Eutrophication
Eutrophication is the enrichment of an ecosystem with chemical
nutrients, typically compounds containing
nitrogen or
phosphorus. Eutrophication is considered a form of pollution because it promotes plant growth, favoring certain species over others and forcing a change in species composition. In aquatic environments, enhanced growth of choking aquatic vegetation or
phytoplankton disrupts normal functioning of the ecosystem, causing a variety of problems. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered.
Encyclopedia
Eutrophication is the enrichment of an ecosystem with chemical
nutrients, typically compounds containing
nitrogen or
phosphorus. Eutrophication is considered a form of pollution because it promotes plant growth, favoring certain species over others and forcing a change in species composition. In aquatic environments, enhanced growth of choking aquatic vegetation or
phytoplankton disrupts normal functioning of the ecosystem, causing a variety of problems. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment. Although traditionally thought of as enrichment of aquatic systems by addition of
fertilizers into
lakes,
bays, or other semi-enclosed waters , terrestrial ecosystems are subject to similarly adverse impacts. Increased content of nitrates in soil frequently leads to undesirable changes in vegetation composition and many plant species are endangered as a result of eutrophication in terrestric ecosystems, e.g. majority of orchid species in Europe. Ecosystems are overgrown by faster growing and more competitive species-poor vegetation, like tall grasses, that can take advantage of unnaturally elevated nitrogen level and the area may be changed beyond recognition and vulnerable species may be lost. Eg. species-rich fens are overtaken by reed or reedgrass species, spectacular forest undergrowth affected by run-off from nearby fertilized field is turned into a thick nettle and bramble shrub.
Eutrophication was recognized as a
pollution problem in European and North American lakes and reservoirs in the mid-20th century. Since then, it has become more widespread. Surveys showed that 54% of lakes in
Asia are eutrophic; in
Europe, 53%; in
North America, 48%; in
South America, 41%; and in
Africa, 28%.
Concept of eutrophication
Eutrophication can be a natural process in lakes, as they age through geological time.
Estuaries also tend to be naturally eutrophic because land-derived nutrients are concentrated where run-off enters the marine environment in a confined channel and mixing of relatively high nutrient
fresh water with low nutrient
marine water occurs.
Human activities can accelerate the rate at which nutrients enter ecosystems. Runoff from
agriculture and development, pollution from
septic systems and
sewers, and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems . Elevated atmospheric compounds of
nitrogen can increase
soil nitrogen availability.
Chemical forms of nitrogen are most often of concern with regard to eutrophication because plants have high nitrogen requirements so that additions of nitrogen compounds stimulate plant growth . Nitrogen is not readily available in soil because N
2, a gaseous form of nitrogen, is very stable and unavailable directly to higher plants. Terrestrial ecosystems rely on
microbial nitrogen fixation to convert N
2 into other physical forms . However, there is a limit to how much nitrogen can be utilized. Ecosystems receiving more nitrogen than the plants require are called
nitrogen-saturated. Under-saturated terrestrial ecosystems contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient. However, in marine environments,
phosphorus may be limiting because it is leached from the soil at a much slower rate than nitrogen, which are highly insoluble.
Ecological effects
Adverse effects of eutrophication on lakes, reservoirs, rivers and coastal marine waters
- Increased biomass of phytoplankton
- Toxic or inedible phytoplankton species
- Increases in blooms of gelatinous zooplankton
- Increased biomass of benthic and epiphytic algae
- Changes in macrophyte species composition and biomass
- Decreases in water transparency
- Taste, odor, and water treatment problems
- Dissolved oxygen depletion
- Increased incidences of fish kills
- Loss of desirable fish species
- Reductions in harvestable fish and shellfish
- Decreases in perceived aesthetic value of the water body
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Many ecological effects can arise from stimulating
primary production, but there are three particularly troubling ecological impacts: decreased biodiversity, changes in species composition and dominance, and toxicity effects.
Decreased biodiversity
When an ecosystem experiences an increase in nutrients, primary producers reap the benefits first. In aquatic ecosystems, species such as
algae experience a population increase . Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water.
Oxygen is required by all respiring plants and animals and it is replenished in daylight by
photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off. In extreme cases, anaerobic conditions ensue, promoting growth of bacteria such as
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that produces
toxins deadly to birds and mammals. Zones where this occurs are known as dead zones.
New species invasion
Eutrophication may cause competitive release by making abundant a normally limiting nutrient. This process causes shifts in the species composition of ecosystems. For instance, an increase in nitrogen might allow new,
competitive species to invade and outcompete original inhabitant species. This has been shown to occur in New England
salt marshes.
Toxicity
Some
algal blooms, otherwise called "nuisance algae" or "harmful algal blooms," are
toxic to plants and animals. Toxic compounds they produce can make their way up the
food chain, resulting in animal mortality. Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten, neuro- and hepatotoxins are released which can kill animals and may pose a threat to humans.
An example of algal toxins working their way into humans is the case of
shellfish poisoning. Biotoxins created during algal blooms are taken up by shellfish , leading to these human foods acquiring the toxicity and poisoning humans. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Other marine animals can be vectors for such toxins, as in the case of ciguatera, where it is typically a predator fish that accumulates the toxin and then poisons humans.
Nitrogen can also cause toxic effects directly. When this nutrient is leached into
groundwater, drinking water can be affected because concentrations of nitrogen are not filtered out.
