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Eutrophication is defined as the enrichment of a habitat or environment with inorganic materials including phosphorus and nitrogen that support and encourage the growth of plants and algae in the affected environment. It is usually characterized by excessive plant and algal growth in water bodies due to the increased availability of one or more limiting growth factors such as nitrogen and phosphorus needed for photosynthesis. Eutrophication is very common in water bodies including lakes, streams, rivers and ponds; and it supports the growth of aerobic photosynthetic organisms in the affected water body. The unusual enrichment of aquatic habitat with inorganic materials that support the overgrowth of photosynthetic organisms and plants leads to the formation of algal bloom – which changes the aesthetics of the affected water body and makes it unfit for domestic or industrial uses.

An algal bloom is a rapid increase in the population of algae in a particular water system. Algal blooms result when water temperatures are warm and when nutrients, such as nitrogen and phosphorus, are present in the water. Some algal blooms are the result of the introduction of an excess amount of nutrients (particularly phosphorus and nitrogen) into water bodies and higher concentrations of these nutrients in water causes increased growth of algae and green plants. Algal blooms causes the discolouration of water bodies (i.e. turn water bodies green); and they prevent oxygen from penetrating effectively into water systems, thus killing aquatic life (Figure 1). The discolouration of the water system is as a result of the high density of pigmented (dead) algal cells that form in the water. The decay of organic matter in the body of water leads to the death of algae that eventually cause algal bloom to form on the surface of the water (Figure 2). Algal blooms can cause taste-and-odor problems in drinking water, and reduction of the aesthetic and recreational value of water systems or bodies. Algal blooms occur in both freshwater and marine habitats. The colours usually observed in algal bloom formations are green, yellowish-brown, or red.

Chemical fertilizers and organic fertilizers are often used in farming, and when fertilizers are applied in excess, they run-off into nearby water bodies causing an increase in nutrient levels. This increase in nutrient level of the receiving water body causes phytoplankton’s and other photosynthetic organisms to grow and reproduce more rapidly, resulting in algal blooms. Algal blooms disrupts normal ecosystem functioning and causes many environmental problems. The algae in the bloom use up all the oxygen in the water, leaving none for other marine life; and this result in the death of many aquatic organisms such as fish, which need the dissolved oxygen in the water to live. Algae blooms block or prevent sunlight from reaching photosynthetic marine plants under the water surface; and some poisonous algae produce toxins that are harmful to higher forms of life including humans.

Figure 1. Algal blooms in water. Runoffs from agriculture and development, pollution from septic systems and sewers, and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems; and this encourages the formation of algal blooms. Algal blooms present many problems for aquatic ecosystems and even for the general public. Water systems affected by algal blooms cannot be used or domestic or industrial purposes; and such water bodies are foul-smelling due to the death of aquatic animals killed from suffocation or lack of oxygen. Nitrogen and phosphorus are found naturally in low quantities in healthy water systems, and they are essential for aquatic plants and other forms of aquatic life. However, the introduction of excess nitrogen and phosphorus into water systems through human activities such as discharge of municipal and industrial effluents and washing-off of fertilizers can cause an overgrowth of algae in a short period of time, faster than even the ecosystems can handle.

Eutrophication is a threat to potable drinking water sources, fishing activities and recreational activities in water systems. Thus, it is critical to prevent its occurrence in order to retain and sustain the aesthetics of our water systems. The primary cause of eutrophication is an excessive concentration of plant nutrients originating from agriculture and sewage treatment. Eutrophication can be prevented by reducing the excess loading or introduction of limiting plant nutrients especially phosphorus and nitrogen into water systems – since these substances encourages the growth of algal blooms that makes water bodies to lose their perceived aesthetic values in the face of eutrophication.      

Figure 2. Eutrophication processes. Nitrogen and phosphorus are growth factors required for the optimal growth of plants. However, these important nutrients required for plant growth are limiting in aquatic environments including freshwater and marine habitat. But human activities and some natural causes, increases the amount of these growth factors in the water systems, thus spurring the process of photosynthesis that causes phytoplanktons and algae to grow in a sporadic manner. The introduction of inorganic substances such as nitrogen and phosphorus into a body of water causes an increase in the overgrowth of phytoplanktons and other photosynthetic organisms. This leads to hypoxia (a condition in which there is an inadequate supply of oxygen due to lack of oxygen) as a result of reoccurring cyanobacterial (algal) bloom that prevent atmospheric oxygen from penetrating the water system. When algal blooms die, microbial decomposition severely depletes the amount of dissolved oxygen in the water, and this condition creates hypoxia – in which the oxygen required to support aquatic life is insufficient. Generally, eutrophication has grave consequences for drinking water sources, recreational waters and fishery activities due to the loss of the aesthetics of the affected water body. Eutrophication reduces the clarity and quality of affected water bodies; and algal blooms produces harmful toxins that poses public health risk.


