Studies of nutrient runoff have shown a mixture of inputs into most river and lake catchments: both point source such as sewage treatment works and diffuse source such as agriculture. Point sources are usually most important in the supply of phosphorus, whereas nitrogen is more likely to be derived from diffuse sources. Using the data presented in Figure 1.
In terms of SRP the form of phosphorus that affects ecosystems most directly , we assume levels have fallen from 0. Comparing these figures with those in Table 2.
This suggests that some recovery of macrophyte species would be possible. Actual re-colonization may be a slow process, however.
Ecosystems can take many years to come back to equilibrium after a perturbation, and if an algal-dominated state has established, it will inhibit macrophyte recovery.
The degree to which eutrophication is considered a problem depends on the place and people concerned. A small lake in South-East Asia, heavily fertilized by village sewage, can provide valuable protein from fish.
In other parts of the world, a similar level of nutrients would be regarded as damaging, making water undrinkable and unable to support characteristic wildlife. In Europe, nitrates in drinking water are regarded as a potentially serious threat to health. Eutrophication has also damaged important fisheries and caused significant loss of biodiversity.
Worldwide, efforts to reduce the causes and symptoms of eutrophication cost huge sums of money. There is no single piece of existing legislation dealing comprehensively with the problem of eutrophication in the UK. This should help to reduce the problem of eutrophication in coastal waters where large discharges contribute significant nutrient loads.
In the UK, 62 rivers and canals totalling km , 13 lakes and reservoirs and five estuaries have been designated as sensitive areas eutrophic under this directive, and there are requirements for reducing nutrient loads from sewage treatment works in these areas Figure 4. However, the majority of nature conservation sites classified as sensitive are affected by smaller, rural STWs for which such equipment is not yet required.
Phosphorus stripping involves the use of chemicals such as aluminium sulfate, which react with dissolved phosphates, causing them to precipitate out of solution. Another piece of European legislation that has some bearing on problems of nutrient enrichment is the EC Nitrates Directive.
This is intended to reduce nitrate loadings to agricultural land, particularly in areas where drinking water supplies have high dissolved nitrate levels. The directive requires member states to monitor nitrate levels in water, set up 'nitrate vulnerable zones' NVZ , and produce and promote a 'code of good agricultural practice' throughout the countryside.
This should include measures to control the storage, handling and disposal of slurry, for example Figure 4. However, the legislation designed to curb nutrient inputs from agricultural sources is primarily directed towards reducing nitrate levels in drinking water rather than protecting nature conservation sites.
The EC Nitrates Directive defines eutrophication only in terms of nitrogen compounds, and therefore does nothing to help protect the majority of aquatic sites where many eutrophication problems are attributable to phosphorus loading.
The Fifth Conference in went further, aiming to eliminate eutrophication and create a healthy marine environment by Fine words. The UK Environment Agency has developed a eutrophication strategy that promotes a coordinated framework for action, and a partnership approach at both national and local levels. The management of eutrophication requires targets and objectives to be agreed for different water bodies. Analysis of preserved plant and animal remains in sediments can be used to estimate the levels of nutrients that occurred in the past, when the water bodies concerned were less affected by eutrophication.
These reference conditions can then be used to determine which waters are most at risk, or have already been damaged by eutrophication, and to prioritize sites for restorative action. The ability to measure and monitor levels of eutrophication has therefore become increasingly important.
During the s there was increased demand in the UK for effective methods of monitoring eutrophication. There was also considerable interest in the development of monitoring systems based on biotic indices. Several 'quality indices' based on a variety of organisms were explored.
For monitoring tools to have practical application, they must satisfy certain requirements:. Within-year variability in nutrient concentrations can be high, particularly for enriched waters. A high sampling frequency may therefore be required to provide representative annual mean data. In nutrient-enriched lakes, annual means are more likely to provide appropriate estimates of phosphorus than winter-spring means, due to the importance of internal cycling of nutrients in summer.
This is an important consideration when designing sampling strategies for use in predictive models of trophic status.
The large group of algal species collectively known as diatoms has been used as indicators of eutrophication in European rivers. Individual species of diatom vary in their tolerance of nutrient enrichment, some species being able to increase their growth rates as nutrients become more available, whilst others are outcompeted and disappear.
As diatoms derive their nutrients directly from the water column, and have generation times measured in days rather than months or years, the species composition of the diatom community should be a good indicator for assessing eutrophication. Convincing correlations have been demonstrated between aqueous nutrient concentrations and diatom community composition, but there are a number of other physical and chemical factors that also affect diatom distribution, such as water pH, salinity and temperature, which also need to be taken into account.