Nitrate has been shown to be toxic to human babies. This is because bacteria can live in their
digestive tract that convert nitrate to
nitrite . Nitrite reacts with
hemoglobin to form methemoglobin, a form that does not carry oxygen. The baby essentially
suffocates as its body receives insufficient oxygen.
Sources of high nutrient runoff
Characteristics of point and nonpoint sources of chemical inputs
Point Sources
- Wastewater effluent
- Runoff and leachate from waste disposal systems
- Runoff and infiltration from animal feedlots
- Runoff from mines, oil fields, unsewered industrial sites
- Overflows of combined storm and sanitary sewers
- Runoff from construction sites >20,000 mē
Nonpoint Sources
- Runoff from agriculture/irrigation
- Runoff from pasture and range
- Urban runoff from unsewered areas
- Septic tank leachate
- Runoff from construction sites <20,000 mē
- Runoff from abandoned mines
- Atmospheric deposition over a water surface
- Other land activities generating contaminants
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In order to gauge how to best prevent eutrophication from occurring, specific sources that contribute to nutrient loading must be identified. There are two common sources of nutrients and organic matter: point and nonpoint sources.
Point sources
Point sources are directly attributable to one influence. In point sources the nutrient waste travels directly from source to water. For example, factories that have waste discharge pipes directly leading into a water body would be classified as a point source. Point sources are relatively easy to regulate.
Nonpoint sources
Nonpoint source pollution is that which comes from ill-defined and diffuse sources. Nonpoint sources are difficult to regulate and usually vary spatially and temporally .
It has been shown that nitrogen transport is correlated with various indices of human activity in watersheds, including the amount of development.e are three reasons that nonpoint sources are especially troublesome:=Soil retention
Nutrients from human activities tend to accumulate in soils and remain there for years. It has been shown that the amount of phosphorus lost to surface waters increases linearly with the amount of phosphorus in the soil. Thus much of the nutrient loading in soil eventually makes its way to water. Nitrogen, similarly, has a turnover time of decades or more.
Runoff to surface water and leaching to groundwater
Nutrients from human activities tend to travel from land to either surface or ground water. Nitrogen in particular is removed through storm drains, sewage pipes, and other forms of surface runoff.
Nutrient losses in runoff and leachate are often associated with agriculture. Modern agriculture often involves the application of nutrients onto fields in order to maximise production. However, farmers frequently apply more nutrients than are taken up by crops or pastures. Regulations aimed at minimising nutrient exports from agriculture are typically far less stringent than those placed on sewage treatment plants and other point source polluters.
Atmospheric deposition
Nitrogen is released into the air because of ammonia volatilization and nitrous oxide production. The combustion of fossil fuels is a large human-initiated contributor to atmospheric nitrogen pollution. Atmospheric deposition can also effect nutrient concentration in water, especially in highly industrialized regions.
Other causes
Any factor that causes increased nutrient concentrations can potentially lead to eutrophication. In modeling eutrophication, the rate of water renewal plays a critical role; stagnant water is allowed to collect more nutrients than bodies with replenished water supplies. It has also been shown that the drying of wetlands causes an increase in nutrient concentration and subsequent eutrophication booms.
Prevention and reversal
Eutrophication poses a problem not only to ecosystems, but to humans as well. Reducing eutrophication should be a key concern when considering future policy, and a sustainable solution for everyone, including farmers and ranchers, seems feasible. While eutrophication does pose problems, humans should be aware that natural runoff is common in ecosystems and should thus not reverse nutrient concentrations beyond normal levels.
Effectiveness
Cleanup measures have been mostly, but not completely, successful. Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. These efforts, which involved removal of phosphorus, have had a 90% removal efficiency. Still, some targeted point sources did not show a decrease in runoff despite reduction efforts.
Minimizing nonpoint pollution: future work
Nonpoint pollution is the most difficult source of nutrients to manage. The literature suggests, though, that when these sources are controlled, eutrophication decreases. The following steps are recommended to minimize the amount of pollution that can enter aquatic ecosystems from ambiguous sources.
Riparian buffer zones
Studies show that intercepting non-point pollution between the source and the water is a successful mean of prevention . Riparian buffer zones have been created near waterways in an attempt to filter pollutants; sediment and nutrients are deposited here instead of in water. Creating buffer zones near farms and roads is another possible way to prevent nutrients from traveling too far. Still, studies have shown that the effects of atmospheric nitrogen pollution can reach far past the buffer zone. This suggests that the most effective means of prevention is from the primary source.
Prevention policy
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems,=Nitrogen testing and modeling
Soil Nitrogen Testing is a technique that helps farmers optimize the amount of fertilizer applied to crops. By testing fields with this method, farmers saw a decrease in fertilizer application costs, a decrease in nitrogen lost to surrounding sources, or both. By testing the soil and modeling the bare minimum amount of fertilizer needed, farmers reap economic benefits while the environment remains clean.
Natural state of algal blooms
Although the intensity, frequency and extent of algal blooms has tended to increase in response to human activity and human-induced eutrophication, algal blooms are a naturally-occurring phenomenon. The rise and fall of algae populations, as with the population of other living things, is a feature of a healthy ecosystem.ee also
- Hypoxia for links to articles dealing with environmental hypoxia or anoxia
- list of environment topics
- oligotrophic
References
