Eutrophication- Definition, Causes, Types, Process, Examples

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Eutrophication Definition

Eutrophication is the condition of a gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in a water body resulting in excessive plant and algal growth.

  • Eutrophication is one of the most widespread challenges faced by freshwater systems and also has a long history in terms of water management.
  • It is a natural process that occurs in different water sources over centuries, but human activities have accelerated the process by increasing the rate and extend of eutrophication.
  • The condition arises as to the productivity or fertility of aquatic ecosystems increases due to the rise in the organic material that can be broken down into simpler usable nutrients.
  • The most prominent effect of eutrophication of water resources is the formation of blooms of foul-smelling phytoplankton. These microorganisms reduce water clarity and might degrade the water quality.
  • The growth of such blooms disturbs light penetration and destroys the growth of plants in coastal zones.
  • Eutrophication threatens the ecological stability of the system as the nutrient enhancement can interact with source-specific conditions like the presence of other contaminants and infectious agents.
  • Eutrophic water resources usually harbor fewer larger animals like fishes and birds when compared to non-eutrophic waters.
  • Destruction of water quality and loss of ecological diversity is among the most harmful consequences of eutrophication. These have redirected the focus of various studies on the control of eutrophication.
  • The excess of nutrients in water sources usually results due to the runoff from land that carries products from terrestrial ecosystems into the sources.
  • For decades, eutrophication has been considered an irreversible process, but for the last few years, eutrophication in several lakes has been reversed by managing both the human nutrient emission and cutting off nutrient loads to the sources.
  • The growth of algal blooms like other plants requires nutrients in certain ratios that are different from the natural concentrations.
  • Eutrophication is an urgent threat as more than 30% of the lakes and reservoirs in the world have been affected by eutrophication.


Image Source: USEPA Environmental-Protection-Agency.

Causes of Eutrophication

Natural eutrophication is a natural process that occurs through centuries as a part of the cycle of the ecosystem. However, the process has been enhanced by human activities which cause the process to occur at a much faster rate with immediate consequences. The primary cause of eutrophication is the entry of a large amount of easily available nutrients to the water resources, which increases the fertility and excessive growth of various plants and algae. Some of the factors that enhance the process of eutrophication by increasing the nutrient content of the water resources are;

1. Fertilizers

  • The use of fertilizers containing phosphates and nitrates in order to increase the productivity of crops is one of the primary causes of cultural eutrophication.
  • The fertilizers used on the land near water resources eventually are ultimately deposited into the water resources during rain and other natural processes.
  • The runoffs from different areas eventually make their way into lakes, rivers, and oceans, which increases the nutrient availability of the water resources.
  • Since the fertilizers contain easily available nutrients, they are taken up by planktons, algae, and aquatic plants of the aquatic ecosystem.
  • The photosynthetic activity of the water resource increases, which further increases the concentration of organic matter in the ecosystem.
  • The increase in nutrient content causes the formation of a dense growth of algal blooms and plant life, resulting in eutrophication.

2. Concentrated animal feeding operations

  • Concentrated animal feeding operations are agricultural practices where a large number of animals are confined to a certain area for a certain period of time in order to increase the productivity and quality of the animals.
  • Operations like these produce millions of tons of manure each year, all of which eventually find their way into water resources.
  • The manure primarily contains nitrogen and phosphorus, both of which are essential elements of algal blooms.
  • Nitrogen and phosphorus occur as limiting factors in algal blooms as these are present in the least proportion in the molecular formula of algae (C106H262O110N16P).
  • Thus, in many ecosystems, the biomass of phytoplankton is limited by the low availability of phosphorus and nitrogen.
  • However, the release of animal manure rich in nitrogen and phosphorus to such ecosystems causes the increased production of phytoplankton, inducing eutrophication.

3. Sewage and Industrial waste Discharge

  • In many developing countries, household sewage, as well as industrial discharge, are released into water resources like lakes, ponds, and rivers.
  • The wastewater entering from different sources tend to have high amounts of chemical nutrients which stimulate the dense growth of algal blooms in such resources.
  • The wastewater from households and communities has high concentrations of nitrogen and phosphorus which have a direct effect on the health of the ecosystem.
  • Industrial sewage, even when treated, contains small concentrations of many chemicals. Over time, the continuous deposition of such chemicals leads to increased nutrient availability in the water resource, eventually resulting in eutrophication.

4. Environmental Factors

  • Different environmental factors like temperature, salinity, and atmospheric conditions play essential roles in eutrophication, but the exact mechanism of their influence is not yet completely understood.
  • Algal blooms usually occur between the temperature of 23°C and 28°C and salinity level of 23% and 28%. The changes in these factors affect the rate of algal growth, especially when the temperature increases and the salinity decreases.
  • The concentration of carbon dioxide also affects the growth of cyanobacteria as these microbes can utilize low levels of carbon dioxide and becomes buoyant to keep themselves on the surface of the water and obtain sunlight.
  • The free carbon dioxide concentrations in water also affect the pH of the water resource, further influencing the growth of different microbes.
  • Similarly, the growth is also favored by increasing light intensity with 4000 lux being the most favorable intensity.