The UK Environment Agency has assessed the extent of eutrophication on the basis of concentrations of key nutrients primarily nitrogen and phosphorus in water, and the occurrence of obvious biological responses, such as algal blooms. There is an intention to rely more heavily in future on biological assessment schemes. One such system is based on surveys of the aquatic plant populations in rivers.
Known as the mean trophic rank MTR approach, this uses a scoring system based on species and their recorded abundances at river sites. Each species is allocated a score its species trophic rank, STR dependent on its tolerance to eutrophication Table 4.
Tolerant species have a low score, so a low MTR tends to indicate a nutrient-rich river. In Britain, rivers in the north and west tend to have the highest MTR scores, whereas rivers in the south and east of England have the lowest.
These scores reflect the influence of numerous factors, such as differences in river flow, patterns of agricultural intensification and variations in population density. In Britain, water supply companies have tended to regard eutrophication as a serious problem only when it becomes impossible to treat drinking water supplies in an economic way.
Threshold concentrations at which action is taken to reduce nutrient loadings thus depend on economic factors, as well as wildlife conservation objectives.
Reduce the source of nutrients e. Reduce the availability of nutrients currently in the system e. Europe is the continent that has suffered most from eutrophication, and increasing efforts are being made to restore European water bodies damaged by nutrient enrichment. If the ultimate goal is to restore sites where nature conservation interest has been damaged by eutrophication, techniques are required for reducing external loadings of nutrients into ecosystems. Although algal production requires both nitrogen and phosphorus supplies, it is usually sufficient to reduce only one major nutrient.
An analogy can be drawn with motor cars, which require lubricating oil, fuel and coolant to keep them moving and are likely to stop if they run short of any one of these, even if the other two are in plentiful supply. As phosphorus is the limiting nutrient in most freshwater systems, phosphorus has been the focus of particular attention in attempts to reduce inputs.
In addition, nitrogen is less easily controlled: its compounds are highly soluble and can enter waterways from many diffuse sources. It can also be 'fixed' directly from the atmosphere. Phosphorus, on the other hand, is readily precipitated, usually enters water bodies from relatively few point sources e.
However, efforts to reduce phosphorus loadings in some lakes have failed due to ongoing release of phosphorus from sediments. In situations where phosphorus has accumulated naturally e. In some circumstances it may be possible to divert sewage effluent away from a water body in order to reduce nutrient loads. Lake Washington is surrounded by Seattle and its suburbs, and in a cyanobacterium, Oscitilloria rubescens , became dominant in the lake.
The sewerage system was redesigned to divert effluent away from the lake, for discharge instead into the nearby sea inlet of Puget Sound. The transparency of the water in the lake, as measured by the depth at which a white disc could be seen, quickly increased from about 1 to 3 m, and chlorophyll concentrations decreased markedly as a result of reduced bacterial populations.
Diversion of effluent should be considered only if the effluent to be diverted does not constitute a major part of the water supply for the water body. Otherwise, residence times of water and nutrients will be increased and the benefits of diversion may be counteracted. The effluent is run into a tank and dosed with a product known as a precipitant, which combines with phosphate in solution to create a solid, which then settles out and can be removed.
It is possible to use aluminium salts as a precipitant, but the resulting sludge contains toxic aluminium compounds that preclude its secondary use as an agricultural fertilizer. There are no such problems with iron salts, so Fe II ammonium sulfate is frequently chosen as a precipitant. Despite its effectiveness, however, phosphate stripping is not yet used universally in sewage treatment.
The interface between aquatic ecosystems and the land is an ecotone that has a profound influence on the movement of water and water-borne contaminants.
Vegetation adjacent to streams and water bodies can help to safeguard water quality, particularly in agricultural landscapes.
Buffer strips are used to reduce the amounts of nutrients reaching water bodies from runoff or leaching. They usually take the form of vegetated strips of land alongside water bodies: grassland, woodland and wetlands have been shown to be effective in different situations.
The vegetation often performs a dual role, by reducing nutrient inputs to aquatic habitat and also providing wildlife habitat.
The plants take up nitrogen directly, provide a source of carbon for denitrifying bacteria and also create oxidized rhizospheres where denitrification can occur. Uptake of nitrogen by vegetation is often seasonal and is usually greater in forested areas with sub-surface water flow than in grassland with predominantly surface flow.
