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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 If more nutrients are made available, from
All living organisms depend on a supply of fertiliser runoff or from sewage, then:
water. Many organisms actually live in Algae and other surface plants grow very
water. Most of these aquatic organisms rapidly, and block out light to plants
respire aerobically and so require oxygen rooted on the bottom of the river or pond.
from their environment. Any change that The rooted plants die, and their bodies
alters the amount of oxygen in the water provide even more nutrients.
can seriously affect the suitability of the The population of bacteria increases
water as a habitat. The two pollutants that rapidly. As they multiply, the bacteria
most often reduce oxygen in water are: consume oxygen for aerobic respiration.
Fertilisers - nitrates and phosphates There is now a biological oxygen
are added to soil by farmers. Some of demand (or BOD) in the water because of
the fertiliser is washed from the soil by oxygen consumed by these microbes.
rain into the nearest pond, lake or Other living creatures cannot obtain
river. This process is called leaching. enough oxygen. They must leave the area,
Sewage - this contains an excellent if they can, or they will die. Their bodies
source of organic food for bacteria, and provide even more food for bacteria, and
also contains phosphates from the situation becomes even worse. This is
detergents. an example of positive feedback - the
change from ideal conditions causes an
How fertilisers and sewage affect even greater change from ideal
conditions.
the oxygen concentration
The lower diagram opposite shows what
Water that contains few nutrients is rich in
happens if a pond or river receives too many
oxygen and supports a wide variety of
nutrients. The process is called
living organisms. The oxygen enters the
eutrophication. The pond or river soon
water from the atmosphere by diffusion
becomes depleted of living organisms. Only a
and from photosynthesising aquatic plants.
few animals, such as Tubifer (sewage
Simpler forms of life, such as algae and
worms), can respire at the very low oxygen
bacteria, are controlled because the low
concentrations that are available. The
concentration of nutrients such as nitrate
solution to this problem is straightforward -
is a limiting factor for their growth.
do not allow excess nutrients into the
water.
Learning Module Organisms and Their Environment | Class 11 26
