EUTROPHICATION: PROCESS, CAUSES, CONSEQUENCES, SOLUTIONS - ENVIRONMENT - 2024

The Eutrophication is an excess of nutrients contributed to a body of water, creating a population explosion of algae, cyanobacteria and aquatic plants. This phenomenon, in turn, causes the decrease in dissolved oxygen in the water and, therefore, affects underwater life.

Dissolved oxygen decreases due to reduced photosynthesis by submerged algae and plants. This occurs both due to the blockage of sunlight due to surface plant proliferation, as well as the higher rate of oxygen consumption by these populations.

Accumulation of algae by eutrophication.

Eutrophication can occur due to natural causes when the size or flow of a body of water is reduced or exceptional rains that carry nutritional loads to bodies of water. However, in most cases eutrophication occurs from human-generated (anthropic) causes.

The problem of eutrophication is growing worldwide and more than half of the world's lakes suffer from it at some level. Similarly, dead zones proliferate in the seas due to the occurrence of this phenomenon, identifying more than 700 of them.

Process of

Eutrophication begins with the contribution to the water of unusual amounts of nutrients, especially nitrogen (N) and phosphorus (P). These contributions can come from both natural and human (anthropic) sources, but in any case they are the product and cause of an ecological imbalance.

The favored organisms

The exceptional intake of essential nutrients for plant growth favors the development of invasive aquatic plants and algae. It can be both phytoplankton and macroalgae and the proliferation of cyanobacteria also occurs.

Proliferation of algae by eutrophication. Source: Felix Andrews (Floybix)

These organisms have high reproductive rates and are fast growing, so with enough nutrients, they have competitive advantages. They grow on the surface of the body of water, and cover the entire surface.

The light

Floating algae and aquatic plants that cover the surface prevent sunlight from penetrating to the lower layers of water. Because of this, the submerged algae and plants cannot photosynthesize and begin to die.

Dissolved oxygen

The blockage of underwater photosynthesis by this superficial proliferation of plant mass also implies the decrease of dissolved oxygen in the water. In addition, other sources of dissolved oxygen, such as exchange with the atmosphere, are limited due to the blockage exerted by the superficial plant mass.

Oxygen demand

Below the surface, the activity of decomposing organisms, especially bacteria and archaea, increases. This implies an increase in the demand for dissolved oxygen, which is reduced.

The increase in decomposing activity is a consequence of the greater supply of organic matter as a result of the surface population explosion. The increase in the death of underwater organisms also influences.

The internal contribution of organic matter and the external one by contaminated effluents requires more oxygen for its biochemical stabilization (Biochemical Oxygen Demand). When oxygen is scarce, semi-decomposed organic matter accumulates, the entire ecosystem goes into crisis and life disappears.

Causes

Eutrophication is caused by a nutrient supply higher than the usual balance of the ecosystem. Which can happen both by natural and artificial causes, although the former are rare.

- Natural causes

At sea

Deep cold water currents can cause phytoplankton population bursts by pulling nutrients from the ocean floor to the surface (upwellings). In the same way, algal blooms or blooms are natural phenomena caused by different factors that favor the growth of phytoplankton.

Under such favorable conditions, unusual population increases of dinoflagellates and diatoms occur. Some species of microalgae are harmless and beneficial, but others are toxic algae, such as dinoflagellates of the genus Alexandrium.

This toxicity is due to the emission by microalgae of toxins such as saxitoxin, ciguatoxin and gonyatoxins.

In fresh water

The rivers in their derivation along the channel generate meanders or marked curves, which sometimes remain isolated. In this way horseshoe lagoons are produced, which are fed by rainwater, runoff or the periodic overflows of the river.

In these water reservoirs there is a higher concentration of nutrients than the river, because there is no outflow. Therefore, natural eutrophication phenomena occur with the proliferation of algae and aquatic vegetation.

- Anthropic causes

The most important cause of eutrophication today is the contamination of bodies of water by human activities. Especially wastewater without adequate treatment and diffuse pollution from agriculture, which is the most important worldwide.

Fertilizers and pesticides

Chemical fertilizers consist of compounds specially formulated to provide essential nutrients for plant growth. The main fertilizer formula includes nitrogen (N), phosphorus (P), and potassium (K).

