Eutrophication (Greek:
eutrophia—healthy, adequate nutrition, development; German:
Eutrophie) or more
precisely hypertrophication, is the ecosystem response to the addition
of artificial or natural substances, mainly phosphates,
through detergents,
fertilizers,
or sewage, to an
aquatic system.]
Eutrophication is extremely costly to society and recovery from eutrophication
has been estimated to take a thousand years. One example is the
"bloom" or great increase of phytoplankton
in a water body as a response to increased levels of nutrients. Negative
environmental effects include hypoxia, the depletion of oxygen in the
water, which causes a reduction in specific fish and other animals.
Mechanism of eutrophication
Eutrophication arises from the oversupply of nutrients,
which induces explosive growth of plants and algae which, when such organisms
die, consume the oxygen in the body of water, thereby creating the state of
hypoxia.
According to Ullmann's Encyclopedia, "the primary
limiting factor for eutrophication is phosphate." The availability of
phosphorus generally promotes excessive plant growth and decay, favouring
simple algae and plankton over other more complicated plants, and causes a
severe reduction in water quality. Phosphorus is a necessary nutrient for
plants to live, and is the limiting factor for plant growth in many freshwater
ecosystems. Phosphate adheres tightly to soil, so it is mainly transported by
erosion. Once translocated to lakes, the extraction of phosphate into water is
slow, hence the difficulty of reversing the effects of eutrophication.
The source of this excess phosphate are detergents,
industrial/domestic run-off, and fertilizers. With the phasing out of
phosphate-containing detergents in the 1970s, industrial/domestic run-off and
agriculture have emerged as the dominant contributors to eutrophication.
Sodium triphosphate, once a component of many
detergents, was a major contributor to eutrophication.
Lakes and rivers
The addition of phosphorus increases algal growth, but not
all phosphates actually feed algae.Algal blooms are associated with assimilate
the other necessary nutrients needed for plants and animals. When algae die
they sink to the bottom where they are decomposed and the nutrients contained
in organic matter are converted into inorganic form by bacteria. The
decomposition process consumes oxygen, depriving fish and other organisms. Also
the nutrients are concentrated at the bottom of the aquatic ecosystem and if
they are not brought up closer to the surface, where there is more available
light allowing for photosynthesis for aquatic plants, a serious strain is
placed on algae populations.
Enhanced growth of aquatic vegetation or phytoplankton
and algal
blooms disrupts normal functioning of the ecosystem, causing a variety of
problems such as a lack of oxygen needed for fish and shellfish to
survive. The water becomes cloudy, typically coloured a shade of green, yellow,
brown, or red. Eutrophication also decreases the value of rivers, lakes and
aesthetic enjoyment. Health problems can occur where eutrophic
conditions interfere with drinking water
treatment.
Human activities can accelerate the rate at which nutrients
enter ecosystems.
Runoff from agriculture and development, pollution from septic
systems and sewers, sewage
sludge spreading, and other human-related activities increase the flow of
both inorganic nutrients and organic substances into ecosystems. Elevated
levels of atmospheric compounds of nitrogen can
increase nitrogen availability. Phosphorus
is often regarded as the main culprit in cases of eutrophication in lakes
subjected to "point source" pollution from sewage pipes. The concentration
of algae and the trophic state of lakes correspond well to phosphorus levels in
water. Studies conducted in the Experimental Lakes Area in Ontario have shown a
relationship between the addition of phosphorus and the rate of eutrophication.
Humankind has increased the rate of phosphorus
cycling on Earth by four times, mainly due to agricultural fertilizer
production and application. Between 1950 and 1995, an estimated 600,000,000 tonnes of phosphorus
were applied to Earth's surface, primarily on croplands. Policy changes to
control point sources of phosphorus have resulted in rapid control of
eutrophication.
Natural eutrophication
Although eutrophication is commonly caused by human
activities, it can also be a natural process particularly in lakes. Eutrophy
occurs in many lakes in temperate grasslands, for instance. Paleolimnologists
now recognise that climate change, geology, and other external influences are
critical in regulating the natural productivity of lakes. Some lakes also
demonstrate the reverse process (meiotrophication), becoming
less nutrient rich with time.The main difference between natural and
anthropogenic eutrophication is that the natural process is very slow,
occurring on geological time scales.
Ocean waters
Eutrophication is a common phenomenon in coastal waters. In
contrast to freshwater systems, nitrogen is more commonly the key limiting
nutrient of marine waters; thus, nitrogen levels have greater importance to understanding
eutrophication problems in salt water. Estuaries tend to
be naturally eutrophic because land-derived nutrients are concentrated where
run-off enters a confined channel. Upwelling in coastal systems also promotes
increased productivity by conveying deep, nutrient-rich waters to the surface,
where the nutrients can be assimilated by algae.
