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Eutrophication Process Steps

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Published: Tue, 27 Jun 2017

Eutrophication is world-wide environmental issue environmental problems that are related to high concentration nutrients. It is the process due to increment of algae productivity which affects adversely aquatic life and also human and animal health. It is mainly influenced by humankind activities that include agriculture and sewage effluent due to creating high amount of nutrients.

The mechanism of eutrophication is briefly described in Figure 1. Large amount of nutrient input to the water body is the main effect and high level of phytoplankton biomass results that lead to algal bloom. Consumption of oxygen close the bottom of the water body is the result. The other effects of the process can be divided two categories that are related to:

  • nutrient dispersion,
  • phytoplankton growth

Nitrogen and phosphorus are two main nutrients for aquatic life. In addition, A silica is also necessary for the diatoms. Nutrient concentration in the water body changes during eutrophication. The nutrient is the limiting factor, if it is not be available for algae develop.

The sufficient factor to determine limiting factor is the ratio of nitrogen to phosphorus compounds in the water body is an important factor for control mechanism. (Table 1). Phosphorus is generally limiting factor for phytoplankton in fresh waters. For large marine areas frequently have nitrogen as the limiting nutrient, especially in summer. Intermediate areas such as river plumes are often phosphorus-limited during spring,but may turn to silica or nitrogen limitation in summer.

The enrichment of water by nutrients can be of natural origin but it is often dramatically increased by human activities. This occurs almost everywhere in the world. There are three main sources of anthropic nutrient input: runoff, erosion and leaching from fertilized agricultural areas, and sewage from cities and industrial wastewater. Atmospheric deposition of nitrogen (from animal breeding and combustion gases) can also be important. According to the European Environment Agency, “the main source of nitrogen pollutants is run-off from agricultural land, whereas most phosphorus pollution comes from households and industry, including phosphorus- based detergents. The rapid increase in industrial production and in in-house consumption during the 20th century has resulted in greater volumes of nutrient-rich wastewater. Although there has been recently a better management of nitrogen and phosphorus in agricultural practices, saturation of soils with phosphorus can be noted in some areas where spreading of excessive manure from animal husbandry occurs. Nutrient removal in sewage treatment plants and promotion of phosphorus-free detergents are vital to minimize the impact of nitrogen and phosphorus pollution on Europe’s water bodies”7.

Since 1980, nitrate concentrations in major EU rivers have generally remained constant. There is no evidence that reduced application of nitrogen fertilizers to agricultural land has resulted in lower nitrate concentrations in rivers. Indeed, concentrations in some regions in Europe, such as Brittany, or Poitou in France, and Catalunya in Spain, are still increasing.

More detailed information on nitrates are to be found in the companion pamphlet in this series “nitrate and health” and in the E.C. report mentioned in (6).

wastewater treatment and less phosphorus in household detergents. Phosphorus release from industry has also fallen sharply (Figure 3) whereas phosphorus from agriculture, despite a reduction in the consumption of phosphate fertilizers in the EU, remains an important source of phosphorus pollution. 

Unfortunately, due to the main role of nitrogen in the eutrophication process in summer in the coastal zone, the reduction in the discharge of phosphorus from rivers into the sea has not been visible, except in very specific sites. In most cases the phosphorus released by the sediments into the open sea is sufficient to allow eutrophication to occur, although external inputs have sharply decreased. In fact, only the Dutch coast has benefited from the improvement of the water of the Rhine, everywhere else the situation is stable or has worsened.

Some activities can lead to an increase in adverse eutrophication and, although they are very specific, they should be noted:

  • Aquaculture development: Expansion of aquaculture contributes to eutrophication by the discharge of unused animal food and excreta of fish into the water;
  • The transportation of exotic species: Mainly via the ballasts of big ships, toxic algae, cyanobacteria and nuisance weeds can be carried from endemic areas to uncontaminated ones. In these new environments they may find a favourable habitat for their diffusion and overgrowth, stimulated by nutrients availability;
  • Reservoirs in arid lands: The construction of large reservoirs to store and manage water has been taking place all over the world. These dams are built in order to allow the collection of drainage waters through huge hydrographic basins. Erosion leads to the enrichment of the waters of these reservoirs by nutrients such as phosphorus and nitrogen

Factors supporting the development of eutrophication

Besides nutrient inputs, the first condition supporting eutrophication development is purely physical – it is the containment (time of renewal) of the water. The containment of water can be physical, such as in a lake or even in a slow river that works as a batch (upstream waters do not mix with downstream waters), or it can be dynamic. 

