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7. Pollution of Water:
Eutrophication
OBJECTIVES
> To recall why water is important to living organisms
> To understand that water supplies oxygen to living organisms
> To know how excess nutrients in water lead to depletion of oxygen levels
> To recall other aspects of water pollution
The causes of oxygen depletion
All living organisms depend on a supply of
water. Many organisms actually live in
water. Most of these aquatic organisms
respire aerobically and so require oxygen
from their environment. Any change that
alters the amount of oxygen in the water
can seriously affect the suitability of the
water as a habitat. The two pollutants that
most often reduce oxygen in water are:
Fertilisers - nitrates and phosphates
are added to soil by farmers. Some of
the fertiliser is washed from the soil by
rain into the nearest pond, lake or
river. This process is called leaching.
Sewage - this contains an excellent
source of organic food for bacteria, and
also contains phosphates from
detergents.
How fertilisers and sewage affect
the oxygen concentration
Water that contains few nutrients is rich in
oxygen and supports a wide variety of
living organisms. The oxygen enters the
water from the atmosphere by diffusion
and from photosynthesising aquatic plants.
Simpler forms of life, such as algae and
bacteria, are controlled because the low
concentration of nutrients such as nitrate
is a limiting factor for their growth.
Learning Module Organisms and Their Environment | Class 11
If more nutrients are made available, from
fertiliser runoff or from sewage, then:
Algae and other surface plants grow very
rapidly, and block out light to plants
rooted on the bottom of the river or pond.
The rooted plants die, and their bodies
provide even more nutrients.
The population of bacteria increases
rapidly. As they multiply, the bacteria
consume oxygen for aerobic respiration.
There is now a biological oxygen
demand (or BOD) in the water because of
oxygen consumed by these microbes.
Other living creatures cannot obtain
enough oxygen. They must leave the area,
if they can, or they will die. Their bodies
provide even more food for bacteria, and
the situation becomes even worse. This is
an example of positive feedback - the
change from ideal conditions causes an
even greater change from ideal
conditions.
The lower diagram opposite shows what
happens if a pond or river receives too many
nutrients. The process is called
eutrophication. The pond or river soon
becomes depleted of living organisms. Only a
few animals, such as Tubifer (sewage
worms), can respire at the very low oxygen
concentrations that are available. The
solution to this problem is straightforward -
do not allow excess nutrients into the
water.
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