Nutrients are essential chemical elements in food, necessary for the growth and survival of plants and animals. Nitrogen and phosphorus are particularly crucial and abundant among numerous nutrients. However, an excessive supply of it can harm water quality and biodiversity, a process known as eutrophication.
Eutrophication originates from the Greek words eutrophos, “eu” meaning well, and “trophos” meaning fed. Thus, the word eutrophication means well-fed or nutrient-rich. Further, eutrophication can be defined as a process in which a body of water, such as a lake, river, or coastal area, becomes overly enriched with nutrients, particularly nitrogen, and phosphorus, leading to phytoplankton’s plentiful growth. It has been found that more than 30% of lakes and reservoirs worldwide are affected by eutrophication. One of Nepal’s most familiar examples of eutrophication is the excess phytoplankton growth in the few lakes of Pokhara.
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Causes of Eutrophication
Eutrophication can be caused by human activities or by natural phenomena. Although eutrophication is also caused by natural wonders but human activities contribute the most that are as follows;
- Agricultural drainage: Nowadays, for productive cultivation, human uses fertilizer that is rich in nitrogen and phosphorus compound; when rain occurs, these compounds drain into the lakes or ponds. As a result, causing increasing in its nutrient that, in the end, results in eutrophication.
- Urban drainage: Urban areas contribute to eutrophication by discharging sewage, stormwater runoff, and industrial waste. Such waste, rich in nutrients, finds its way to the water bodies.
- Deforestation: Deforestation is a significant factor leading to soil erosion and sedimentation, transporting nutrients into water bodies and promoting eutrophication.
- Aquaculture and Fish Farming: Using excess nutrients to cultivate fish in a lake or a pond also increases water bodies’ nutrient content, leading to eutrophication.
- Atmospheric deposition: Emission of nitrogen from industries, burning of fossil fuel, animal breeding, etc., can be deposited into land and subsequently to water bodies. This emission results in nutrient enrichment (eutrophication).
Process of Eutrophication
Eutrophication occurs in four steps. These include;
- Excess nutrients: The process begins by adding excessive nutrients, particularly nitrogen, and phosphorus, into a water body. This addition can occur naturally or due to human activities such as agricultural drainage, sewage discharges, industrial waste, and atmospheric deposition.
- Algae bloom: Algae are the first organisms to take advantage of the increased nutrient availability and start to increase rapidly. This results in the overgrowth or bloom of algae in the water body. These blooms can manifest as thick green or brown mats on the water’s surface.
- Oxygen depletion: When algae form a thick layer on the water’s surface, it blocks sunlight from entering the water and uses oxygen. Eventually, water becomes oxygen-depleted (such a condition is also known as hypoxia).
- Dead zones: The excessive growth of algae and the subsequent oxygen depletion can negatively impact sensitive species, leading to a decline in biodiversity and the dominance of certain species adapted to high-nutrient conditions.
Types of Eutrophication
There are two main types of eutrophication: natural eutrophication and cultural (or anthropogenic) eutrophication.
A process occurring as a lake or river ages hundreds or thousands of years is called natural eutrophication. It happens gradually by addition of nutrients from decaying plants and animals in a water body. This type of eutrophication occurs slowly and results from the natural aging of the water body.
It is a type of eutrophication that is caused by human activities and the input of nutrients. This type occurs through increased nutrients, especially nitrogen, and phosphorus, in the water body due to human activities such as fertilizer use, sewage discharge, etc. It is a rapid type of eutrophication that is accelerated by human activities.
Eutrophication causes various effects on human health, the ecosystem, socio-economic sector, which are as follows;
- Due to the excess growth of phytoplankton, water transparency decreases, and water turbidity increases. The water may become unsuitable for drinking even after treatment.
- Phytoplankton utilizes all of the oxygen present, which results in the depletion of oxygen content in water bodies.
- The presence of excess nutrients in water bodies promotes algal growth. This increased growth leads to the formation of dense algal blooms on the water surface, which can reduce light penetration and limit oxygen availability for other organisms.
- It causes a change in the aesthetic value of the water body.
- It causes a decrease in species diversity in the water body due to the algal bloom.
- Some harmful algae can also produce toxins. The release of cyanotoxins by cyanobacteria is said to cause the deaths of wild animals, farm livestock, pets, fish, and birds in many countries.
- Increased vegetation may impede water flow and the movement of boats. This prevention can lead to a loss of tourism/recreation (swimming, boating).
- It can also lead to the disappearance of commercially important species such as trout.
Eutrophication is one of the most widespread problems faced by freshwater systems that need to be controlled or managed by implementing various strategies and practices that are as follows;
- Management of source: Nitrogen and phosphorus are significant nutrients that get excess in water bodies through human activities such as; sewage runoff and agricultural drainage. Therefore, to prevent eutrophication, best management practices in agriculture, such as reducing excessive fertilizer application, implementing buffer zones, and optimizing irrigation techniques, can help minimize nutrient runoff.
- Application of physical and chemical methods: Several methods are introduced to control eutrophication. These methods include; the use of algicides that kill the algae, the use of chemical flocculants, and the use of modified clay (e.g., nano silicate platelets, lanthanum-modified clay, etc.). Similarly, destratification, physical removal or harvesting, and sediment dredging are also possible to control methods, although they contain the significant drawback that they are costly.
- Public awareness: Raising awareness among the general public, policymakers, and stakeholders about the causes and impacts of eutrophication can help foster support for implementing necessary measures.
- Biological control: Phytoplankton can be controlled more inherently. One of the methods is using aquatic macrophytes like Eichhornia crassipes and Salvinia auriculata, which helps decrease the concentration of nitrogen and phosphorus compounds in water. Besides this, duckweed can also be used to treat wastewater.
- S. R. Carpenter, D. Ludwig and W. A. Brock . Management of Eutrophication for Lakes Subject to Potentially Irreversible Change. JSTOR, 9(3):751-771, 1999.
- T. Wagner, L.E. Erickson. Sustainable Management of Eutrophic Lakes and Reservoirs. Journal of Environmental Protection. 2017,8(4). DOI: 10.4236/jep.2017.84032