The notion of dynamic containment is mostly relevant for marine areas. Geological features such as the shape of the bottom of the sea, the shape of the shores, physical conditions such as streams, or large turbulent areas, and tidal movements, allow some large marine areas to be really “contained”, exhibiting very little water renewal. This is known as dynamic containment. In other cases, due to tidal effects, and/or streams, some areas that would seem to be prone to containment see their waters regularly renewed and are not contained at all and are therefore very unlikely to become eutrophic.

Other physical factors influence eutrophication of water bodies. Thermal stratification of stagnant water bodies (such as lakes and reservoirs), temperature and light influence the development of aquatic algae.

Increased light and temperature conditions during spring and summer explain why eutrophication is a phenomenon that occurs mainly during these seasons.

Eutrophication itself affects the penetration of light through the water body because of the shadow effect coming from the development of algae and other living organisms and this reduces photosynthesis in deep water layers, and aquatic grass and weeds bottom development.

Main consequences of eutrophication

The major consequence of eutrophication concerns the availability of oxygen. Plants, through photosynthesis, produce oxygen in daylight. On the contrary, in darkness all animals and plants, as well as aerobic microorganisms and decomposing dead organisms, respire and consume oxygen. These two competitive processes are dependent on the development of the biomass. In the case of severe biomass accumulation, the process of oxidation of the organic matter that has formed into sediment at the bottom of the water body will consume all the available oxygen. Even the oxygen contained in sulphates (SO4 2-) will be used by some specific bacteria. This will lead to the release of sulphur (S2-) that will immediately capture the free oxygen still present in the upper layers. Thus, the water body will loose all its oxygen and all life will disappear. This is when the very specific smell of rotten eggs, originating mainly from sulphur, will appear. 

In parallel with these changes in oxygen concentration other changes in the water environment occur:

  • Changes in algal population: During eutrophication, macroalgae, phytoplankton (diatoms, dinoflagellates, chlorophytes) and cyanobacteria, which depend upon nutrients, light, temperature and water movement, will experience excessive growth. From a public health point of view, the fact that some of these organisms can release toxins into the water or be toxic themselves is important. 
  • Changes in zooplankton, fish and shellfish population: Where eutrophication occurs, this part of the ecosystem is the first to demonstrate changes. Being most sensitive to oxygen availability, these species may die from oxygen limitation or from changes in the chemical composition of the water such as the excessive alkalinity that occurs during intense photosynthesis.

Ammonia toxicity in fish for example is much higher in alkaline waters.

Effects of eutrophication

The effects of eutrophication on the environment may, have deleterious consequences for the health of exposed animal and human populations, through various pathways. Specific health risks appear when fresh water, extracted from eutrophic areas, is used for the production of drinking water. Severe impacts can also occur during animal watering in eutrophic waters.

Macroalgae, phytoplankton and cyanobacteria blooms

Algae display varying degrees of complexity depending on the organization of their cells. Macroalgae, phytoplankton and cyanobacteria may colonize marine, brackish or fresh waters wherever conditions of light, temperature and nutrients are favourable. Cyanobacteria have been largely studied in fresh water systems, due to their ability to proliferate, to form massive surface scums, and to produce toxins that have been implicated in animal or human poisoning.

Some species of algae may also contain toxins, but incidents where fresh water algae are at the origin of cases of human or animal illness have very seldom been reported.

Coloured toxic tides caused by algal overgrowth have been known to exist for many centuries. In fact the Bible (Exodus, 7: 20-24) states “all the water of the Nile river became red as blood and fish which were in the river died. And the river was poisoned and the Egyptians could not drink its waters”.

