Why is it dark in the deepsea?

You have probably noticed that coral reefs are colourful places. When filmed in natural light, however, much of the vibrant colour is not visible below 10 metres. The underwater world becomes increasingly blue (and eventually black) the deeper you go. Why is this?

The amount of light that penetrates through seawater depends upon many factors including the time of day, season, geographic location and the clarity of the water.

When light passes through water, it is absorbed and scattered by water molecules and particles in the water.

As water depth increases, the longer (red) wavelengths are the first to be absorbed and scattered. By about 10 m below the surface, most of the red and orange wavelengths of visible light are no longer present. A source of artificial light must be used to view reds and oranges. For this reason, many divers carry a torch even during the day.

As depth increases the scattering and absorption of shorter wavelengths (yellows and greens) becomes evident. By about 150 m depth, even in the clearest water, human eyes can only see blue light.

Beyond about 800 m the human eye can detect no visible light from the surface. At these depths the only visible light is made by living organisms.

The eyes of some deepsea fishes are 15 to 30 times more light-sensitive than human eyes. Some deepsea fishes can detect light in depths down to 1300 m.

Further reading

1. Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
2. Lucas, J & P. Critch. 1974. Life in the Oceans. Thames and Hudson. Pp. 216.