The World Resources Institute has identified
375 hypoxic coastal zones in the world, concentrated in coastal areas in
Western Europe, the Eastern and Southern coasts of the US, and East Asia,
particularly Japan.
In addition to runoff from land, atmospheric fixed
nitrogen can enter the open ocean. A study in 2008 found that this could
account for around one third of the ocean's external (non-recycled) nitrogen
supply, and up to 3% of the annual new marine biological production. It has
been suggested that accumulating reactive nitrogen in the environment may prove
as serious as putting carbon dioxide in the atmosphere.
Terrestrial ecosystems
Terrestrial ecosystems are subject to similarly adverse
impacts from eutrophication. Increased nitrates in soil are frequently
undesirable for plants. Many terrestrial plant species are endangered as a
result of soil eutrophication, such as the majority of orchid species in
Europe. Meadows, forests, and bogs are characterized by low nutrient content
and slowly growing species adapted to those levels, so they can be overgrown by
faster growing and more competitive species. In meadows, tall grasses that can
take advantage of higher nitrogen levels may change the area so that natural
species may be lost. Species-rich fens can be overtaken by reed or reedgrass species. Forest
undergrowth affected by run-off from a nearby fertilized field can be turned
into a nettle and bramble thicket.
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 will stimulate plant growth. Nitrogen is
not readily available in soil because N2, a gaseous form of
nitrogen, is very stable and unavailable directly to higher plants. Terrestrial
ecosystems rely on microbial nitrogen
fixation to convert N2 into other forms such as nitrates.
However, there is a limit to how much nitrogen can be utilized. Ecosystems
receiving more nitrogen than the plants require are called nitrogen-saturated.
Saturated terrestrial ecosystems then can contribute both inorganic and organic
nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is
also typically a limiting nutrient. This is also the case with
increased levels of phosphorus. However, because phosphorus
is generally much less soluble than nitrogen, it is leached from the soil at a much slower rate
than nitrogen. Consequently, phosphorus is much more important as a limiting nutrient
in aquatic systems.
Ecological effects
Eutrophication was recognized as a water
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%.
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.
- 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 (increased turbidity)
- Colour, smell, 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
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 (called an algal bloom).
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 aerobically 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 Clostridium botulinum 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 out-compete
original inhabitant species. This has been shown to occur in New England
salt
marshes. In Europe and Asia, the Common carp
frequently lives in naturally Eutrophic or Hypereutrophic areas, and is adapted
to living in such conditions. The eutrophication of areas outside its natural
range partially explain the fish's success in colonising these areas after
being introduced.
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
(mussels, oysters), 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.
Sources of high nutrient runoff
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Characteristics of point and nonpoint sources of
chemical inputs ( modified from Novonty and Olem 1994)
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Point sources
Nonpoint sources
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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. Point sources are relatively easy to regulate.
Nonpoint sources
Nonpoint source pollution (also known as 'diffuse' or
'runoff' pollution) is that which comes from ill-defined and diffuse sources.
Nonpoint sources are difficult to regulate and usually vary spatially and
temporally (with season,
precipitation, and other irregular
events).
It has been shown that nitrogen transport is correlated with
various indices of human activity in watersheds, including the amount of
development.[19]
Ploughing in 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. 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.
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. It should
be also noted that lakes within forested land are also under surface runoff
influences. Runoff can wash out the mineral nitrogen and phosphorus from
detritus and in consequence supply the water bodies leading to slow, natural
eutrophication.
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 (e.g., in the form of acid rain)
can also affect 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 blooms.
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 (which causes algal blooms
in the wild) is common in ecosystems and should thus not reverse nutrient
concentrations beyond normal levels. 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 have had a 90% removal efficiency. Still, some
targeted point sources did not show a decrease in runoff despite reduction
efforts.
Shellfish in estuaries: unique solutions
One proposed solution to eutrophication in estuaries is to
restore shellfish populations, such as oysters and mussels. Oyster
reefs remove nitrogen
from the water column and filter out suspended solids, subsequently reducing
the likelihood or extent of harmful algal blooms or anoxic conditions.
Filter feeding activity is considered beneficial to water quality by
controlling phytoplankton density and sequestering nutrients, which can be
removed from the system through shellfish harvest, buried in the sediments, or
lost through denitrification. Foundational work toward the idea of
improving marine water quality through shellfish cultivation was conducted by
Odd Lindahl et al., using mussels in Sweden. In the United States, shellfish
restoration projects have been conducted on the East, West and Gulf coasts. See
nutrient pollution for an extended explanation
of nutrient remediation using shellfish.
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 means of prevention. Riparian buffer
zones are interfaces between a flowing body of water and land, and 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, but it is generally agreed that
a policy regulating agricultural use of fertilizer
and animal waste must be imposed. In Japan the amount of nitrogen produced by
livestock is adequate to serve the fertilizer needs for the agriculture
industry. Thus, it is not unreasonable to command livestock owners to clean up
animal waste—which when left stagnant will leach into ground water.