Algal blooms were observed in 1638 by fishermen in north west of Iceland. Fjords were reported to be stained blood red and during the night produced a kind of phosphorescence. The fishermen thought that the colours could be due to the blood of fighting whales or to some marine insects or plants (Olafsson and Palmsson, 1772). The first scientific report of domestic animals dying from poisoning as a consequence of drinking water that was affected by a blue/green algae bloom was in 1878 in lake Alexandrina, Australia.

In coastal and estuarine systems, however, where conditions are less favourable to the proliferation of cyanobacteria, which need oligo-elements such as iron, toxic algae such as dinoflagellates have been observed and have been at the origin of health troubles. There is growing evidence that nutrients, especially nitrogen, favour the duration and frequency of such toxic “blooms”, and concentrations of toxin in the cells.

Health effects linked to toxins of cyanobacteria in fresh waters

Some cyanobacteria have the capacity to produce toxins dangerous to human beings. Toxins can be found either free in the water where the bloom occurs or bound to the algal or cyanobacterial cell. When the cells are young (during the growth phase), 70 to 90% of the toxins are cell bound, whereas when the cells Cyanobacteria have been largely studied in fresh water systems, due to their ability to proliferate, to form massive surface scums, and to produce toxins that have been implicated in animal or human poisoning.

Some species of algae may also contain toxins, but incidents where fresh water algae are at the origin of cases of human or animal illness have very seldom been reported.

Coloured toxic tides caused by algal overgrowth have been known to exist for many centuries. In fact the Bible (Exodus, 7: 20-24) states “all the water of the Nile river became red as blood and fish which were in the river died. And the river was poisoned and the Egyptians could not drink its waters”.

Algal blooms were observed in 1638 by fishermen in north west of Iceland. Fjords were reported to be stained blood red and during the night produced a kind of phosphorescence. The fishermen thought that the colours could be due to the blood of fighting whales or to some marine insects or plants (Olafsson and Palmsson, 1772). The first scientific report of domestic animals dying from poisoning as a consequence of drinking water that was affected by a blue/green algae bloom was in 1878 in lake Alexandrina, Australia.

In coastal and estuarine systems, however, where conditions are less favourable to the proliferation of cyanobacteria, which need oligo-elements such as iron, toxic algae such as dinoflagellates have been observed and have been at the origin of health troubles. There is growing evidence that nutrients, especially nitrogen, favour the duration and frequency of such toxic “blooms”, and concentrations of toxin in the cells.

Health effects linked to toxins of cyanobacteria in fresh waters

Some cyanobacteria have the capacity to produce toxins dangerous to human beings. Toxins can be found either free in the water where the bloom occurs or bound to the algal or cyanobacterial cell. When the cells are young (during the growth phase), 70 to 90% of the toxins are cell bound, whereas when the cells fresh waters. People may be exposed to toxins through the consumption of contaminated drinking water, direct contact with fresh water or the inhalation of aerosols. Toxins induce damage in animals and humans by acting at the molecular level and consequently affecting cells, tissues and organs (Table 3).

The nervous, digestive, respiratory and cutaneous systems may be affected. Secondary effects can be observed in numerous organs. Age or physiological conditions of the affected individual may determine the severity of the symptoms. A variety of symptoms, depending on the toxins implicated, are observed such as fatigue, headache, diarrhoea, vomiting, sore throat, fever and skin irritations. Cyanotoxins can be classified into three groups:

• Hepatotoxins.

These are the most frequently observed cyanotoxins. Experiments using mice indicate that they cause liver injury and can lead to death from liver haemorrhage and cardiac failure within a few hours of exposure at acute doses. Chronic exposure induces liver injury and promotes the growth of tumours.

Questions remain concerning the effects of repeated exposures to low levels of toxins. Animal experiments have shown liver injury from repeated oral exposure to microcystins, the most frequently observed cyanotoxins.

It is thought that the high prevalence13 of liver cancer observed in some areas of China could be due to the presence of microcystins in water supplies.

• Neurotoxins.

These are generally less common and act on the nervous system. In mice and aquatic birds, they cause rapid death by respiratory arrest, sometimes occurring in a few minutes.

• Dermatotoxins.

These induce irritant and allergenic responses in tissues by simple contact.

