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.
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 ( refer to the 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.
development 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.
addition, 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:
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.
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.
Although 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.
Eutrophication 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 control.
of eutrophication in the world
In 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)
Lake 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 breathe.