The balance between surface flow and sub-surface flow, and the redox conditions that result, are critical in determining rates of nitrate removal in buffer strips Figure 4. The dynamics of nitrogen and phosphorus retention by soil and vegetation can alter during succession. In newly constructed wetlands, nitrogen retention commences as soon as emergent vegetation becomes established and soil organic matter starts to accumulate: usually within the first years.
Accumulation of organic carbon in the soil sets the stage for denitrification. Under higher nitrogen loading, the amount of nitrogen stored in accumulating organic matter may double, and nitrogen removal by denitrification may increase by an order of magnitude or more.
Accumulation of organic nitrogen and denitrification can therefore provide for reliable long-term removal of nitrogen regardless of nitrogen loading. Phosphorus removal, on the other hand, tends to be greater during the first years of succession when sediment deposition and sorption absorption and adsorption and precipitation of phosphorus are greatest.
However, as sedimentation decreases and sorption sites become saturated, further phosphorus retention relies upon either its accumulation as organic phosphate in plants and their litter, or the precipitation with incoming aqueous and particulate cations such as iron, aluminium and calcium.
Nevertheless, in general, retention of phosphorus tends to be largely regulated by geochemical processes sorption and precipitation which operate independently of succession, whereas retention of nitrogen is more likely to be controlled by biological processes e. Surface retention of sediment by vegetated buffer strips is a function of slope length and gradient, vegetation density and flow rates. Construction of effective buffer strips therefore requires detailed knowledge of an area's hydrology and ecology.
Overall, restoration of riparian zones in order to improve water quality may have greater economic benefits than allocation of the same land to cultivation of crops. Wetlands can be used in a similar way to buffer strips as a pollution control mechanism. They often present a relatively cost-effective and practical option for treatment, particularly in environmentally sensitive areas where large waste-water treatment plants are not acceptable.
For example, Lake Manzala in Egypt has been suffering from severe pollution problems for several years. Land reclamation projects have reduced the size of the lake from an estimated km 2 to km 2. The lake is shallow, with an average depth of around 1. Five major surface water drains discharge polluted waters into the lake. These waters contain municipal, industrial and agricultural pollutants, which are causing water quality to deteriorate and fish stocks to decline.
Recently, efforts have been made to improve water quality in the most polluted of the five drains. This carries waste water from numerous sources, including sewage effluent from Cairo, waste water from industries, agricultural discharges from farms, and discharges and spills from boat traffic. Several methods for drain water treatment have been proposed, including conventional waste-water treatment plants and other chemical and mechanical methods for aerating the drain water.
There are also proposals for construction of a wetland to treat approximately 25 m 3 per day of drain water and discharge the treated effluent back to the drain. The treatment process involves passing the drain water through basins and ponds, designed to have specific retention times.
The pumped water first passes through sedimentation basins to allow suspended solids to settle out primary treatment , followed by a number of wetland ponds secondary treatment. The ponds are cultivated with different types of aquatic plants, such as emergent macrophytes e. Phragmites with well-developed aerenchyma systems to oxygenate the rhizosphere, allowing the oxidation of ammonium ions to nitrate.
Subsequent denitrification removes the nitrogen to the atmosphere. The waste-water treatment mechanism depends on a wide diversity of highly productive organisms, which produce the biological activity required for treatment.
These include decomposers bacteria and fungi , which break down particulate and dissolved organic material into carbon dioxide and water, and aquatic plants. Some of the latter are able to convey atmospheric oxygen to submerged roots and stems, and some of this oxygen is available to microbial decomposers.
Aquatic plants also sequester nitrogen and phosphorus. Species such as common reed Phragmites australis , Figure 4. Another highly productive species is the water hyacinth Eichhornia crassipes , Figure 4. This species is regarded as a serious weed on the lake and is regularly harvested to reduce eutrophication. However, it has a potential role in water treatment due to its high productivity and rapid rates of growth.
The resultant biomass could possibly be harvested and used for the production of nutrient-rich animal feed, or for composting and the production of fertilizer. Further research is required to develop practical options. The passage of water through emergent plants reduces turbidity because the large surface area of stems and leaves acts as a filter for particulate matter. Transmission of light through the water column is improved, enhancing photosynthesis in attached algae.
These contribute further to nutrient reduction in through-flowing water. The mixture of floating plants and emergent macrophytes contributes to removal of suspended solids, improved light penetration, increased photosynthesis and the removal of toxic chemicals and heavy metals. Estimates for the removal of total suspended solids TSS , biological oxygen demand BOD , total phosphorus and total nitrogen by the different wetland components are provided in Table 4.