Eutrophication Types

Eutrophication can be divided into two types based on the root cause of the process;

1. Natural Eutrophication

  • Natural eutrophication is a process that occurs as a result of a gradual buildup of nutrients and organic matter in water resources over a very long period of time.
  • Natural eutrophication can take up to 100 years as the natural deposition and increase in organic matter deposits requires a long time.
  • The process of natural eutrophication is enhanced by natural conditions like floods and landslides, where the organic matter from the land is washed off into water resources.
  • Besides, environmental factors like temperature, carbon dioxide concentration, and light also play essential roles in natural eutrophication.
  • The process of natural eutrophication begins in an oligotrophic water resource where the productivity increases as the nutrients accumulate to reach a stable state of eutrophy.
  • As the process of nutrient accumulation and utilization continues, the state might shift towards eutrophication, but it takes hundreds of years. The time period depends on the nature of the water resource, the land areas around the resource, and the climate.

2. Cultural (anthropogenic) Eutrophication

  • Cultural eutrophication of anthropogenic eutrophication is the process of accumulation of excess nutrients in water ecosystems as a result of human activities.
  • Cultural eutrophication is the process that speeds up natural eutrophication, resulting in severe conditions within a short period of time.
  • The primary cause of cultural eutrophication is the series of human activities that primarily increase the concentration of phosphorus and nitrogen in the ecosystem.
  • This type of eutrophication occurs within a shorter period of time and usually has significant consequences to the health of all living beings.
  • Many human activities like overfertilization, industrial and agricultural expansion and the release of sewage into water resources are among the prime causes of the process.
  • Cultural eutrophication can occur in both fresh water and saltwater bodies, among which shallow waters are the most susceptible.
  • Shallow lakes and ponds experience wind waves which cause the suspension of huge nutrients to the overlying water.
  • Eutrophication restricts the use of such water resources for various purposes like drinking, aquatic life, and industrial use as a result of the increased growth of undesirable algae.

Process / Steps of Eutrophication

The overall process of eutrophication can be explained in the following steps;

1. Accumulation of nutrients

  • The first step in the process of eutrophication is the accumulation of nutrients in different ecosystems.
  • These nutrients are usually rich in nitrogen and phosphorus, which are the two limiting elements in an algal bloom.
  • The accumulation can be obtained by various natural as well as anthropogenic means. Natural processes like soil erosion, landslide, rain, and storms move the soil from adjacent land to water resources which significantly increase the nutrient content of the respective resources.
  • Anthropogenic activities like the release of domestic and industrial sewage into water resources and the expansion of agricultural and residential areas directly or indirectly result in the accumulation of nutrients in the nearby water resource.
  • At the beginning of the process, the aquatic ecosystem is oligotrophic with fewer nutrients available. As the concentration of nutrients begins to grow, the microbes and plant species utilize the nutrients to increase their productivity.

2. Increase in productivity

  • The increased nutrient concentration in the aquatic system causes an increased production of phytoplankton as well as plant species.
  • An aquatic ecosystem consists of a diverse group of microorganisms that can utilize a variety of complex as well as simple nutrients. 
  • The biomass of the microorganisms as well as plant species increased significantly in the ecosystem. As the organisms begin to die, further biomass is accumulated in the ecosystem.
  • The process continues as long as the concentration of nutrients in the ecosystem is sufficient. 

3. Algal bloom formation

  • With the increased productivity of algae, algae blooms begin to form on the surface of water resources.
  • Algal bloom formation also results in the vicious algal bloom cycle which releases more nutrients in the water.
  • Algae in water receive enough sunlight, which enables them to produce oxygen as well as nutrients via photosynthesis. This causes an increase in algal population, resulting in algal blooms on the surface of the water.
  • Eventually, these algae begin to cover most of the water surface, causing a lack of penetration of sunlight into the water. This affects the process of photosynthesis, resulting in oxygen depletion.
  • As a result, algae die which are then decomposed by various bacteria using up any remaining oxygen for respiration.
  • All of this eventually decreases the health of aquatic life on such water resources as well as on the quality of water.

Examples of Eutrophication

Eutrophication of Potomac river

Eutrophication of Potomac river

Figure: Eutrophication of the Potomac River evident from the bright green water, caused by a dense bloom of cyanobacteria. Image Source: Alexandr Trubetskoy.

  • The Potomac River in Washington County in Maryland is one of the classic examples of eutrophication of water resources.
  • The river was initially identified as being in eutrophic condition based on the analysis of dissolved oxygen values, biological oxygen demand, and the health of the aquatic lives.
  • Various studies found that the nutrient from the land around the river resulted in the accumulation of chemicals like nitrogen and phosphorus.
  • The river is surrounded by land that is primarily used for forest land use, agricultural use and also supports livestock operations.
  • It is believed that the manure from the livestock and the washed off fertilizers from the land are the primary cause of eutrophication.
  • The river still harbors some aquatic life forms like fishes and arthropods, but the biodiversity has been decreasing every year.