The global toxicity of a cyanobacterial proliferation is not constant in time or space, making it difficult to assess the health threat although some acute poisonings have led to death (Tables 3 and 4).

The release of cyanotoxins in water has been at the origin of several outbreaks affecting animal or human health (Case studies p. f12). About 75% of cyanobacterial blooms are accompanied by toxin production.

The presence of cyanobacterial toxins after potabilization treatment represents a health threat for patients undergoing renal dialysis treatment.

Monitoring of eutrophication 

Monitoring is useful if it is performed for a purpose. The main reasons for monitoring a water body for eutrophication are:

• To prevent the occurence of eutrophication;

• Early warning purposes. Public health authorities need to know when eutrophication is likely to start in order to allow them to implement preventive actions;

• To know the level of development of the process, and have a precise picture of the quality of the water.

This is mostly relevant for water companies, which have to deal with eutrophic waters;

• Research.

The reality is that monitoring systems are often multipurpose.

Monitoring and management of cyanobacterial growth in fresh waters for public health purposes

Chorus and Bartram (1999) have proposed the following monitoring and management scheme to water treatment plant operators and managers as an alert level framework. It provides a graduated response to the onset and progress of a cyanobacteria bloom.

This tool initially comes from Australia. Three response levels are defined:

• Vigilance Level is defined by the detection of one colony, or five filaments, of a cyanobacterium in a 1 ml water sample. When the Vigilance Level is exceeded, it is recommended that the affected water body is sampled more frequently – at least once a week, so that potentially rapid changes in cyanobacteria biomass can be monitored.

• Alert Level 1 is initiated when 2,000 cyanobacterial cells per ml or 0.2 mm3/l biovolume23 or 1 μg/l chlorophyll- a24 are detected. Alert Level 1 condition requires an assessment to be made of the total toxin concentration in the raw water. A consultation should be held with the health authorities for on-going assessment of the status of the bloom and of the suitability of treated water for human consumption. Monitoring should be conducted at least once per week. It may also be appropriate at this time to issue advisory notices to the public through the media or other means. Government departments or interested authorities or those with legal responsibilities should also be contacted, as should organizations that treat or care for members of the public with special needs.

• Alert Level 2 is initiated when 100,000 cells per ml or 10-mm3/l biovolume or 50 μg/l chlorophyll-a are detected, with the presence of toxins confirmed by chemical or bioassay techniques. This density of cells corresponds to an established, toxic bloom with high biomass and possibly also localized scums. In this situation there is a need for effective water treatment systems and an assessment of the performance of the system. Hydro-physical measures to reduce cyanobacteria growth may still be attempted. If efficient water treatments are not available (see technical annex), a contingency water supply plan should be activated. In extreme situations, safe drinking water should be supplied to consumers in tanks and bottles. Media releases and contact with consumers should be undertaken via mail of leaflets informing that water may present danger for human consumption but is still suitable for the purposes of washing, laundry and toilet flushing.

National water quality monitoring programs

Few national water quality monitoring programmes include parameters which indicate eutrophication or a risk of algal or cyanobacterial overgrowth. In Europe, North America, Japan and Australia, local monitoring plans which check the occurrence of toxic species in areas where shellfish or fish are consumed, are implemented. This is based on sampling at strategic points and analysis of phytoplankton and/or shellfish. The frequency of sampling generally depends on the sea- son. Table 6 summarizes the monitoring systems in some EU Member States. They only allow the monitoring of toxic blooms, which are only a part of the eutrophication consequences.

Technologies such as satellite imaging can be used to monitor large water bodies. The same technique can be applied to monitor the extent of high chlorophyll-a concentrations reflecting the phytoplankton biomass of the upper layers of the eutrophic area.

Possible parameters used for monitoring purposes

According to the definition of eutrophication, it is clear that formulae such as “an increase of x grams of bottom macrophytes per square meter” or “y micrograms chlorophyll-a per litre” are not suitable to define a threshold, which, when exceeded, will describe eutrophication.

