Eutrophication: Causes, Classification, Impacts, Process

Eutrophication is the excessive enrichment of a water body with nutrients, specifically nitrogen and phosphorus. resulting in the abundant proliferation of basic plant organisms. The proliferation of algae and plankton in a water body serves as an indication of this phenomenon. Eutrophication is a significant environmental issue due to its negative impact on water quality and the reduction of dissolved oxygen levels in aquatic ecosystems. Eutrophic waters have the potential to transform into “dead zones” where life cannot be sustained.

Eutrophication Causes, Classification, Impacts, Process
Eutrophication Causes, Classification, Impacts, Process [Image source: thespruce]

Certain lakes are naturally eutrophic and may undergo progressive eutrophication as they mature. Eutrophication is often linked to human activities, particularly the introduction of plant nutrients. This has led to changes in communities and the deterioration of water quality in different freshwater ecosystems. Eutrophication’s significance has increased due to population growth, agricultural expansion, and its detrimental effects on natural ecosystems. Anthropogenic activities such as deforestation, global warming, ozone layer depletion, and large-scale environmental disturbance are now recognized as significant factors.

Aquatic ecosystems provide a habitat for a wide range of plant and animal species, encompassing both simple and complex organisms. Eutrophication disrupts ecosystem equilibrium by promoting the growth of simple plant species. This process leads to a significant decline in ecological diversity within ecosystems as it results in the extinction of numerous species that are considered desirable.

Causes of Eutrophication

Nutrient pollution in freshwater and coastal areas originates from various sources such as agriculture, aquaculture, septic tanks, urban wastewater and rainwater runoff, industry, and fossil fuel combustion. Nutrients are introduced into aquatic ecosystems through three main pathways: atmospheric deposition, surface water runoff, and groundwater inflow.

Chemical Fertilizer

  • The global consumption of synthetic nitrogen and phosphate fertilizers experienced a significant increase, with a growth rate of sevenfold and threefold, respectively, during the period from 1960 to 1990. Research findings indicate that there is a tendency to overutilize fertilizers.
  • The removal of excessive nutrients takes place through processes such as nitrogen volatilization into ammonia, surface discharge, and groundwater leaching. Approximately 20% of nitrogen fertilizer is lost through surface runoff and groundwater leaching.
  • The application of synthetic nitrogen fertilizer and manure onto agricultural fields has the potential to cause volatilization. The process of volatilization has the potential to result in the loss of approximately 60% of the nitrogen that is provided to crops.
  • However, it is typically observed that volatilization leads to a loss ranging from 40% to 60% of the nitrogen. Streams receive a portion of volatilized ammonia through atmospheric deposition. Soil erosion originating from agricultural regions results in the depletion of phosphorus that is bound to the soil.


Over the last century, cattle farming has changed dramatically, elevating nutritional levels. Animal production is growing farther from feedstock suppliers, making manure spreading tougher. These processes produce a lot of manure, which is used as fertilizer, and kept in feedlots, or lagoons. Overapplying manure to crops increases nutrient runoff and leaching.


Aquaculture—fish farming—is a growing source of nutrient pollution.

  • Aquaculture production increased 600% from 8 million tons in 1985 to 48.2 million tons in 2005.
  • 43% of aquaculture production is in marine or brackish waters, while the rest is in freshwater lakes, streams, and man-made ponds.
  • Marine fish and shrimp aquaculture usually take place in net pens or cages in enclosed bays. Excrement, uneaten food, and other organic waste on these farms produce high nitrogen and phosphorus levels.
  • Aquaculture facilities that directly release fertilizer waste into neighboring waters can harm aquatic ecosystems.
  • Aquaculture produces 42–66 kilos of nitrogen and 7.2–10.5 kilograms of phosphorus for every ton of fish.

Urban And Industrial Waste

The most prevalent urban source of nutrient pollution is human sewage, though its importance varies by region and country. Sewage is estimated to contribute 12 percent of riverine nitrogen input in the United States, 25 percent in Western Europe, 33 percent in China, and 68 percent in the Republic of Korea.

  • Human sewage is the primary urban source of nutrient pollution, although its significance may differ across regions and countries. Sewage is a significant source of riverine nitrogen input in various regions. Sewage is estimated to contribute 12 percent of riverine nitrogen input in the United States, 25 percent in Western Europe, 33 percent in China, and 68 percent in the Republic of Korea.
  • Waste percolates through the soil in septic systems. They discharge 14 kg of nitrogen per system annually, most of which enters groundwater or neighboring surface waters.
  • Rainfall carries nutrients from home lawns and impermeable surfaces to nearby rivers and streams. CSOs worsen urban stormwater runoff. Combined Sewer Overflows (CSOs) are pipelines that carry rainwater, home wastewater, and industrial wastewater. The combined sewer overflow (CSO) system and wastewater treatment plant can reach capacity during heavy rain or snowmelt. Thus, surplus wastewater—including untreated sewage—is dumped into nearby watercourses.