Effects/Problems of Eutrophication

Eutrophication is considered a water pollution problem that affects about 30-40% of all the water bodies in the world. In addition to water pollution, there are several other effects of eutrophication on the ecosystem as well as the lives of various living beings. The following are some of the effects of eutrophication;

1. Increased biomass of phytoplanktons

  • Increased biomass of phytoplankton, especially in the form of algal blooms is one of the most prominent effects of eutrophication.
  • The algal blooms might even develop toxic strains of cyanobacteria which harm both aquatic organisms as well as humans.
  • The blooms also result in the reduction of water clarity and a decrease in water quality. Some of the phytoplanktons might even form foul-smelling algal blooms which further affect the ecosystem.

2. Oxygen depletion

  • The formation of algal blooms on water surfaces reduces the availability of sunlight to living beings present in the water source.
  • The lack of sunlight causes a decrease in photosynthesis and the eventual death of plant species present in the water.
  • The dead plants are then decomposed by various bacteria that utilize the remaining oxygen in the water.
  • All of this results in oxygen depletion in the water source, which brings other sets of problems. Lack of oxygen results in an anoxic process that produces various toxic and foul-smelling gases.
  • The oxygen depletion causes the death of various aquatic plants as well as animals.

3. Decrease in biodiversity

  • The biodiversity of aquatic ecosystems decreases as a result of eutrophication. As the population of algae increases as a result of increased nutrients, the population of other living beings like plants and animals decreases.
  • The algal blooms limit the availability of sunlight to bottom-dwelling organisms and result in a decrease in variation of a population.

4. Toxicity

  • Various algal blooms are known as harmful algal blooms which produce a toxic compound that can make its way up the food chain to different trophic levels.
  • Blooms in freshwater can be harmful to livestock when eaten by various animals. Toxic compounds then pass on from the animals to humans, causing a threat to human lives.
  • One of the common examples of algal toxins affecting the human body is shellfish poisoning, where the toxins produced by algae are taken up by shellfish and are transmitted to humans that feed on such shellfishes.

5. Water pollution

  • The formation of algal blooms decreases the clarity of the water, resulting in extensive water pollution.
  • It makes the water unsafe from drinking or any other recreational activities, ultimately decreasing the aesthetic value of the water body.

Solutions of Eutrophication

Because eutrophication affects a large portion of water bodies throughout the world, it is important that the process should be controlled. Since anthropogenic activities are the primary causes of nutrient enrichment and eutrophication, it is important to address such issues. The following are some of the solutions or control measures to minimize eutrophication;

1. Biological Control

  • Phosphorus is one of the important elements that induce the process of eutrophication. Thus, it is important to remove phosphorus from various sources.
  • One of the tools for the removal of phosphorus naturally is through periphytons. These microbes are involved in phosphorus removal by phosphorus uptake and deposition and by filtering phosphorus from water,
  • Various methods of phytoremediation have also been suggested for the effective reduction of toxicity of the water.
  • Aquatic macrophytes like Eichhornia crassipes and Salvinia auriculata decrease the concentration of nitrogen and phosphorus compounds in water. 
  • Besides, duckweeds have also been used for wastewater treatment as these remove effluent from shrimp farm and remove high amounts of ammonia from the water.

2. Reduction of excessive fertilization

  • Fertilizers are one of the important sources of nutrients causing eutrophication thus reducing the use of fertilizers can be a good strategy to reduce nutrient inputs.
  • Optimized use of fertilizer also requires regular soil testing to ensure that optimal fertilizers are used.
  • Similarly, the use of an alternative source of fertilizers that can supply phosphorus in a slow-release form can also be used.
  • In soil with high phosphorus content, other chemicals like potassium and sulfur can be used to achieve the most production.

3. Public Awareness and legislations

  • Public awareness determines the ability of the public to understand the surrounding world, sensitivity to changing environment, and understanding the cause-effect relationship between environment and human behavior.
  • Public awareness can enable the use of organic fertilizers and the treatment of sewage water before releasing into water sources.
  • Besides, different organizations can put forward crucial pieces of legislation in order to reduce the extent of eutrophication.
  • Some of these legislative orders include those by the European Union which aim to protect the water quality of ground and surface waters and to decrease the adverse effects of urban wastewater discharges.


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  • JoAnn M. Burkholder, Patricia M. Glibert. Eutrophication and Oligotrophication, Encyclopedia of Biodiversity (Second Edition). Academic Press, 2013, Pages 347-371.
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  • Potomac River Washington County WQA – Eutrophication Document version: September 30, 2010.

About Author

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Anupama Sapkota

Anupama Sapkota has a bachelor’s degree (B.Sc.) in Microbiology from St. Xavier's College, Kathmandu, Nepal. She is particularly interested in studies regarding antibiotic resistance with a focus on drug discovery.

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