These suggest that wetlands, combined with sedimentation and ancillary water treatment systems, could play an important part in reducing nutrient loadings. An important aspect of efforts to reduce nutrient inputs to water bodies is the modification of domestic behaviour. Public campaigns in Australia have encouraged people to:. These campaigns have combined local lobbying with national strategies to tackle pollution from other sources. Once nutrients are in an ecosystem, it is usually much harder and more expensive to remove them than tackle the eutrophication at source.
The main methods available are:. In severe cases of eutrophication, efforts have been made to remove nutrient-enriched sediments from lakes. Lake Trummen in Sweden accumulated thick black sulfurous mud after years of receiving sewage effluent. Drastic action was needed. Eventually nutrient-rich sediment was sucked from the lake and used as fertilizer.
The water that was extracted with the sediment was treated with aluminium salts and run back into the lake. This action reduced phosphorus concentrations and improved the clarity and oxygenation of the water. However, removal or sealing of sediments is an expensive measure, and is only a sensible option in severely polluted systems, such as the Norfolk Broads, England. Removal of fish can allow species of primary consumers, such as the water-flea, Daphnia , to recover and control algae.
Once water quality has improved, fish can be re-introduced. Mechanical removal of plants from aquatic systems is a common method for mitigating the effects of eutrophication Figure 4. Efforts may be focused on removal of existing aquatic 'weeds' such as water hyacinth that tend to colonize eutrophic water. Each tonne of wet biomass harvested removes approximately 3 kg N and 0.
Alternatively plants may be introduced deliberately to 'mop up' excess nutrients. Although water hyacinth can be used in water treatment, the water that results from treatment solely with floating macrophytes tends to have low dissolved oxygen. Addition of submerged macrophytes, together with floating or emergent macrophytes, usually gives better results.
Submerged plants are not always as efficient as floating ones at assimilating nitrogen and phosphorus due to their slower growth, resulting from poor light transmission through water particularly if it is turbid and slow rates of CO 2 diffusion down through the water column. However, many submerged macrophytes have a high capacity to elevate pH and dissolved oxygen, and this improves conditions for other mechanisms of nutrient removal.
At higher pH, for example, soluble phosphates can precipitate with calcium, forming insoluble calcium phosphates, so removing soluble phosphates from water. Various species have been used in this way. One submersed macrophyte, Elodea densa , has been shown to remove nitrogen and phosphorus from nutrient-enriched water, its efficiency varying according to loading rate. In terrestrial habitats, removal of standing biomass is an important tool in nature conservation. Reduction in the nutrient status of soils is often a prerequisite for re-establishment of semi-natural vegetation, and the removal of harvested vegetation helps to reduce the levels of nutrients returned to the soil Figure 4.
However, if the aim is to lower the nutrient status of a nutrient-enriched soil, this can be a very long-term process Figure 4.
A short reach of the River Great Ouse in Bedfordshire was found to contain the following species:. Using the trophic rank scores in Table 4. Assume all the species recorded are of similar abundance and therefore there is no need to weight scores according to relative abundance, as you would do in a real situation. The River Great Ouse has a MTR of 3, suggesting it is enriched with nutrients and therefore eutrophic, but it is on the mildest edge of this category so the eutrophication is not severe.
The River Eden has an MTR of 6, indicating that its plant community is composed of species that are moderately sensitive to enrichment, so it can be assumed that this stretch has not undergone substantial eutrophication.
The most sensitive species are absent, suggesting that the waters may naturally carry a moderate concentration of nutrients or that some very mild enrichment has occurred. Prevention of eutrophication at source compared with treating its effects or reversing the process has the following advantages. Technical feasibility. In some situations prevention at source may be simply engineered by diverting a polluted watercourse away from the sensitive ecosystem, while removal of nutrients from a system by techniques such as mud-pumping is more of a technical challenge.
Nutrient stripping at source using a precipitant is relatively cheap and simple to implement. Chorus, I. Toxic cyanobacteria in water: a guide to their public health consequences, monitoring, and management. Crews, J. Agriculture and natural resources U.
Auburn AL: Auburn University Diaz, R. Spreading dead zones and consequences for marine ecosystems. Science , Dodds, W. Eutrophication of U. Environmental Science and Technology 43 , Downing, J. Predicting cyanobacteria dominance in lakes. Canadian Journal of Fisheries and Aquatic Sciences 58 , Edmondson, W. Phosphorus, nitrogen, and algae in Lake Washington after diversion of sewage.
