Seaweed
Seaweeds are simple plant like structures, also known as marine algae, that are adapted to living in
quite difficult conditions.
In this series of activities and investigations we start off in the classroom and then take you out and
about to look at seaweeds, providing you with an opportunity to explore how they have adapted to live
in harsh conditions. We also have a look at the many ways in which we use seaweeds.
Health and safety
CREATE — Seaweed Weaving
If you decide to take your class to the beach
you will need to make sure that you have done
a full risk assessment beforehand. You will also
need to make sure that you know the area well,
you are aware of tide times and the hazards
that you may encounter at the coast.
You will need: Drift wood, loose seaweed found on the beach,
biodegradable string and scissors.
Environmental impact
Seaweeds are living organisms that support
many other organisms. We suggest that you
carry out investigations on seaweeds that have
been washed up on the strandline (the area at
the top of a beach where debris is deposited
and rotting seaweed collects and is no longer
attached to rocks).
If you are looking at seaweeds on rocks or in
rock pools remember not to detach the
seaweeds from the rock and not to take
seaweed out of the water as there may be
living creatures on it who will not like being out
of the water.
Always put everything back where you found it. What to do:
Find two pieces of drift wood, roughly the same size
(approximately the length of a forearm), and some loose seaweed
(please do not pick live seaweed). Using the two bits of drift wood
as the top and bottom of the weaving ‘loom’, wrap string around to
make a weaving frame and a handle to hang it with. Then weave
the various coloured seaweed in and out of the vertical string loom.
Things to talk about:

where green, brown and red seaweed is found in the sea and
on the seashore and the different colours and textures of
seaweeds

how flexible the seaweed was – how was that flexibility
affected by how dry it was?

how seaweeds photosynthesis (brown and red seaweeds have
extra pigments to chlorophyll which helps them absorb light
of a different wave length (light that is altered in water)

how they can float when the tide comes in – what can you
see on the seaweed that may help the seaweed to float?
IN THE CLASSROOM
Where do seaweeds live and what conditions do they need to survive?
Discuss what plants need to survive eg air, water, nutrients and warmth. Seaweeds need all these things too. They
photosynthesise (make energy from sunlight) just like land based plants.
Now think about what it might be like to live in the sea. Encourage pupils to think about:



the type of water (salt)
the effect of tides
the effect of waves.
Perhaps they can think about what happens when a wave washes up
and down the shore and how it moves rocks and sand. How would this affect seaweeds?
Explain that to put up with these conditions seaweeds have adapted.
INVESTIGATE - Seaweed adaptations in the classroom
Investigation 1: Why do living
organisms need to adapt to live in
seawater ?
Investigation 2: What happens to
light when you shine it through
water?
Investigation 3: What happens to
seaweed when the tide goes in
and out?
All living organisms are made of
cells which have water in them to
help the cells thrive and survive.
Because seaweeds need sunlight to
photosynthesise, it is important
that they live in conditions in the
water where there is enough light.
Some seaweeds live above the low
water mark.
1)
2)
3)
4)
Place a sieve over a clear
bowl on a sheet of A4 paper
labelled WITHOUT SALT.
Then place another sieve
over a clear bowl on a sheet
of A4 paper labelled WITH
SALT.
Grate or cut up into thin
strips cucumber or courgette
slices. Divide the mixture in
two placing half in each
sieve.
Add a generous 2-3
tablespoons of table salt to
the WITH SALT sieve and
gently mix the salt with your
hands into the strips or
grated mixture.
After twenty minutes gently
press the grated mixture in
each sieve. Liquid should
come out. Which bowl has
the most liquid in it?
What conclusions can you draw
about the effect of saltwater on
living organisms?
To stop sea water drawing water
from seaweed the seaweed has a
tough skin and also is covered in a
slime which acts as a barrier.
As light passes through water it
gets weaker as it is refracted
(bent).
This means as water gets deeper
there is less sunlight for seaweed
to photosynthesise.
You can explore refraction here
and find out more about light in
water here
This means that when the tide goes
out they are exposed to the air and
sun and start to dry out.
To stop them from drying out
completely they have adapted in
several ways.
1)
Take two tea towels and
soak them in water.
2)
Gently wring one tea towel
and hang it out on a line or
over a chair in the sunlight.
Do the same with another
tea towel but this time roll
it up well and leave it
outside in the sunlight.
3)
After 2 hours what has
happened to the tea towels?
4)
Which tea towel is the driest
(lightest)? Why do you think
one tea towel is drier than
the other?
Seaweed has adapted to low
levels of light in a number of
ways:



Seaweed only grows in
certain depths of water or
the whole seaweed floats on
the surface of the sea
Floats (air bladders) help the
parts of the seaweed that
photosynthesise to stay close
to the surface. You can see
the floats quite clearly on
bladder wrack
Certain pigments – which
make some seaweeds red
and brown – are especially
useful for capturing light
that is weaker or has been
altered by the water.
How do seaweeds reproduce?
Seaweeds have no flowers and do not require pollination to
reproduce. Instead they reproduce using several different
methods.
In some cases parts of some seaweeds can break off and then
grow to form whole new seaweeds. These new seaweeds are
clones and are genetically identical to their parent.
In other cases the seaweed produces male sperm and female
eggs which are released into the water. When the egg meets
the sperm the egg becomes fertilised and a new seaweed
grows.
Reducing the surface area by rolling
up the tea towel reduces the rate at
which moisture (water) is lost or
evaporates from the tea towel.
Some seaweeds which live closer to
the high tide line do this in order to
survive. Follow this link to find out
more
INVESTIGATE - Seaweed adaptations on the beach
DOWN AT THE BEACH
Investigation 1: What colour is seaweed?
In these activities seaweed may be collected. It is
important to collect seaweed from the strandline
(seaweed that has been washed up on the beach)
and not to collect live seaweed – seaweed that is
attached to rocks or another substrate and
submerged. We ask you to wash seaweed in
washing up liquid.
Walk along the beach and look at the rocks and sea.
Every time you come to a seaweed mark its colour in
the table below with a tick. What colour is the
seaweed? Seaweed can be divided into three groups:
green, red and brown. Which colour seaweed did you
find was the most common?
Please make sure you dispose of washing up liquid in
a safe way for example take it back home with you.
Investigation 2: What are the
different parts of seaweed for?
1) Find some seaweed on the
strandline and lay it out.
2) Now use the labels to the right
and place them in the correct
place. These labels are available
on another document in a bigger
format.
3) Make a dichotomous or
branching key
Find at least 3 - 5 different
seaweeds. Give each different
seaweed a name or label them
a,b,c,d and e.
Now make a key using questions
that include the seaweed colour
and the definitions? above. Test
your key on others.
Green Red Brown STIPE The stipe is the stem or
stalk of a seaweed. Its function
is to support the rest of
seaweed plant. The structures
of the stipe varies among the
different types of seaweed, they
can be flexible, stiff, solid, gasfilled, very long, short or
sometimes even completely
absent.
HOLDFAST The holdfast is a
root like structure that holds it
to the rocky bottom. Holdfast IS
NOT necessary for water and
nutrients uptake, but it is
needed as an anchor.
HAPTERA The holdfast is made
up of many fingerlike
projections called haptera.
You can tell the
age
of
some
seaweeds by the
number of air
bladders.
For
example each air
bladder on a frond
of
egg
wrack
represents
one
year of growth.
© Anne Burgess-ShareAlike 2.0 Generic Note not all seaweeds have air
bladders.
BLADE / FROND The blade or
blades are the ‘leaves’ of the
seaweed. Its main function is to
provide a large surface for
sunlight to be absorbed. Some
seaweeds have only one blade,
which may be divided while
other species have a number of
blades. Some blades or fronds
may have a mid-rib running
down the centre.
Investigation 3: What can seaweeds
tell us about climate change?
Did you know?
Air bladders can provide a clue as to
how exposed the seashore gets ie how
shallow the water becomes. Seaweeds
living on exposed shores may have
fewer air bladders than seaweeds
living on less exposed shores.
AIR BLADDER These are hollow,
gas filled structures which help
to keep the baldes buoyant so
they are able to absorb energy
from
the
sun
and
photsosyntheise. You can tell
the age of some seaweeds by
the number of air bladders.
In the photo above you can see all
the little air pockets that give
bladder wrack it’s funny name! It is
these air pockets that allow the
seaweed to float in deep water so
that the blades can reach the light
and photosynthesise.
The Natural History Museum are
running the Big Seaweed Search and
would like you to get out and about
letting them know which seaweeds
you have found on the coast near
you. The survey is easy to complete
and the resources can be printed
from their website
You will need to print off:

The identification guide

The recording form

Instructions for carrying out
the survey are here
Investigation 4: Why are seaweeds important in
ecosystems?
Investigation 5: How does seaweed protect itself
from dessication (drying out in the air)?
Find some seaweed in a rock pool or on the beach.

Find two pieces of fresh seaweed on the
strandline which have a slime coating.

Lay one piece out flat to dry in the air. Wash the
other piece with fresh water and washing up
liquid, removing all the slime and then lay it out
to dry.

After an hour is there a difference between the
seaweeds? The one which had the slime washed
off should be drier (much lighter and more
brittle).
Can you find any creatures living on or around it?
How do you think seaweed may be useful to them? (eg
for food, shelter and protection).
Seaweeds photosynthesise. This means they use the
sun’s energy to make food – they are primary
producers. When they photosynthesise they take in
carbon dioxide and produce oxygen which is released
into the water. Why might this be useful for other
sea creatures?
Bladder wrack ‐ why do you think it got its name? Sea lettuce ‐ why do you think it got its name? DEBATE— The importance of seaweeds
Supermarket sweep
The importance of seaweeds
Seaweed is used by people all over the world in many
different ways. Extracts from seaweed such as agar,
carrageenan and alginate are found in many of our
foods such as ice cream as well as things like
toothpaste.

Can you remember
important?

Climate change may be causing some areas of
the UK to experience more storms and rough
waves. How do you think this may affect
seaweeds?

How do you think pollution could affect
seaweed? - for example if the water becomes
murkier?

If seaweed populations decrease what affect do
you think this may have on other marine
organisms?
Why not do a supermarket sweep to see how many
products you can find with carrageenan, alginate or
agar in?
Which of the products do you have in your house?
How many products containing seaweed extract do you
have in your house?
Draw a graph to show how many products containing
seaweed are found in each room in the house.
why
seaweeds
were