            Eutrophication is majorly caused by the unusual increase of nutrient supply to a particular water body. And this phenomenon makes the water body more eutrophic (i.e. richer in nutrients such as phosphorus and nitrogen that support the growth of aerobic photosynthetic organisms such as phytoplanktons and/or cyanobacteria). Both human activities and natural occurrences contribute to the increase in the nutrient enrichment of freshwater habitat and marine ecosystems. The most common biological effects of increased nitrogen and phosphorus supplies on aquatic ecosystems is increase in the abundance of the growth of algae, cyanobacteria and aquatic plants which all culminate to the formation of algal bloom in water systems. This leads to sever consequential aesthetic loss of the quality of the affected water body, making it unfit for industrial, domestic or recreational activities. The supply of nutrients that leads to the eutrophication of a particular water body is caused by several factors as elucidated in this section.      

  • Municipal wastewater effluents.
  • Effluents from industrial wastewater.
  • Leachate and runoff from waste/refuse disposal sites.
  • Runoff from oil fields and mines.
  • Runoff of fertilizers from farms into water systems.
  • Runoff from construction or building sites.
  • Runoff or overflows from sanitary sewers.
  • Deposition of atmospheric substances over a particular water surface.
  • Runoff from failed septic tanks.
  • Leakage from septic tank. 
  • Runoff from farms and other lands where agricultural practices such as irrigation are carried out.
  • Runoff from animal grazing sites and lands.
  • Runoff from abattoirs and animal feedlots.


            The excessive addition or introduction of inorganic chemicals such as nitrogen and phosphorus into a body of water leading to eutrophication has far-reaching effects on the biology and chemistry of water bodies including lakes, streams, rivers and ponds. Water systems that receive unusual amount of nitrogen and phosphorus from either natural sources or human activities lose their aesthetics and usefulness due to the growth of algal blooms. The effects of eutrophication on water bodies are highlighted in this section.

  • Eutrophication pollutes aquatic ecosystems including rivers, streams, lakes and ponds. 
  • Eutrophication leads to increased death of aquatic organisms especially fishes.
  • It reduces the clarity of water.
  • It impacts negatively on the odour, taste and quality of water.
  • Eutrophication poses potential health risks to water supplies especially people that use water covered with algal blooms.
  • It leads to increase in the pH of the affected water body.
  • Eutrophication depletes the amount of dissolved oxygen in water.
  • It decreases the aesthetic value of the water body.
  • Eutrophication impacts negatively on the environment especially on the affected water body which it renders useless for domestic and industrial usage.    
  • It impacts negatively on aquatic plants by changing their physiology and species compositions.
  • Eutrophication changes the species of fish in the affected water body to less desirable species of fish.
  • Eutrophication alters the food web or food chain of aquatic ecosystems.
  • It prevents the usage of affected water bodies for swimming and other recreational activities.
  • It disrupts water treatment processes such as flocculation and chlorination.
  • Eutrophication leads to loss of aquatic biodiversity.

Further reading

Jee C and Shagufta (2007). Environmental Biotechnology. APH Publishing Corporation, Darya Ganj, New Delhi, India.

Latha C.D.S and Rao D.B (2007). Microbial Biotechnology. First edition. Discovery Publishing House (DPH), Darya Ganj, New Delhi, India.

Maier R.M, Pepper I.L. and Gerba C.P (2000). Environmental Microbiology. Academic Press, San Diego.

Mishra B.B, Nanda D.R and Dave S.R (2009). Environmental Microbiology. First edition. APH Publishing Corporation, Ansari Road, Darya Ganj, New Delhi, India.

Paul E.A (2007). Soil Microbiology, ecology and biochemistry. 3rd edition. Oxford: Elsevier Publications, New York.

Pelczar M.J., Chan E.C.S. and Krieg N.R. (2003). Microbiology of Soil.  Microbiology, 5th Edition. Tata McGraw-Hill Publishing Company Limited, New Delhi, India.

Pepper I.L and Gerba C.P (2005). Environmental Microbiology: A Laboratory Manual. Second Edition. Elsevier Academic Press, New York, USA. 

Roberto P. Anitori (2012). Extremophiles: Microbiology and Biotechnology. First edition. Caister Academic Press, Norfolk, England.

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