James Fraser of Dunedin asks :-

Why is deep ocean water blue but coastal waters may be milky white, green or blue?

Nick Strickland, an optical physicist at Industrial Research Ltd in Lower Hutt, responded.

It is often claimed that the sea is blue because it reflects the colour of the blue sky. While the prevailing light conditions do of course have some influence, this isn't the main reason. Water really is blue.

Red light is absorbed much more strongly and will only pass a few metres through pure water, whereas blue light will travel about 100 metres. Thus a glass of water appears clear, but white objects viewed underwater down the length of a swimming pool will really look blue.

Of course the oceans don't contain pure water, so in addition to this unequal absorption, light is also scattered (reflected to all directions) from particles in the water. If the particles are very small then blue light is scattered much more strongly than red (this is known as Rayleigh scattering and is also the reason why the sky is blue). Deep, clear, water will appear blue because the blue light tends to be scattered back to the surface without being absorbed whereas the red light is scattered less and absorbed more.

On the other hand if the particles are quite large then red and blue light will be scattered more equally. This will give a milky-white appearance if the scattering takes place near the surface, or a greenish-blue colour for slightly deeper scattering where there is some red absorption. These colours can be observed in coastal areas around the South Island of New Zealand, where rivers are fed by glaciers containing finely-ground rock.

Other types of particles may also actually reflect light with some colour preference. Particles of biological origin, such as algae and phytoplankton, give a green colour due to the presence of chlorophyll (the same stuff that makes plants green).