Fossil Fuel Consumption

  • The combustion of fossil fuels results in the emission of nitrogen oxides (NOx) into the Earth’s atmosphere.
  • Nitrogen oxides (NOx) play a significant role in the generation of smog and the occurrence of acid rain.
  • Nitrogen oxides (NOx) can be reintroduced to terrestrial and aquatic environments via precipitation in the form of rain and snow, known as wet deposition, or through the process of dry deposition, where they settle out of the atmosphere.
  • The main contributors to nitrogen oxides (NOx) are coal-fired power plants and vehicular emissions, specifically those from cars, buses, and trucks.

Classification of Eutrophication

The phenomenon of eutrophication can be classified into two distinct types, distinguished by their underlying causes. Both of these categories are elucidated in the present subsection.

Anthropogenic Eutrophication

  • Humans produce anthropogenic eutrophication. Humans fertilize fields, golf courses, lawns, etc. Rain washes fertilizers into lakes and rivers.
  • Fertilizers feed algae and plankton, atrophying the water. Overpopulation drives industrial and agricultural expansion, which deforests. This makes soil disintegrate more easily, depositing soil in bodies of water. Eutrophication and water body ecological degradation can result from phosphorus-rich soil.
  • The nutrients in sewage lines and waste from factories accelerate eutrophication in water bodies.

Natural Eutrophication

  • Natural eutrophication enriches water bodies excessively. Floods are capable of transporting soil nutrients into lakes and rivers. Algae and other basic plants flourish excessively in these nutrient-rich water bodies.
  • Natural eutrophication is slower than anthropogenic. Climate further impacts this process. Global warming may enhance it.

Process of Eutrophication

The phenomenon of eutrophication typically advances through four identifiable stages, which will be comprehensively looked over in the parts that follow.

Over Fertilization

  • Excessive application of fertilizers in aquatic environments leads to the proliferation of algae on the water’s surface. When fertilizer is introduced into water bodies, it serves as a nutrient source for the growth and proliferation of algae.
  • Eutrophication, which stimulates algae growth, is a common cause of the occurrence of dense green blooms in water bodies. However, a notable concern associated with algae is its capacity to absorb sunlight, thereby impeding its penetration to lower depths.
  • Cyanobacteria, commonly referred to as blue-green algae, have the potential to pose risks to both plant life and human health. For instance, ingestion of the substance can lead to toxicity. The proliferation of this particular species of algae has emerged as a significant ecological concern across various regions globally.

Algae Blooms And Hypoxia

  • The growth of algae leads to the obstruction of sunlight penetration in aquatic ecosystems such as ponds, lakes, and rivers. The phenomenon of eutrophication perpetuates itself through a cyclic process of algal bloom, wherein an increased influx of nutrients into the water leads to the release of additional nutrients.
  • When exposed to sufficient sunlight, algae engage in the process of photosynthesis, wherein they generate oxygen and subsequently release it into the surrounding water. However, in the absence of light, algae cease their production of oxygen and instead engage in its consumption.
  • Upon the death of algae, bacteria initiate the process of decomposition, thereby consuming oxygen for the purpose of respiration. Over time, the process of decomposition leads to a reduction in the oxygen content of the water. Over a period of time, this phenomenon leads to a reduction in the oxygen content of the water.

Dead Zones

  • Dead zones refer to regions in the Earth’s oceans and lakes that are characterized by low levels of oxygen, also known as hypoxic conditions. Due to the essentiality of oxygen for the survival of the majority of organisms, only a limited number are capable of enduring hypoxic environments. Hence, these regions are commonly referred to as dead zones.
  • Dead zones are a consequence of eutrophication, a phenomenon characterized by excessive nutrient accumulation, particularly phosphorus, and nitrogen, within a water body.
  • On a global scale, scientists have identified a total of 415 dead zones. The prevalence of hypoxic areas has exhibited a substantial escalation over the course of the last five decades, with the number of documented instances rising from approximately 10 in 1960 to a minimum of 169 by 2007.

Impacts of Eutrophication

Impact on Aquatic Ecosystem

  • Aquatic ecosystems undergo a phenomenon called algal blooms when there is an influx of nutrients, resulting in the rapid growth of phytoplankton and other photosynthetic plants. The proliferation of algal blooms restricts the availability of dissolved oxygen necessary for respiration among various aquatic animal and plant species. Oxygen depletion occurs as a result of the decomposition of algae and plant life.
  • When the concentration of dissolved oxygen decreases to hypoxic levels, the organisms inhabiting the aquatic environment, including shrimp, fish, and other aquatic biota, experience suffocation and ultimately perish. In severe instances, the anaerobic conditions foster the proliferation of bacteria that generate toxins, which pose a lethal threat to marine mammals and avian species. This phenomenon can lead to the formation of aquatic dead zones and result in a reduction in biodiversity.