Huisman J. Changes in turbulent mixing shift competition for light between phytoplankton species. Ecology 85 , Jeppesen, E. Top-down control in freshwater lakes: the role of nutrient state, submerged macrophytes and water depth. Lehtiniemi, M. Turbidity decreases anti-predator behaviour in pike larvae, Esox Lucius. Environmental Biology of Fishes 73 , Morris, J. Harmful algal blooms: an emerging public health problem with possible links to human stress on the environment.
Annual Review of Energy and the Environment 24 , Paerl, H. Nuisance phytoplankton blooms in coastal, estuarine, and inland waters. Limnology and Oceanography 33 , Climate change: a catalyst for global expansion of harmful cyanobacterial blooms.
Environmental Microbiology Reports 1 , Climate change: links to global expansion of harmful cyanobacteria. Water Research 46 , Porter, K. The plant-animal interface in freshwater ecosystems. American Scientist 65 , Schindler, D.
Eutrophication and recovery in experimental lakes: implications for lake management. Recent advances in the understanding and management of eutrophication. Limnology and Oceanography 51, Shapiro, J. Biomanipulation: An ecosystem approach to lake restoration. In Water quality management through biological control pp. Brezonik, P. Gainesville, FL: University of Florida Smith, V.
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Home page News What is eutrophication? Causes, effects and control. All the topics All the topics facts News and events reports. All the subjects All the subjects air earth ecosystems energy life space water. Causes, effects and control Algal bloom in along the coast of Qingdao, eastern China nationalgeographic. Example of fertiliser spreading on agricultural land.
Example of discharge of waste water into a reservoir. Algal bloom. Fish mortality. Download " What is eutrophication? Causes, effects and control " pdf file. Eni S. The growth of phytoplankton causes increased turbidity or decreased penetration of light into the lower depths of the water column. In lakes and rivers this can inhibit growth of submerged aquatic plants and affect species which are dependent on them fish, shellfish.
Oxygen is required for all life forms on the planet. Oxygen is produced by plants during photosynthesis. At night animals and plants, as well as aerobic micro-organisms respire and so consume oxygen which results in a decrease in dissolved oxygen levels. Algal blooms may cause strong fluctuations in dissolved oxygen levels. When the algae population is growing at a fast rate, it may block sunlight from reaching other organisms and cause a decrease of dissolved oxygen levels.
When algae die, they are decomposed by bacteria which in this process consume oxygen so that the water can become temporarily hypoxic. Oxygen depletion, or hypoxia , is a common consequence of eutrophication, both in fresh water and seawater. The direct effects of hypoxia include fish kills , especially the death of fish that need high levels of dissolved oxygen Fig.
There is some evidence that hypoxic conditions promote the growth of cyanobacteria as a consequence of enhanced phosphorus release [3]. Many cyanobacteria species produce toxins that are lethal to birds and animals. The respiration of bacteria that decompose dead algae not only causes hypoxia due to oxygen consumption, but also produces CO 2 and thus causes an increase in the carbonic acid content of the water.
A sharp increase in acidity pH even below 7 has been observed in highly eutrophic estuaries and lagoons along the east coast of the US, coinciding with hypoxia [4]. Acidification is an annual feature of these estuaries during summer when algae decay and oxygen depletes. Lowest pH levels undersaturated with respect to aragonite occur near the bottom, especially in poorly flushed stratified systems.
This poses a serious threat to calcifying benthic organisms, for example scallops and other bivalves that spawn in mid-to-late summer. Zones with extreme hypoxia are called dead zones. Upwelling of nutrient-rich waters may produce 'natural' dead zones in some ocean regions where water mass circulation is weak.
These so-called 'oxygen minimum zones' occur below the photic zone, between about - m depth, and are related to microbial mineralization of sinking organic material [5]. However, most severe dead zones occur as a result of anthropogenic eutrophication of stratified water masses.
Stratification is mainly caused by temperature-induced density differences between surface waters and deeper waters in the ocean; stratification in estuaries and coastal waters is mainly due to the salinity-induced density difference between seawater and river water. When an algal bloom in the surface layer decays, the deeper water layers become laden with sinking dead algae that are decomposed by oxygen consuming bacteria. Stratification inhibits turbulent mixing of the oxygen-rich surface layer into the underlying water layer that therefore becomes depleted from oxygen.
The size of dead zones decreases during severe storms when strong wind-driven turbulent water motion partially neutralizes stratification. Climate change is expected to worsen oxygen depletion in the ocean and coastal waters by increasing nutrient supply [6] and increasing ocean surface temperature and stratification [7].
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