Policy concerning the prevention and reduction of
eutrophication can be broken down into four sectors: Technologies, public
participation, economic instruments, and cooperation. The term technology is
used loosely, referring to a more widespread use of existing methods rather
than an appropriation of new technologies. As mentioned before, nonpoint
sources of pollution are the primary contributors to eutrophication, and their
effects can be easily minimized through common agricultural practices. Reducing
the amount of pollutants that reach a watershed can be achieved through the
protection of its forest cover, reducing the amount of erosion leeching into a
watershed. Also, through the efficient, controlled use of land using
sustainable agricultural practices to minimize land degradation, the amount of
soil runoff and nitrogen-based fertilizers reaching a watershed can be reduced.
Waste disposal technology constitutes another factor in eutrophication
prevention. Because a major contributor to the nonpoint source nutrient loading
of water bodies is untreated domestic sewage, it is necessary to provide
treatment facilities to highly urbanized areas, particularly those in
underdeveloped nations, in which treatment of domestic waste water is a
scarcity. The technology to safely and efficiently reuse waste water, both from
domestic and industrial sources, should be a primary concern for policy
regarding eutrophication.
The role of the public is a major factor for the effective
prevention of eutrophication. In order for a policy to have any effect, the
public must be aware of their contribution to the problem, and ways in which
they can reduce their effects. Programs instituted to promote participation in
the recycling and elimination of wastes, as well as education on the issue of
rational water use are necessary to protect water quality within urbanized
areas and adjacent water bodies.
Economic instruments, "which include, among others,
property rights, water markets, fiscal and financial instruments, charge
systems and liability systems, are gradually becoming a substantive component
of the management tool set used for pollution control and water allocation
decisions." Incentives for those who practice clean, renewable, water
management technologies are an effective means of encouraging pollution
prevention. By internalizing the costs associated with the negative effects on
the environment, governments are able to encourage a cleaner water management.
Because a body of water can have an effect on a range of
people reaching far beyond that of the watershed, cooperation between different
organizations is necessary to prevent the intrusion of contaminants that can
lead to eutrophication. Agencies ranging from state governments to those of
water resource management and non-governmental organizations, going as low as
the local population, are responsible for preventing eutrophication of water
bodies. In the United States, the most well known inter-state effort to prevent
eutrophication is the Chesapeake Bay.
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. By testing the soil
and modeling the bare minimum amount of fertilizer needed, farmers reap
economic benefits while reducing pollution.
Organic farming
There has been a study that found that organically
fertilized fields "significantly reduce harmful nitrate leaching"
over conventionally fertilized fields. However, a more recent study found that
eutrophication impacts are in some cases higher from organic production than
they are from conventional production.
Cultural eutrophication
Cultural eutrophication is the process that speeds up
natural eutrophication because of human activity. Due to clearing of land and
building of towns and cities, land runoff is accelerated and more nutrients such as phosphates
and nitrate
are supplied to lakes and rivers, and then to coastal estuaries and
bays. Extra nutrients are also supplied by treatment plants, golf courses,
fertilizers, and farms.
These nutrients result in an excessive growth of plant life
known as an algal bloom. This can change a lake's natural food web,
and also reduce the amount of dissolved oxygen in the water for organisms to
breathe. Both these things cause animal and plant death rates to increase as
the plants take in poisonous water while the animals drink the poisoned water.
This contaminates water, making it undrinkable, and sediment quickly fills the
lake. Cultural eutrophication is a form of water
pollution.
Cultural eutrophication also occurs when excessive
fertilizers run into lakes and rivers. This encourages the growth of algae
(algal bloom) and other aquatic plants. Following this, overcrowding occurs
and plants compete for sunlight, space and oxygen. Overgrowth of water plants
also blocks sunlight and oxygen for aquatic life in the water, which in turn
threatens their survival. Algae also grows easily, thus threatening other water
plants no matter whether they are floating, half-submerged, or fully submerged.
Not only does this cause algal blooming, it can cause an array of more long
term effects on the water such as damage to coral reefs
and deep sea animal life. It also speeds up the damage of both marine and also
affects humans if the effects of algal blooming is too drastic. Fish will die
and there will be lack of food in the area. Nutrient pollution is a major cause of algal
blooming, and should be minimized.
The Experimental Lakes Area (ELA), Ontario, Canada is a fully
equipped, year-round, permanent field station that uses the whole ecosystem approach and long-term, whole-lake
investigations of freshwater focusing on cultural eutrophication. ELA is
currently cosponsored by the Canadian Departments of Environment and Fisheries
and Oceans, with a mandate to investigate the aquatic effects of a wide variety
of stresses on lakes and their catchments
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