Why is the sea blue?

‘why is the sea blue?’ is one of the more frequently asked questions from thoughtful children and the braver adults.

Watching the sea and the way it changes colours and moods is no doubt one of the most enjoyable aspects of being near or on the ocean.

The many factors involved in the changing colours of the ocean include: the sun and it’s light, the time of the day, the part of the planet you are located on, the substrate of the ocean floor, and what is and what is not in the water.

The dominant blueish hue of the ocean is due to the absorption of light. What we as humans perceive as light is a spectrum of different wavelengths of energy.

Different ‘colours’ are from different ends of the spectrum, and have different wavelengths; the shorter wavelengths are towards the red end of the spectrum, while the longer wavelengths are towards the blue end of the spectrum.

When light enters water, such as the ocean, the shorter wavelengths are absorbed much more quickly, and the longer wavelengths can penetrate further.

Thus, the shorter wavelengths of light that make up ‘red’ are lost in the first few metres of the water surface, followed by yellows, then greens, finally leaving blue.

Eventually all the wavelengths are absorbed and there is no light left.

The sunnier the day and the clearer the water, the further light penetrates, but at several hundred metres there is little light discernable to humans, and there is no light beyond 1,300 metres depth.

Variations of blue (and other colours discussed later), are effected by depth.

Thus, if all other factors (such as time of day, substrate and salinity) are equal, shallow water, which allows more light to penetrate and reflect back up from the bottom, will be a lighter blue, while deeper water will be a darker blue.

One of the factors that may reduce the penetration of light, and thus cause significant variation in colour, is what is floating in the water, and how much of it there is.

When there is little in the way of nutrients, the waters are ‘crystal clear’, as in the shallow tropical waters around coral reefs, especially when over sand.

Oceans in colder temperate or polar areas that are richer in plankton, may be a deeper blue.

At the other end of the scale, when land based sediment enters the ocean, the lighter material, such as mud or fine dust, may float for some time before it eventually mixes and dissolves, or sinks to the bottom.

Water carrying this ‘suspended’ material varies from a milky blue-grey to brown.

Sediment can enter the ocean in two ways; it can be placed in there by water coming off the land, or the water itself can invade the land and take it out.

The latter occurs in places of higher tidal movement, where the more severe tides sweep up into ocean edge habitats, such as mangroves, and come back with sediment such as mud.

Fresh water going into the ocean from rivers in high rainfall areas carries much sediment.

Sometimes the contrast can even be seen as a clear line, often far out to sea.

This fresh water can carry large amounts of mud, as in large tropical rivers such as the Sepik River from New Guinea, or the Amazon River from Brazil, or when running off particular types of forests it may have a brown tea colour, due to high tannins and saponins.

Where the freshwater is entering in many places at once over an area, such as off the high rainfall sea cliffs and into the ocean in New Zealand’s Fiordland, there may develop a lens of darker freshwater on top of the salt water.

The actual composition of the bottom of the ocean, or the substrate, can also influence colour, especially in shallow waters. The same low nutrient and thus relatively clear waters in the tropics can appear dark if over volcanic sand, or clear if over brightly reflecting broken coral or shell sand.

For more information and pictures about the Ocean, refer to the "Ecosystem Guides" book "Ocean Surfaces of Australasia".