Why aren't all Seaweeds Green?
Most land plants are green. This is caused by Chlorophyll, a green pigment that traps light energy to help the plant make its food by photosynthesis.
Seaweeds can be green, red or brown. The brown and red weeds do contain chlorophyll, but this is hidden by other pigments that let them absorb even more light, which is handy where light levels are reduced.
You can see the green chlorophyll in a red or brown seaweed when it dies. The other colours break down first, revealing the green colour beneath. Eventually the chlorophyll goes too leaving behind a white leaf.
Seawater absorbs light, and so does the stuff floating around in it. The deeper you go, the darker it gets. The light that gets absorbed fastest is red light. At depth red objects appear to be black. Red weeds absorb all colours of light except red, and since there isn't much red light they are not missing much. Red weeds can grow deeper than other weeds for this reason.
Clear water lets more light through it than cloudy water. The sea round St Kilda is very clear because it is a long way from rivers or streams that bring down sediment, and the seabed is rock or sand, with no mud to stir up. Kelp in St Kilda grows down to depths of 30m or more, whereas on the Harris coast is rarely grows below 15m; much less than that in muddy sealochs such as the Bays where the water is cloudier.
(Photo: Paul Tyler)
Red weed becomes green as it diesNext Section