The enrichment of lake waters through the discharge of run-off carrying excessive fertilizers from agricultural land, and human waste from settlements. The inflow of phosphate and nitrogen-rich waters can result in the loss of lake flora and fauna, as once-clear waters become turbid with microscopic algae. These are better able to live in the enriched conditions, and cause oxygen depletion for other flora and fauna.
Eutrophication is caused by the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen or phosphorus. Eutrophication is considered a form of pollution, despite the fact that it is a natural process, because it promotes excessive plant growth and decay, favors certain weedy species over others, and causes severe water quality problems. 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. Ecosystems (like some meadows, forests and bogs that are characterized by low nutrient content and species-rich, slowly growing vegetation adapted to lower nutrient levels) 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.
Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the mid-20th century.
Concept of eutrophication
Eutrophication can be a natural process in lakes, as they fill in through geological time, though other lakes are known to demonstrate the reverse process, becoming less nutrient rich with 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.
Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to point source pollution from sewage.
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 (including coral reefs). 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 (primary production). Nitrogen is not readily available in soil because N2, a gaseous form of nitrogen, is very stable and unavailable directly to higher plants. Saturated terrestrial ecosystems contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient.
Ecological effects
Adverse effects of eutrophication on lakes, reservoirs, rivers and coastal marine waters (from Carpenter et al., 1998; modified from Smith 1998)| 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 |
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.
Oxygen is required by all respiring plants and animals and it is replenished in daylight by photosynthesizing plants and algae.
New species invasion
Eutrophication may cause competitive release by making abundant a normally limiting nutrient.
Toxicity
Some algal blooms, otherwise called "nuisance algae" or "harmful algal blooms," are toxic to plants and animals.
An example of algal toxins working their way into humans is the case of shellfish poisoning.
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.
Sources of high nutrient runoff
Characteristics of point and nonpoint sources of chemical inputs (from Carpenter et al, 1998; modified from Novonty and Olem 1994)|
Point Sources Wastewater effluent (municipal and industrial) 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 m²
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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.
Nonpoint sources
Nonpoint source pollution (also known as 'diffuse' or 'runoff' pollution) is that which comes from ill-defined and diffuse sources.
It has been shown that nitrogen transport is correlated with various indices of human activity in watersheds, including the amount of development. Agriculture and development are activities that contribute most to nutrient loading.
There 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. Thus much of the nutrient loading in soil eventually makes its way to water.
Runoff to surface water and leaching to groundwater
Nutrients from human activities tend to travel from land to either surface or ground water.
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. Regulations aimed at minimising nutrient exports from agriculture are typically far less stringent than those placed on sewage treatment plants (Carpenter et al., 1998) and other point source polluters.
Atmospheric deposition
Nitrogen is released into the air because of ammonia volatilization and nitrous oxide production. Atmospheric deposition (e.g., in the form of acid rain) 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. 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. While eutrophication does pose problems, humans should be aware that natural runoff (which causes algal blooms in the wild) is common in ecosystems and should thus not reverse nutrient concentrations beyond normal levels.
Effectiveness
Cleanup measures have been mostly, but not completely, successful.
Minimizing nonpoint pollution: future work
Nonpoint pollution is the most difficult source of nutrients to manage.
Riparian buffer zones
Studies show that intercepting non-point pollution between the source and the water is a successful mean of prevention (Carpenter et al., 1998). 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.
Prevention policy
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems, but it is generally agreed that a policy regulating agricultural use of fertilizer and animal waste must be imposed.
Nitrogen testing and modeling
Soil Nitrogen Testing (N-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.
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.
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