Eutrophication is a process which
occurs both in fresh and marine waters, where excessive nutrient enrichment of
waters that causes structural changes to the ecosystem. According to the
Survey of the State of the World’s Lakes, a project promoted by the
International Lake Environment Committee, eutrophication affects 54% of Asian
lakes, 53% of those in Europe, 48% of those in North America, 41% of those in
South America and 28% of those in Africa. All water bodies are subject to a
natural and slow eutrophication process, which in recent decades has undergone
a very rapid progression due to the presence of man and his activities, that’s
why it also known as cultural eutrophication.
Oxygen Demand (B.O.D.)
Demand refers to the amount of oxygen that is utilised when all of the organic
matter in one litre of water is oxidised by living organisms in the water. The
measurement of B.O.D. is an indication of the level of water pollution. If
there is large amount of organic waste in the water supply, the quality of
aerobic bacteria working to decompose this waste will also be great. In this
case, the biochemical demand for oxygen value will be high and dissolved oxygen
level in the water declines rapidly. Generally, an increase in the B.O.D. level
corresponds to a decrease in the dissolved oxygen levels. The dissolved oxygen
level is an indication of the level of water pollution. It also shows how well
the water can support aquatic plant and animal life. A higher dissolved oxygen
level indicates better water quality. A lower dissolved oxygen level indicates
poor water quality.
Draining of excess nutrient into
ponds, lakes, or rivers encourages the rapid growth of photosynthesis organism,
especially algae. This results in a population explosion known as an algae
bloom ( diagram 1.1 ). The excessive growth of algae restricts the penetration
of light into the water. As a result, the photosynthesis of other aquatic
plants is greatly reduced and this further decreases the supply of oxygen in
the water. The algae also grow faster than their consumers. As a result, most
of the algae die without being consumed. As the photosynthetic organisms die
and organic material accumulates at the bottom of the lake, decomposing
microorganisms, especially aerobic bacteria, grow rapidly and use up oxygen in
the deeper waters at a fast rate. The aerobic bacteria use up oxygen faster
than it can be replenished. An increase in the biochemical oxygen demand causes
the oxygen level to decrease rapidly. The low level of oxygen concentration
kills larger aerobic organisms such as fish. Organisms that require higher
oxygen levels will not survive. In extreme cases, if the oxygen level continues
to drop and the water become completely deoxygenated, anaerobic bacteria will
grow and release toxic gases such as hydrogen sulphide, causing the death of many
aquatic organisms including fish. During the growth of algae certain blue-green
bacteria also produce toxins that may cause the death of fish.
nutrients enrichment of water can be naturally but it is normally increased by
the men’s activities. There are three main sources of anthropic nutrient which
are erosion from agriculture areas, waste water from the industry and sewage
from the city. Besides, some activities will also cause an increase in
Ø Development of aquaculture:
waste matter from the fish and discharge of unused animal food into the water
will contribute to eutrophication.
Ø Storage tank in arid land: usage of large reservoirs to manage and store
the water has been used all over the world. These dams are constructed to allow
collection of drainage waters through hydrographic basins. This action will
result in enrichment of water by nitrogen and phosphorus.
Ø The transportation:
through the ballast of big ships, toxic algae will be carried to uncontaminated
place. They may start to find a habitat for their growth which are stimulated
by nutrient availability.
of eutrophication can be caused physically. Geological issues such as shape of
the shores and bottom of the sea, physical conditions such as tidal movement or
streams. Due to tidal effects, some areas that would seem to be prone to
containment see their waters regularly renewed and are not contained at all and
at last become eutrophic.
thermal conditions of stagnant water bodies, light and temperature affect the
growth of aquatic algae. This phenomena influence the penetration of light
through the water surface because of the development of the algae and this
reduces photosynthesis in deep water layers.
The main impact of
eutrophication is oxygen availability. All the plants require oxygen to carry
out photosynthesis in daylight. In contrast, all plants and animals respire and
consume oxygen. These two processes are dependent on development of the
biomass. Accumulation of biomass will cause organic matter undergoes oxidation
and formed into sediment at the bottom of water will consume all the available
oxygen. Even the oxygen in sulphates will be used by the bacteria and cause sulphur
immediately capture oxygen in the upper layers. The water body will loose all
the oxygen and cause all life disappear. The changes in concentration of oxygen
cause changes in water ecosystem:
· Changes in algal population : When
eutrophication occur, algae obtain sufficient light, nutrients and experience
growth. These cause toxin to be released into the water or be toxic themselves.
· Changes in zooplankton : During
eutrophication, being sensitive to availability of oxygen, these species may
die from limitation of oxygen or changes in the environment in the water.
The best method to cope with eutrophication is
to reduce the input of nutrients into the water surface. The authorities should
eutrophic water be the only source available for the production of drinking
water, all necessary preventive measures should be taken, among which the most
important are to limit the maximum extent possible the amount of organic
matters present in the water before final chlorination. Besides we need to
ensure that there is free chlorine available at the distribution point.
Chlorination itself can ensure the destruction of most of the toxins released
by algae blooms and is necessary for bacteriological safety.
water bodies do not have the same susceptibility to become eutrophic, action
should be taken to reduce the discharge of nutrients into the water surface. This
can be done by reducing usage of chemicals and carry out advanced treatments of
waste waters before their discharge into the environment.
can also be avoided by implementing good practice at farm level. For example,
instead of temporary forage crops, permanent grassland should be promoted. Then,
fertilization plans, regular soil nutrients analysis and registers at plot
level. Moreover, irrigation management should be precised such as soil moisture
of eutrophication in the world
the 1960s and 1970s, Lake Erie was the most suitable example of eutrophication
because of the heavily developed agricultural and urban lands. Algae bloom
occur and causes other species living in the lake die. (diagram 1.2)
Dianchi (diagram 1.3) near Kunming in China, and Lake Taihu, near Wuxi in China
are lakes which have extreme eutrophication. These lakes are covered by the algal
bloom, fishes in the lake are almost die because there is no oxygen for them to