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In
general, eutrophication can be classified into two categories, natural
eutrophication and manmade eutrophication. Natural eutrophication occurs over a
period of centuries while manmade eutrophication takes place over a decade, in
between 8-10 years. Natural eutrophication is a slow process and the
eutrophication that occurs is temporary. However, manmade eutrophication occurs
very quickly. Other than that, natural eutrophication occurs due to the
addition of nutrients by nature itself whereas manmade eutrophication is due to
interference with the ecosystem.

What
are the sources of eutrophication? The sources can also be divided into two
respective categories, point sources and nonpoint sources. Point sources are
sources that can contribute nutrients directly to the water bodies while
nonpoint sources are sources that provide nutrients in a ‘diffuse’ manner.
Point sources are easy to regulate but nonpoint sources are the opposite. It is
difficult to regulate and it varies spatially. At times, it is also said that
nonpoint sources can be very troublesome. Some examples of point sources are
untreated sewage  from sewage treatment
plants, leaching of inorganic fertilisers especially phosphates and nitrates
from agricultural lands, run-offs of animal wastes from farms and run-off from
mines as well as oil field. Next, examples of nonpoint sources includes run-offs
from agriculture, pastures, construction sites and also atmospheric deposition
over a water surface. For a detailed difference of sources, please refer to
Figure 2.0.1 in the Appendices section.

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As
stated earlier, eutrophication occurs due to the presence of nutrients in the
water bodies. Phosphorus is often seen as the main reason of eutrophication in
lakes although there are many other nutrients found dispelled into the water. As
the concentration of nutrients in water sources increases, the concentration of
algae in the same water sources also increases. Researches carried out in the
Experimental Lakes Area in Ontario have shown a relationship between the
addition of phosphorus into water sources and the rate of eutrophication. It
has also been learnt that phosphorus is the limiting factor for the growth of plants
in freshwater bodies. Besides that, phosphorus anchors tightly to the soil.
This leads us to consider that phosphorus is mainly transported via soil
erosion. Once phosphorus has been translocated into water sources, the
extraction of phosphate is slow. Therefore, it will be much more difficult to
reverse the effect of eutrophication. Saying this, the usage of detergents can
be further explained. Some detergents contain too much of phosphate. However,
this phosphate is not removed by sewage treatment and so, it is discharged into
rivers. This increases the concentration of phosphate in the water, thus,
increases the growth of microscopic algae.

Besides
the usage of detergents, farming also somehow contributes to the rise in
eutrophication. Ever since the Second World War, more and more land has been ploughed
to grow arable crops such as wheat, barley and maize. As the soil is being
exposed this way, the bacteria produces soluble nitrates with the aid of oxygen
and water. These nitrates are then disposed into water where they encourage the
growth of algae. If, by any chance, the nitrates reach the underground stores,
they might increase the concentration of nitrates in drinking water. This is considered
unsafe for babies, infants and individuals who have a very sensitive immune
systems.

Moving
on, the usage of fertilizers in agriculture will also promote eutrophication.
As we know, chemical fertilizers contains various minerals and nutrients that
will help in the crop yield. However, once these nutrients reach high
concentration levels and the ground is no longer able to assimilate them, they
will flow into the water source and cause algae to be produced at a faster
rate. Fertilizers are not only used for crops but are also used in aquaculture.
For instance, aquaculture scientists and pond managers will usually eutrophy
water bodies intentionally, that is, by adding fertilizers directly to the water
source. This is to enhance the primary productivity and increase the density
and biomass of economically important fishes. However, they
later realised the result of their actions, when drinking water became
undrinkable at one point. The estimated damage cost caused by eutrophication in
the U.S. is said to be around $2.2 billion yearly.

Besides
that, factory farming is also one of the leading cause of eutrophication. Pigs,
chickens and calves are usually reared in large sheds. Their wastes are washed
out by water and this forms ‘slurry’. If the ‘slurry’ gets in contact with
rivers or any water source, it will provide an excess of nitrates and
phosphates for the growth and development of microscopic algae.

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