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

Newsletter and Technical Publications

Lakes and Reservoirs vol. 3

Water Quality: The Impact of Eutrophication

Why Is Eutrophication Such a Serious Pollution Problem?

Eutrophication is one of the most widespread environmental problems of inland waters, and is their unnatural enrichment with two plant nutrients, phosphorus and nitrogen.

One important result of lake and reservoir enrichment is increased growth of microscopic floating plants, algae, and the formation of dense mats of larger floating plants such as water hyacinths (Photos 1 and 2) and Nile cabbage. Growth results from the process of photosynthesis which is how the plants generate organic compounds and biomass through the uptake of nutrients (nitrogen, phosphorus and others) from the soil and water. In the process light acts as the energy source and carbon dioxide dissolved in water as the carbon source. As a result of the photosynthetic process oxygen is also produced.

Photo 1: Algal bloom in a lake.
Photo 2: Overgrowth of floating aquatic plants.

When the plants die they decompose due to bacterial and fungi activity; in the process oxygen is consumed and the nutrients are released together with carbon dioxide and energy. In many lakes and reservoirs in the world plants growing in the surface during spring and summer will die during autumn and sink to the bottom where they decompose.

During spring and summer, lakes and reservoirs are often supersaturated with oxygen due to the amount of plants. The oxygen surplus is released to the atmosphere and no longer available to decompose organic matter. This causes oxygen depletion or anoxia in the deeper layers of lakes, particularly in autumn. Oxygen depletion is therefore caused by the shifts in time and space between photosynthesis and decomposition. In tropical areas the same process takes place, but seasonally speaking it is not as representative as in temperate areas because temperature and daylight duration is very similar throughout the year.

Fig. 1: Thermocline and the relationship between temperature/oxygen and depth in lakes within temerpate regions.
Photo 3: Black sediment from the bottom of a lake.

At certain times, lakes may form a thermocline some metres below the surface (Fig. 1). In the thermocline the temperature declines several degrees over a few metres and divides the lake into two zones an upper warmer one ( epilimnion), and a lower colder one ( hypolimnion). Lakes in temperate regions are lakes with a depth of about five to 10 meters or more and typically form a thermocline during the summer time, therefore they stratify. Shallow tropical lakes can also stratify, but stratification can be broken down by strong winds.

A thermocline prevents the upper and lower layers of the lake from mixing. The result is a change in vertical oxygen concentrations, as shown in Fig. 1, where concentration is high in the upper layer or epilimnion and very low in the lower layer or hypolimnion (the low oxygen concentration may degrade water quality downstream of the lake or reservoir, particularly downstream of reservoirs with short retention times, as mentioned in Volume 1, p.15).

Photo 4: Fish mortality due to lack of oxygen in an Indonesian lake.

Oxygen depletion often leads to complete deoxygenation or anoxia in the deep layers of the lake or reservoirs also because oxygen poorly dissolves in water. In shallow lakes and where plant production is high, deoxygenation of the sediment and water occur frequently too (black sediment, Photo 3). Such conditions kill fish and invertebrates (Photo 4). Moreover, ammonia and hydrogen sulfide originated from bacterial activity can be released from sediments under conditions of anoxia, and their concentrations can rise to levels which adversely affect plants and animals as they act as poisonous gases (also hydroelectrical power facilities in reservoirs often suffer because of the corrosive power of hydrogen sulfide). Phosphorus and ammonia may also be released into the water, further enriching it with nutrients.

Some particular type of algae, which grow in highly nutrient enriched lakes and reservoirs (blue-green algae or cyanobacteria, Photos 5 and 6 and so-called dinoflagelates which produces red tide, Photos 7 and 8), release in the water very powerful toxins which are poisonous at very low concentrations. Some of the toxins produce negative effects on the liver of life stock at minimal concentrations but they can lead to the death of cattle and other animals even to humans when ingested in drinking water at higher concentrations. Although one way to treat and disinfect surface waters where these algae grow and/or to prevent high concentration of organic matter is to use chlorine, unfortunately this leads to the formation of compounds which may produce or induce cancer -a serious threat to the safety of drinking water supplies.

Photos 5 & 6: Macro- and micro-scopic viwes of Microcystis aeruginosa.
Photos 7 & 8: Macro- and microscopic views of Uroglena americana, a culprit of red tide.

High concentrations of nitrogen in the form of nitrate in water can also cause public health problems. They can inhibit the ability of infants to incorporate oxygen into their blood and so result in a condition called the blue baby syndrome or methemoglobinemia. For this to occur, nitrate levels must be above 10mg per liter in drinking water. The condition can be life-threatening.

Photo 9: Eutrophied waters (left down) in Barra Bonita reservoir, São Paolo, Brazil.

One of the main problems occurring as a result of algal blooms or other aquatic plants (disproportionate growth, Photo 9) is the reduction in transparency in the water which reduces the recreational value of lakes, particularly for swimming and boating. Water hyacinth and Nile cabbage can cover large areas near the shore and can float into open water spreading at times over the entire surface. These mats can block light to submerged plants and produce large quantities of dead organic matter that can lead to low oxygen concentrations and the emission of unpleasant gases such as methane and hydrogen sulfide due to its decomposition or decay. Masses of these plants can restrict access for fishing or recreational uses of lakes and reservoirs and can block irrigation and navigation channels.

Shifts in the abundance of, and significant reduction in diversity of species (biodiversity) of aquatic organisms within a lake or reservoir may also be caused by eutrophication (Fig. 2). This results from the changes in the water and food quality together with decreased oxygen concentration which often alter the composition of the fish fauna from more to less desirable species. Nevertheless, yields of certain species of fish tend to increase as eutrophication increases since there is more food available. However, oxygen depletion and high ammonia concentrations under hypereutrophic conditions can lead to decreases in fish yields as eutrophication rises.

Fig. 2: Relationship between number of species and volume of chlorophyll a.

In the following Table 1 the general effects of eutrophication in the aquatic environment are presented.

      Table of Contents

  • Brochure
  • IETC Brochure

  • International Year of Forests
  • International Year of Forests

  • World Environment Day
  • ??????

  • UNEP Campaign
  • UNite to Combat Climate Change