Download Plants and light in the ocean

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Plants and Light in
the Ocean
Light and plant growth
Light and depth in the ocean
How plants survive
Summary
Light and plant growth
Light and depth in the ocean
How plants survive
Summary
Plants grow by photosynthesis, which
uses light as an energy source to
make complex molecules from
carbon dioxide and water
WATER
WATER
LIGHT
CARBOHYDRATE
HYDROGEN
HYDROGEN
OXYGEN
CARBON
DIOXIDE
Light is captured by various complex
molecules in the plant cell, especially
the group of green pigments called
chlorophylls
Image courtesy of SeaWiFS/ORBIMAGE
This map of plants in the oceans is made by a
satellite sensor which measures chlorophyll in the
water
The colours code for amount of chlorophyll - red and orange is HIGH, blue and purple is LOW
Light and plant growth
Light and depth in the ocean
How plants survive
Summary
Light is absorbed by water, so the
amount of light decreases as you go
deeper into the ocean
Absorption is exponential
To understand exponential decay, we
will first look at a very simple example
Imagine that you are carrying a bag
containing 100 coins
There is a small hole in the bag.
Each coin has a one-in-ten chance of
falling through the hole for each
metre you walk
We can plot the number of coins
remaining in the bag against the
distance travelled
10% of coins are
lost for each
metre travelled
Number of coins remaining
100
0
The less coins
there are left,
the less are lost
0
5
10
15
Distance (in metres)
20
25
The process of light absorption works
in the same way
Each particle of light - photon - has
the same chance of being stopped by
a water molecule within a certain
distance
To start with, light decreases rapidly
with depth. Further down, light
decreases more slowly
Plants can grow at light levels which
are at least 1% of that at the surface
In clear ocean water, this amount of
light is found at depths down to about
150 metres
In muddy coastal waters, this depth
may be as shallow as 20 metres
Light and plant growth
Light and depth in the ocean
How plants survive
Summary
On land, sunlight is
in plentiful supply in
most places at most
times of year
Plants compete for
light. Trees grow tall
trunks so that they
can stand above
other plants which
might shade them
In the ocean, water
absorbs light
This sets a depth
limit for plant growth
The average depth of the oceans is 3.7 kilometres
75% of the oceans are more than 3 kilometres deep
Only 5% of the oceans are less than 200 metres deep
Plants growing
attached to the
seabed, like this
kelp, can only grow
where the ocean is
shallow enough to let
through enough light
for growth
95% of the oceans are ‘off
limits’ to bottom-dwelling
vegetation
This is a chain-forming diatom - a
common type of plankton alga
Most vegetation in the ocean
is microscopic plankton
Each cell is about 0.05 mm long
surface
Enough light here for growth
100 m
500 m
1000 m
In the open ocean, algal
Too dark for growth
plankton grow only in the top
100 metre-thick layer
3500 m
Plankton algae do not need complex
structures to compete for light or to
retain water
They do need to stay close to the
surface, either by floating, swimming, or
just sinking very slowly
There is a small mass of
algae per unit area
but because they are very
simple, they can grow fast
Light and plant growth
Light and depth in the ocean
How plants survive
Summary
You have seen that Plants need light in order
to grow
Light is absorbed by the
water, producing an
exponential decrease
with depth
You have seen that 95% of the oceans are too
deep for bottom-dwelling
plants to survive
Vegetation in the open
ocean consists of
single-celled algae
NOTES for USERS
The material in this slide show is designed to support the teaching of science at Key Stages 3-4
A full description of the slide show, and linked activities for students, can be found on the
National Marine Aquarium (NMA) web-site:
www.national-aquarium.co.uk
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