwa in Japan (Photo 10), Lake Constance on the border between Germany, Switzerland and Austria (Photo 11), Lake Balaton in Hungary (Photo 12), Lake Mälleran in Sweden (Photo 13), the Great Lakes in North America (See p.18, Photos 25 and 26) and a number of North European lakes.

Photo 10: General view of lake Biwa, Japan.
Photo 12: General view of Lake Balaton, Hungary.
Photo 14: General view of lake Washington, U.S.A.



Photo 11: General view of Lake Constance, Germany, Switzerland and Austria.
Photo 13: General view of Lake Mälleran, Sweden.

In eutrophied lakes and reservoirs when measures have been taken to improve water quality by reducing or removing nitrogen and/or phosphorus without effect, it is largely due to the enormous amounts of nutrients stored in sediments being constantly released into the water. This shows the need to avoid nutrient loading into the water bodies as early as possible by proper management and planning practices. Still, it has often been difficult to reduce nutrient inputs of diffuse source such as drainage water and erosion from agriculture or dumping grounds; this cannot be collected for treatment, unlike point source pollution from industrial or municipal wastewater. Point source pollution can be treated by ‘end-of-pipe- technology’, i.e., environmental technology.

The best examples of success in treating eutrophication are where diversion of wastewater was used from a lake in an area with little agriculture. Lake Washington is an example (Photo 14). Figure 4 shows declining phosphorus concentrations in this lake after the diversion which was completed in 1967. It is important to realize that on occasions diversions do not solve the problem as it removes it downstream.

Fig. 4: Diversion of wastewaters proves to be effective in the reduction of phosphorus in freshwater bodies.

Almost all lakes still show increasing eutrophication, including most lakes in developing countries, which lack pollution abatement because they cannot afford it. Lake Dianchi (Photo 15) near Kunming in China, and Lake Taihu, near Wuxi in China are lakes suffering from extreme eutrophication or are hyper-eutrophied. In these lakes vast areas are covered by dense algal blooms like green dye and fish-breeding has been almost totally abandoned because there is no oxygen for them to breath, mainly in autumn. Almost all native water plants and many fish species have been killed. Snails die from lack of oxygen in the bottom water and in addition due to the poor water quality it is very difficult to supply water for domestic use that meets legal standards.

Photos 15 & 16: excessive growth of water hyacinth in lake Dianchi, China, and the weevil N. eichhornaiae
utilized in Lake Victoria to control the growth of this plant.

Even large lakes suffer from eutrophication. In Lake Victoria in Africa, for example, dense mats of water hyacinth float on huge surface areas. This problem has been somewhat reduced by biological methods -a beetle feeding on water hyacinths was introduced (Photo 16, beetle). Parts of the lake, near Kisumu and Kampala show oxygen depletion and reduced transparency. Many Haplochromis (Photo 17) fish species known locally as “furu” have become extinct, and the fish catch overall is declining with unfortunate results as fish is the major protein source for the lake’s coastal populations (Photo 18).

Photos 17 & 18: Fish such as the Haplochromis, or 'furu' fish, that used to be caught in Lake Victoria.
Photo 19: General view of Lake Baikal, Russia.

Even Lake Baikal (Photo 19), the largest freshwater body in the world and 1.7km deep, shows signs of eutrophication: decreased transparency and increased concentrations of algae and nutrients.

A solution to eutrophication in the developing countries is urgent since stopping eutrophication becomes more and more difficult and expensive every year it is postponed due to increasing nutrient accumulation in sediments.

      Table of Contents

  • Brochure
  • IETC Brochure

  • International Year of Forests
  • International Year of Forests

  • World Environment Day
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  • UNEP Campaign
  • UNite to Combat Climate Change