The material in this slide show is provided free for educational use only. All other forms of storage or reproduction are subject to copyright- please contact the National Marine Aquarium The slide show was designed and produced for the NMA by STEP, the Science Training & Education Partnership T raining & S cience E ducation P artnership

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 LIGHTOXYGEN HYDROGEN CARBOHYDRATE CARBON 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 Image courtesy of SeaWiFS/ORBIMAGE

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 walk We can plot the number of coins remaining in the bag against the distance travelled

Distance (in metres) Number of coins remaining % 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 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 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 growLight 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 responsibility for modifications, and original material remains copyright of the NMA 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