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The slide show was designed and produced for the NMA by STEP, the

Science Training & Education Partnership

Training &Science

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Plants and Light in the Ocean

Light and plant growth

Light and depth in the ocean

Summary

How plants survive

Light and plant growth

Light and depth in the ocean

Summary

How plants survive

Plants grow by photosynthesis, which uses light as an energy source to

make complex molecules from carbon dioxide and

water

WATER

WATER

LIGHT OXYGENHYDROGEN

CARBOHYDRATECARBON DIOXIDE

HYDROGEN

Light is captured by various complex molecules in the plant cell, especially the group of green pigments

called chlorophylls

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

Imag

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SeaW

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Light and plant growth

Light and depth in the ocean

Summary

How plants survive

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 walkWe can plot the number of coins remaining in the bag against the

distance travelled

0

5 10 15 20 25

Distance (in metres)

Num

ber

of

coin

s re

main

ing

0

100

10% of coins are lost for each

metre travelled

The less coins there are left, the

less are lost

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

surfaceIn 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

Summary

How plants survive

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

Most vegetation in the ocean is microscopic

plankton

This is a chain-forming diatom - a common type of plankton alga

Each cell is about 0.05 mm long

Too dark for growth

surface

100 m

500 m

1000 m

3500 m

Enough light here for growth

In the open ocean, algal plankton grow only in the top 100 metre-thick layer

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

Summary

How plants survive

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

You have seen that -

NOTES for USERS

The material in this slide show is designed to support the teaching of science at Key Stage 1

A full description of the slide show, and linked activities for students, can be found on the National Marine Aquarium (NMA) web-site:

Teachers are free to amend the slide show in whatever way they feel fit, or to use slides in other contexts. However, please note that neither the NMA nor the designers will accept

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Individual images used in the slides are copyright of NMA or STEP, except where acknowledged separately

The slides have been set up to display as A4 landscape format. If they are incorporated into other slide sequences with different display settings, change in aspect ratio and text location

will occur

The slide sequence contains the minimum of effects and transitions. However, there are some automated animations, and teachers will wish to make sure that they are familiar with

the sequence before use in class

Use the PowerPoint notes viewer to obtain additional information for some slides

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