Water Quality Deterioration

  • Algal blooms possess a high level of toxicity, and the presence of anaerobic conditions in the water further facilitates the proliferation of more toxic bacteria. The result is a significant degradation of water quality and a decrease in the accessibility of potable water.
  • The proliferation of algal blooms and photosynthetic bacteria in surface waters can lead to the obstruction of water systems, thereby restricting the accessibility of piped water.
  • In relation to this matter, the occurrence of toxic algal blooms has resulted in the closure of multiple water supply systems worldwide.
  • In 2007, a significant incident occurred in Wuxi, China, where over 2 million residents experienced a prolonged disruption in accessing piped drinking water. A severe algal bloom outbreak on Lake Taihu was the root of this unfortunate situation.

Increase In the Water Toxicity

  • Cyanobacteria, commonly known as dinoflagellates, are responsible for the occurrence of red tide. These organisms release potent toxins that exhibit significant toxicity even at minimal concentrations in the water.
  • The proliferation of plant growth in aquatic environments leads to the establishment of anaerobic conditions, which consequently causes a twofold increase in the presence of toxic compounds.
  • Even at the lowest concentration, ingestion of this substance in drinking water can lead to fatalities in both humans and animals. The presence of algal blooms in freshwater ecosystems has the potential to pose a significant threat to the health and well-being of livestock.
  • The presence of toxic compounds in the food chain can result in detrimental health effects, including the development of cancer.

Impact on the Fishing Industry

  • One of the primary attributes of eutrophication, as previously mentioned, is the heightened proliferation of small suspended flora, such as algae and photosynthetic bacteria, along with the formation of substantial and compact masses of floating vegetation, such as Nile cabbage and water hyacinths.
  • The occurrence of this phenomenon in aquatic environments poses a threat to the sustainability of fishing activities. The act of deploying fishing nets in aquatic environments becomes increasingly challenging due to various factors, such as the presence of aquatic vegetation that restricts the movement of boats and other fishing vessels.
  • Cyanobacteria, commonly known as dinoflagellates, are responsible for the occurrence of red tide. These microorganisms release potent toxins that exhibit significant toxicity in aquatic environments, even at minimal concentrations.

Invasion of New Species

  • The process of eutrophication can result in an increase in the abundance of a limiting nutrient in a water body, leading to alterations in the species composition of the aquatic ecosystem and its surrounding environment.
  • In the event that a water body experiencing nitrogen deficiency undergoes a sudden enrichment of this element, it is plausible that numerous competitive species may migrate to the water body and potentially outcompete the original inhabitants of the ecosystem.
  • An illustrative instance of a newly introduced species thriving in eutrophic environments is the common carp, which has successfully acclimated to such conditions.

Frequently Asked Questions (FAQ)

What are the underlying factors contributing to eutrophication?

Eutrophication is a phenomenon that arises when a water body experiences an excessive accumulation of nutrients. Eutrophication can be attributed to a range of factors, including the overuse of fertilizers, the release of untreated sewage, the presence of detergents containing phosphorous, and the discharge of industrial waste.

What exactly is natural eutrophication?

Natural eutrophication refers to a form of eutrophication wherein water bodies become excessively enriched with nutrients as a result of natural occurrences such as floods. Anthropogenic eutrophication exhibits a higher rate of acceleration in comparison to the phenomenon under consideration.

Can you swim in a eutrophic lake?

The eutrophic body of water is rich in nutrients. The turbidity of the water is attributed to the presence of abundant aquatic vegetation, organisms, as well as algae and plankton that are suspended within it. Hence, swimming in a eutrophic lake is either impossible or exceedingly challenging.

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  • Lürling, Miquel; Oosterhout, Frank van (2013). “Controlling eutrophication by combined bloom precipitation and sediment phosphorus inactivation”. Water Research.
  • Paerl, Hans W.; Valdes, Lexia M.; Joyner, Alan R.; Piehler, Michael F.; Lebo, Martin E. (2004). “Solving problems resulting from solutions: Evolution of a dual nutrient management strategy for the eutrophying Neuse River Estuary, North Carolina”. Environmental Science and Technology.

About Author

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Jyoti Bashyal

Jyoti Bashyal is a dedicated researcher specializing in computational chemistry, enzyme inhibition, in-vitro analysis, and sustainable chemistry. Alongside her scientific pursuits, she finds immense joy in creative writing, approaching her work with unwavering determination and a positive outlook. With an open mind and a thirst for knowledge, she embraces new opportunities to learn and grow, embodying the spirit of curiosity and continuous self-improvement.

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