Environmental Chemistry


Eutrophication is the process in which excessive growth of algae occurs in a water body due to excessive minerals and nutrients. This process may end in oxygen depletion of the water body after the bacterial degradation of the algae. One example is an “algal bloom”. Eutrophication is usually induced by the discharge of nitrate or phosphate-containing detergents, fertilizers, or sewage into an aquatic system.



What is Eutrophication?

Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as increased production of algae and aquatic plants, depletion of fish species, general degradation of water quality and other effects that reduce and preclude use”.

Lake eutrophication has become a worldwide problem of pollution. Chlorophyll-a, total nitrogen, total phosphorus, biological or chemical oxygen demand and Secchi depth are the most indicators to gauge lake eutrophication level.

Mechanism of Eutrophication

Eutrophication most ordinarily arises from the oversupply of nutrients, most ordinarily as nitrogen or phosphorus, which results in overgrowth of plants and algae in aquatic ecosystems. After such organisms die, bacterial degradation of their biomass leads to oxygen consumption, thereby creating the state of hypoxia.

The primary limiting factor for eutrophication is phosphate. The presence of phosphorus normally helps in excessive plant growth and decay. It also favours simple algae and plankton and causes a severe reduction in water quality. Phosphorus may be a necessary nutrient for plants to measure and is that the limiting factor for plant growth in many freshwater ecosystems.

  • Cultural Eutrophication: It is the process that increases the rate of natural eutrophication. Because of clearing of land and building of towns and cities, land runoff is accelerated and more nutrients like phosphates and nitrate are supplied to lakes and rivers, then to coastal estuaries and bays.
  • Natural Eutrophication: Eutrophy occurs in many lakes in temperate grasslands. Paleolimnologists now signifies that global climate change, or change in geology, and other external factors are critical in regulating the natural productivity of lakes. The natural action is extremely slow, occurring on geologic time scales.
  • Lakes and rivers: When algae die, it generally decomposes. Therefore the nutrients in it are converted into inorganic form by microorganisms. This decomposition process takes in a lot of oxygen, which reduces the concentration of dissolved oxygen. The depleted oxygen levels successively may cause fish kills and a variety of other effects reducing biodiversity.
  • Coastal waters: Eutrophication may be a common phenomenon in coastal waters. In contrast to freshwater systems where phosphorus is usually the limiting nutrient, nitrogen is more commonly the key limiting nutrient of marine waters.

Terrestrial Ecosystems

Eutrophication has an adverse effect on the terrestrial ecosystem. Increased nitrates in soil are frequently undesirable for plants. Many terrestrial plant species are endangered as a result of soil eutrophication. Meadows, forests, and bogs have usually low nutrient content and slowly growing species adapt to those levels.

Chemical sorts of nitrogen is a matter of concern with due to eutrophication. Terrestrial ecosystems believe the microbial organic process to convert \(N_{2}\) into other forms like nitrates. However, there’s a limit to what proportion nitrogen are often utilized. Ecosystems that receives more amount of nitrogen than the plants’ needs are nitrogen-saturated. Hence, saturated terrestrial ecosystems are able to contribute to both inorganic and organic nitrogen to coastal, freshwater, and marine eutrophication, where nitrogen is additionally typically a limiting nutrient.

Ecological Effects

Eutrophication became a pollution problem since the mid-20th century in both European and North American lakes and reservoirs. In general, there are particularly three ecological impacts which are troublesome: decreased biodiversity, changes in species composition and dominance, and toxicity effects.

  • Decreased Biodiversity: In aquatic ecosystems, species like algae experience a population increase (called an algal bloom) due to rise in nutrients. Algal blooms stop the daylight from reaching to the bottom-dwelling organisms and cause wide swings within the amount of dissolved oxygen within the water. If we study eutrophic conditions, we will see that dissolved oxygen rises a lot during the day, and also reduced after dark by the respiring algae and by microorganisms that prey on the increasing mass of dead algae. As soon as dissolved oxygen levels reduce to hypoxic levels, fish and other marine animals suffocate. As a result, creatures like fish, shrimp, and particularly immobile bottom dwellers die out.
  • Toxicity: Some algal blooms are also known as “harmful algal blooms”, are toxic to plants and animals. It results from eutrophication. Toxic compounds can make their high organic phenomenon, leading to animal mortality. Dead algae release neuro- and hepatotoxins which may kill animals and hence pose a threat to humans.
  • New species invasion: Increased nitrogen content might allow new, competitive species to invade and out-compete original inhabitant species. This process causes shifts within the species composition of ecosystems.

Prevention and Reversal

Eutrophication poses a drag not only to ecosystems but to humans also. Reducing eutrophication should be a key concern when considering a future policy.  A sustainable solution for everybody, including farmers and ranchers, seems feasible.

Shellfish in estuaries: One proposed solution to prevent and reverse eutrophication in estuaries is to revive shellfish populations, like oysters and mussels. Oyster reefs helps in removing nitrogen from the water column and filter suspended solids. Thus, subsequently reducing the extent of harmful algal blooms or anoxic conditions.

Seaweed farming: It helps to adapt to global climate change. Seaweed, like kelp, also absorbs phosphorus and nitrogen. It is thus useful to get rid of excessive nutrients from polluted parts of the ocean. Some cultivated seaweeds have really high productivity and will absorb large quantities of N, P, \(CO_{2}\), producing a great deal of \(O_{2}\) have a superb effect on decreasing eutrophication.

Minimizing nonpoint pollution: Nonpoint pollution is that the most difficult source of nutrients to manage. The subsequent steps are recommended to attenuate the quantity of pollution which will enter aquatic ecosystems from ambiguous sources.

  • Riparian buffer zones: Studies show that intercepting non-point pollution between the source and therefore the water may be a successful means of prevention. Riparian buffer zones are interfaces between a flowing body of water and land and are created near waterways in an effort to filter pollutants.
  • Prevention policy: New policies and laws to regulate the discharge and treatment of sewage have led to potential nutrient reductions to surrounding ecosystems. But it’s generally agreed that a policy, regulating the agricultural use of fertilizer along with animal waste must be imposed.
  • Organic farming: According to a study it is found that organic farming “significantly reduce harmful nitrate leaching” compared to conventionally fertilized fields.

Geo-engineering in lakes: Geoengineering is the manipulation of biogeochemical processes. Usually, it is the phosphorus cycle, to realize a desired ecological response within the ecosystem. Phosphate is one of the major cause of algal growth, mainly cyanobacteria, so once phosphate is reduced the algal isn’t ready to overgrow.

FAQs about Eutrophication

Q.1. Can eutrophication be reversed?

Answer. Cultural eutrophication is harmful. But yes it can be reversed if the nutrients come from easily identified point sources such as sewage treatment plants or septic systems.

Q.2. Can you swim in a eutrophic lake?

Answer. The eutrophic body of water contains high amounts of nutrients. The water looks cloudy due to the numerous aquatic vegetation, organisms, as well as algae and plankton that drift on it. Therefore, one can’t swim or very difficult to swim in the eutrophic lake.

Q.3. What is the most harmful effect of eutrophication?

Answer. Oxygen depletion, or hypoxia, is a common effect of eutrophication in water. The direct effects of hypoxia include fish kills which in turn impact on the whole aquatic ecosystem.

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