Page 36 - Science Coursebook

2.7 Pollution

In Stage 7, you learnt about pollution. Pollution means adding things to the
environment that harm living organisms.
Let’s look at one example of pollution, and think about how it affects food webs,

decomposers and the populations of living organisms.

Effects of fertiliser pollution in water

Step 1 Fertiliser added to a lake

A farmer sprays fertiliser onto a field to help his crops to
grow better. By mistake, some of the fertiliser gets into
a lake.

Step 2 Increased growth of water plants and algae
The fertiliser makes the algae and plants in the lake grow
faster. (Algae are very simple plant-like organisms.) Their
populations increase. This is known as an algal bloom.

Step 3 Plants die
ae co
v
he big popula
tions of
plants and alg
The big populations of plants and algae cover the water.
T
.
er the wa
ter
They stop light getting to the plants underneath them.
Many plants die.
Step 4 Increase in decomposers
All the dead plants provide extra food for decomposers.
The numbers of decomposers increase. Populations of
bacteria get very large. The bacteria all respire, using up
most of the dissolved oxygen in the water.

Step 5 Death of fish

Now there is so little oxygen in the water that most
animals cannot live there. Fish die. Only animals that
are adapted to live in low oxygen concentrations
can survive.

34 2 Living things in their environment