Discoloured water

Clean water and clean seawater have a blue colour, if the light passes through a thick layer of water. This colour is produced in a similar way to the production of the blue of the sky as a result of the dispersion of sunlight by water molecules. The blue part of the light is dispersed most here. Added to this is the fact that the blue part in clean (sea)water is absorbed less than the other colours. The blue colour is characteristic of the purity of the water, the salt normally dissolved in seawater has no effect on this.

In oceans, particularly in tropical and subtropical regions, the colour of the seawater is deep blue, in the seas at a high latitude the blue colour is less intense. The blue colour of the oceans often changes in many places and in lakes to other colours. This is caused by the presence of dissolved (organic) substances in the water and small suspended particles. The colour of the seawater in these cases tends towards green, with shading to brown or yellow. In shallow water the colour of the bottom may also play a part. However a temporary discoloration of the water may also occur at sea, largely caused by the presence of plant plankton (phytoplankton) and animal plankton (zooplankton) in the water of oceans and lakes. The movement and spread of plankton very much depends on the sea currents.

The organisms mentioned are generally microscopically small and contain colourings in their body (largely: red). The discoloration of the seawater is then only noticed if more than a normal number of these organisms are present in the water (millions of cells per litre). This is called the 'bloom' of plankton. A special feature in this respect is formed by the phenomenon called 'red water' ('red tide'). If this phenomenon takes place the surface water has such a population of organisms that the sea takes on a clear red discoloration and so its name is completely justified. This bloom as a rule only lasts a short time, because the enormous quantity of organisms in the water quickly consume the nutrients present, while pollution of the water occurs due to the biological decomposition products and because perhaps changes also occur in the temperature and the salt content, two factors that among other things determine the bloom. Because of a change in wind or tide the conditions may already change such that the bloom comes to an end. Coastal waters contain more nutrients than open seas, partly because in the latter the waste materials (which in turn produce nutrients) fall to the bottom, so in coastal waters remain within the reach of the plankton; for this reason the quantity of plankton is greater in coastal waters than in open seas. Coasts where the quantity of plankton may be great are those where the phenomenon of 'upsurge' or 'upwash' occurs. In the upsurging water nutrients are brought to the surface and produce an abundance of plankton.

It is generally assumed that upsurge is the main reason for the abundance of plankton and hence for water discoloration. Such phenomena (which are often seasonal) are found near the coasts of Peru and Chile, the coasts of Central America, Mexico and California, the coast of Florida, the south-west coast of India, the coast of Madras, the west coast of Africa, the Red Sea, the Arabian coast off Aden and off Oman, the east coast of Japan and the east coast of Australia. Various water discolourations reported by Dutch ships did then occur in the regions mentioned. Other places where upsurge may prevail are: the Portuguese coast, the east coast of South America and the coast of Somalia. Such a bloom may also occur in places where due to the contact of different water masses a combination is present of conditions favourable for the growth of plankton.

A striking example of this are the sea currents called 'El Niño' and 'Aquaje', near the coast of Peru. As a result of the removal of oxygen from the water by chemical decomposition of dead plankton or as a result of toxins that may be produced by living plankton, the large scale death of fish and all sorts of marine animals may sometimes occur. Hydrogen sulphide may also be released here. This phenomenon is known on the coast of Peru, where the hydrogen sulphide given off can affect the colour of houses and ships, a phenomenon that has been given the name 'El Pintor' or 'Callao Painter' there.