Such unique parameter does not exist. Moreover, in order to define the magnitude of eutrophication, two measurements are required: That of the system in its reference conditions, and in its current or predicted future condition. As baseline data for a site is the exception rather than the rule, this makes it difficult to test eutrophication using a case-by-case

approach. Nevertheless, as the first signs of adverse eutrophication is a decrease in the oxygen concentration in the lower layers of the water body of stagnant waters, and an increase in pH due to photosynthesis (CO2 depletion), these parameters, together with direct microscopic observations, are likely to be the only ones that can help forecast the likelihood of the start of such a process as long as a model integrating physical conditions, nutrient inputs and biological effects has not been locally validated. Prevention25,26

The causes that drive eutrophication are multiple and the mechanisms involved are complex. Several elements should be considered in order to assess the possible actions aimed at counteracting nutrient enrichment of water supplies. The use of computerised models now allows a better understanding of the role of each factor, and forecasting the efficiency of various curative and preventive measures. The best way to avoid eutrophication is to try to disrupt those mechanisms that are under human control; this clearly means to reduce the input of nutrients into the water basins. Such a control unfortunately does not have a linear effect on the eutrophication intensity. Integrated management should comprise:

• Identification of all nutrient sources. Such information can be acquired by studies of the catchment area of the water supply. Knowledge of industrial activities, discharge practices and localization, as well as agricultural practices (fertilizer contribution/plant use and localization of crops) is necessary in order to plan and implement actions aiming at limiting the nutrient enrichment of water.

The identification of sewage discharge points, agricultural practices, the nature of the soil, the vegetation, and the interaction between the soil and the water can be of great help in knowing which areas should be targeted.

• Knowledge of the hydrodynamics of the water body, particularly the way nutrients are transported, and of the vulnerability of the aquifer, will allow determination of the ways by which the water is enriched with nutrients.

Anthropogenic nutrient point sources such as nontreated industrial and domestic wastewater discharge can be minimized by systematic use of wastewater treatments. In sensitive aeras, industries and local authorities should control the level of nutrients in the treated wastewater by the use of specific denitrification or phosphorus removal treatments.

Diffuse anthropogenic nutrient sources can be controlled by soil conservation techniques and fertilizer restrictions.

Knowledge of the agronomic balance (ratio of fertilizer contribution to plant use) is very relevant to optimize the fertilization practice and to limit the loss of nutrients. Diffuse nutrient losses will be reduced by implementation at farm level of good practices such

as:

• Fertilization balance, for nitrogen and phosphorus, e.g. adequation of nutrients supply to the needs of the crop with reasonable expected yields, taking into account soil and atmospheric N supply.

• Regular soil nutrients analysis, fertilization plans and registers at plot level.

• Sufficient manure storage capacities, for spreading of manure at appropriate periods.

• Green cover of soils during winter, use of “catchcrops” in crop rotations.

• Unfertilized grass buffer strips (or broad hedges) along watercourses and ditches.

• Promotion of permanent grassland, rather than temporary forage crops.

• Prevention of erosion of sloping soils.

• Precise irrigation management (e.g. drip irrigation, fertilisation, soil moisture control).

In coastal areas, improvement in the dispersion of nutrients, either through the multiplication of discharge points or through the changing of their localization, can help to avoid localized high levels of nutrients.

Reuse and recycling, in aquaculture and agriculture, of waters rich in nutrients can be optimized in order to avoid discharge into the water body and direct consumption of the nutrients by the local flora and fauna.

Water resources are environmental assets and therefore have a price. There are market-based methods to estimate costs and benefits, and these make it possible to use cost- benefit analysis as a useful tool to assess the economic effects of abatement of eutrophication or other pollution problems. Benefits range from higher quality drinking water and reduced health risks (Photo 29) to improved recreational uses (Photo 30). The effects on human health from the lack of sanitation and the chronic effects of toxic algal blooms are two of the many indirect effects resulting from eutrophication. Numerous cost-benefit analyses of pollution abatement have clearly demonstrated that the total costs to society of ‘no pollution reduction’ is much higher than at least a ‘reasonable pollution reduction’. Consequently, it is necessary to examine the prevention of pollution and restoration of water quality in lakes and reservoirs from an economic standpoint. The result of such examinations should be applied to assess effluent charges and green taxes. International experience shows that these economic instruments are reasonably effective in improving water quality and solving related water pollution problems. Thus, effective planning and management of lakes and reservoirs depends not only on a sound understanding of these water-bodies as ecological systems but also of their value to people as recreational areas and water resources.

