You might have wondered why the sea is blue. Or if you spend a lot of time at British coastlines, you might have wondered (longingly) why it isn’t! Both questions are interesting because the answers draw from our knowledge of physics, chemistry and biology.
On the most basic level, water is blue because of the way it interacts with light. When light hits the surface of the ocean, three things can happen to it:
1. Absorption. Light from the sun contains all colours, which combine to appear white. When this light reaches water, some of the colours are absorbed by water molecules and converted into heat energy, which warms the ocean surface. The diagram below shows that of the colours visible to the naked eye, red and orange are best absorbed. Absorbed light never reaches our eyes, so we don’t see these colours. These wavelengths also don’t penetrate very far into seawater, which is why underwater photographers require extra lighting to capture reds and oranges in photographs.
2. Scattering. When some colours of light reach the water, they bounce back off in every direction. In the diagram, you can see that blue light is not absorbed very effectively by water. Blue light is instead scattered, and some of the scattered light reaches our eyes. This is why we perceive the sea to be blue.
3. Reflection. Some light never penetrates the water itself, but bounces off the surface and directly into our eyes. This is how we see reflections on the surface of water. If this reflected light has already had some colours ‘filtered’ out by other objects such as the sky, you will only see the colours that have not already been absorbed. This is why on a grey day, the sea often looks grey as well.
The sea can appear a multitude of other colours for various reasons. If seawater contains lots of suspended sediment, it can appear brown or yellow according to local sediment types. It takes quite a lot of energy for larger sediment particles to be lifted into suspension, so the effect is most prominent where there are large waves and during storms.
Seawater can be also brown due to the presence of coloured dissolved organic matter (CDOM), often produced from the decay of detritus such as plant material.
From a biological perspective, plankton can have great effect on the colour of seawater. Phytoplankton contain chlorophyll for photosynthesis, and waters heavily laden with these species often appear green. Large phytoplankton blooms occur in spring and can sometimes be seen from space.
Pollution from sewage or fertilisers can provide sufficient nutrients for certain species to multiply extremely quickly. This phenomenon, known as eutrophication, is seen in heavily polluted estuaries and coastlines. The eventual die-off and decay of the bloom causes oxygen levels in the surrounding water to become severely depleted, wreaking havoc on local ecosystems and fishing operations.
Interestingly, not all phytoplankton blooms are green. Some species of plankton (such as certain dinoflagellates) cause red or orange blooms known as red tides. These can also be disastrous for local ecosystems as these plankton sometimes produce toxins.
Check out NASA’s website for more cool pictures of ocean colours taken from space.
If you enjoyed this article, you might also fancy finding out what causes the sea to smell the way it does – Compound Interest has a great article on this which can be found here.