Eutrophication. Source: F. lamiot (own work)

In agricultural systems the application of these products is inefficient, so that close to 60% is washed by runoff water. These nutrients end up in water bodies such as rivers or finally lakes and oceans, transported by runoff water.

Urban effluents

Sewage from homes, offices and factories provides a large amount of organic matter and inorganic substances that promote eutrophication. Domestic wastewater carries a high load of organic matter that, if not treated, reaches the water bodies.

Soaps and detergents used in personal hygiene, household and other facilities, also include phosphates and nitrates.

Acid rain

Industries emit gases containing nitrogen oxides among other compounds into the atmosphere. These gases when reacting with water vapor produce acids that precipitate and reach bodies of water.

These compounds are additional sources of nitrates that are one of the main nutrients that promote plant growth.

Global warming

The rise in global temperatures favors eutrophication, as warm waters promote the proliferation of algae and aquatic plants. On the other hand, the amount of dissolved oxygen decreases in proportion to the increase in water temperature.

- Contributing factors

Other factors contribute to the development of eutrophication, including lack of water mobility, shallow depth, low flow, and high water temperatures. The less water there is in the aquatic ecosystem or the slower its renewal rate, the faster it reaches nutrient saturation.

Consequences

Eutrophication is a serious problem that mainly affects freshwater ecosystems, but it also occurs in marine ecosystems. Worldwide, around half of the lakes have some level of eutrophication and many rivers as well as coastal areas have dead areas due to this cause.

- The disappearance of life in the ecosystem

The main consequence of eutrophication is the decrease of biodiversity in the ecosystem and finally the disappearance of aquatic life. In the early stages, the excess of nutrients favors the proliferation of invading organisms that show rapid growth and reproduction under these conditions.

Eutrophication in the Potomac River (United States). Source: Alexandr Trubetskoy

To the extent that these species predominate, they displace the rest of the species in the ecosystem, decreasing their biological diversity. As eutrophication proceeds, conditions at the underwater level become unfavorable to life and organisms die.

In this way, submerged fish, algae and plants disappear, as well as other aquatic organisms due to the lack of oxygen and light. The final destination, if eutrophication continues, is the almost total disappearance of aquatic life.

Toxic gases

The activity of anaerobic bacteria produces toxic gases such as hydrogen sulfide, causing great deaths of aquatic organisms.

Impact throughout the area

The impact extends to terrestrial ecosystems, since those living beings that depend on the body of water also perish or migrate. This is due both to the loss of the food source and to the degradation of the drinking quality of the water.

- Decrease in food production

In eutrophied areas, food production decreases as fishing decreases because the first organisms to be affected are fish. These, in the case of rivers and seas, are forced to leave the area, while in the lakes they end up disappearing.

- Loss of drinking water sources

Drinking water must meet quality conditions, including dissolved oxygen level, pH and absence of organic and inorganic impurities. When eutrophication occurs, the water in the ecosystem reduces its content of dissolved oxygen and accumulates large amounts of decomposing organic matter.

This in turn is harmful to health and causes bad odors and color changes, incompatible with its use as drinking water. The pH of the water is altered, becoming more acidic due to the presence of organic acids.

Health conditions

Public health can be affected by the consumption of non-potable water from eutrophic bodies of water. In addition, the proliferation of certain microalgae contribute toxins to the water and the organisms that consume it.

Similarly, the increase in anaerobic bacteria generates the emission of toxic gases such as hydrogen sulfide and methane.

- Decrease in tourist activity

Aquatic ecosystems are important poles of tourist attraction, generating sources of work and local development. These benefits are threatened by eutrophication, since it affects both the biodiversity and the landscape values ​​of the area.

- Economic losses

As a consequence of all of the above, eutrophication leads to great economic losses in terms of decreased fishing, tourism income, and water use.

Solutions

Solutions to the problem of eutrophication must be approached from the perspective of preventing the phenomenon and from its remediation.

- Prevention

The starting point for the prevention of eutrophication is citizen awareness of its existence, causes and consequences. In this way, citizens can lobby for the necessary measures to be adopted.

Avoid water contamination

The basic solution to the problem of eutrophication due to anthropic causes is the reduction of water pollution. For this, it is necessary to implement the adequate treatment of domestic and industrial sewage before being discharged into water bodies.