In the past, several management strategies were developed and applied to solve problems of decreasing surface and groundwater quality. These were often a response to acute critical situations resulting in increased costs of water. The demand for good quality fresh water was only solved partially and locally; this was because too few resources were allocated too late to solve the problems. Early prevention is by far the cheapest method to avoid later pollution.

Eutrophication Management

Recognizing that the specific needs of policy-makers and administrators are usually different from those of the strictly technical audience, the primary purpose of this digest is to provide quantitative tools for assessing the state of eutrophication of lakes and reservoirs; to provide a framework for developing cost-effective eutrophication management strategies; to provide a basis upon which strategies can be tailored for each specific case according to the physical, social, institutional, regulatory and economic characteristics of the local area or region; and to provide specific technical guidance and case studies regarding the effective management

of eutrophication. The approach presented in this document (Figure 1) also is sufficiently general that it can be applied, with relative little modification, to the assessment of other environmental problems and to the development of effective management strategies for such problems.

An approach for achieving the basic objectives stated above consists of the following components, applied approximately in the order presented: identify eutrophication problem and establish management goals; assess the extent of information available about the lake/reservoir; identify available options for management of eutrophication; analyze all costs and expected benefits of alternative management/control options; analyze adequacy of existing institutional and regulatory framework for implementing alternative management strategies; select desired control strategy and distribute summary to interested parties prior to implementation; and provide periodic progress reports on control programme to public and other interested parties.

designation of bad (unacceptable) versus good (acceptable) water quality in this digest is based on the specific intended use or uses of the water resource.

That is, water quality management goals for a lake or reservoir should be a function of the major purpose(s) for which the water is to be used.

Obviously, there are water quality conditions to be avoided because of their interference with water uses. Ideally, for example, a lake or reservoir used as a drinking water supply should have water quality as close to an oligotrophy state as possible, since this would insure that only a minimum amount of pre-treatment would be necessary to yield a water suitable for human consumption. For such a waterbody, the content of phytoplankton (and their metabolic products) in the water should be as low as possible to facilitate this goal. Further, if the water is taken from the bottom waters of a lake during the summer (usually the period of maximum algal growth), it should be free of interferring substances resulting from decomposition of dead algal cells. Eutrophic lakes and reservoirs also could be used as a drinking water supply. However, extensive pre-treatment would be necessary before the water was suitable for human consumption.

Some water uses may require no treatment at all, regardless of the existing water quality. Examples are fire-fighting purposes and the transport of commercial goods by ship. Further, in areas with extremely limited water resources, virtually all of the water may be used for various purposes (with or without treatment), regardless of its quality. Therefore, although humans can use water exhibiting a range of water quality, there is a desirable or optimal water quality for virtually any type of water usage. Though it is not quantitative in nature, a summary of intended water uses and the optimal versus minimally-acceptable trophic state for such uses is provided in Table 3. In addition, an example of the values of several commonly measured water quality parameters corresponding to different trophic conditions, based on the international eutrophication study of the Organization for Economic Cooperation and Development (1982), is provided in Table 4. Thus, it is possible to identify acceptable or optimal water quality for given water uses.

Given these factors, a prudent approach in setting eutrophication management goals is to determine the minimum water quality and trophic conditions acceptable for the primary use or uses of the lake or reservoir (Table 1), and attempt to manage the water body so that these conditions are achieved. In a given situation, if the primary use or uses of a waterbody is hindered by existing water quality, or else requires water quality or trophic conditions not being met in the waterbody, this signals the need for remedial or control programmes to achieve the necessary in-lake conditions.

21 the problem?

The governmental role

It is recognized that a range of different forms of government, as well as economic conditions, exist around the world. Consequently it is difficult to provide general guidelines regarding the role of the government in environmental protection efforts that will cover all possible situations. However, virtually all nations also contain some type of civil service infrastructure which, if properly used, can be an effective instrument with which to address governmental concerns. Even so, as noted earlier, not all


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