It is also necessary to control other indirect sources of water pollution, such as gas emissions that generate acid rain.

New agricultural and livestock model

Water pollution caused by agricultural and livestock activities is diffuse, that is, the polluting sources cannot be exactly identified. In such a way that the solution to the problem of the contribution of pollutants by these activities is to change the technological production schemes.

Agriculture is required that uses fewer inputs, especially fertilizers and herbicides, compounds that contribute nitrites and phosphates to water. Thus, agricultural systems with closed circulation of nutrients such as hydroponics are less polluting in terms of contributions to ecosystems.

Attention is also required to the management of organic waste, the collection and treatment of sewage in animal production systems.

- Remediation

Once the eutrophication process of a body of water is underway, there are several alternatives to remedy the problem.

Extraction of the vegetable mass

Given the proliferation of floating aquatic plants such as lenna (Lennaceae), bora (Eichhornia crassipes) or others, it is feasible to implement collection programs. This consists of massively removing the plants from the surface of the body of water using dragging mayas.

Water purification

In severe cases of contamination, water can undergo purification treatments by recirculating from the water body to treatment plants.

Biologic control

There are some organisms that consume the algae and aquatic plants that proliferate in eutrophication. Heterotrophic grazing protists have been used, that is, they consume microalgae to reduce the populations that proliferate in eutrophication.

Likewise, the fungus Cercospora piaropi has been used to control Eichhornia crassipes, a floating angiosperm that proliferates under eutrophic conditions.

Chemical control

Causing the flocculation and precipitation of microalgae is an alternative, especially in cases of toxic algal blooms. This is achieved by adding products such as sodium hypochlorite (NaOCl), clays (kaolinite) or combinations such as polyaluminum chloride with bentonite. These products attract and adhere the microalgae, forming flocs or masses that then precipitate to the bottom.

Examples of areas with eutrophication

Eutrophication in the Caspian Sea. Source: Jeff Schmaltz, MODIS Rapid Response Team, NASA / GSFC

- Marine dead zones or areas

The United Nations Environment Program indicates the existence of more than 700 dead areas in the world's oceans. These dead zones are due to a low concentration of dissolved oxygen due to eutrophication and their estimated area is greater than 240,000 km².

The Dead Zone of the Gulf of Mexico

One of the most extensive marine dead zones in the world is the so-called Dead Zone of the Gulf of Mexico. It is more than 20,700 km² of sea that suffer serious problems of eutrophication, with the proliferation of floating macro and microalgae.

The central cause of the eutrophication of this area is the polluted waters of the Mississippi River that has its mouth in the Gulf. The Mississippi runs through much of the agricultural zone of North America, being a catchment basin with a high load of agricultural pollutants.

- The Black Sea

In this inland sea located between Europe and Asia, an increase in algal blooms has been reported since the 1970s. This has resulted in declining populations of shallow water species.

According to information from the European Environmental Agency, hypoxic and anoxic phenomena occur every summer as a result of the activity of algae. All this due to eutrophication by nutrients provided by agricultural and industrial activities in the basin of this sea.

Sensitivity to eutrophication

Being an inland sea whose only indirect ocean connection is the Bosphorus channel, the renewal of the waters is very slow. This, together with the high levels of contamination in the basins that feed it, makes it especially susceptible to eutrophication.

In fact, the average concentrations of nitrates and phosphates have increased 18 times from 1960 to 1992. The greatest contributions of pollutants come from the basins of the Danube, Dnieper and Dniester rivers.

- The lake of Valencia

This lake, also called Los Tacariguas lake, is located in the north center of Venezuela with an area of ​​344 km². It forms an endorheic basin, that is to say closed, of 3,140 km². Many of its tributaries pass through cities in the center of the country such as Valencia and Maracay, with high populations and industrial zones.

Valencia Lake Basin (Venezuela). Source: Fev

The majority of urban and industrial wastewater is discharged into rivers converted into drainage channels without adequate treatment. This has resulted in huge amounts of pollutants accumulating over decades, including organic and inorganic nutrients.

Since 1974 nitrogen concentrations have doubled and phosphorus tripled in the lake. Being located above what is admitted in 146% for nitrogen and 250% for phosphorus. All this has caused an extensive phenomenon of eutrophication that is now well advanced.

References

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