The Sea-water and its Physical and Chemical Properties 63 



downwards the spectral curve asymptotically. With layers of increasing thickness 

 the increasing saturation of the colour gives a slow displacement towards the blue, 

 while at the same time the brightness of the colour decreases rapidly so that only a 

 relatively thin surface layer is concerned in the colour of the sea. According to Kalle, 

 the result is a colour with a wavelength approaching 492 m/x, a somewhat greenish 

 blue, corresponding to a light path of 38 m. This shows immediately that the deep 

 blue colour of the Sargasso Sea cannot be explained in this way. If selective scattering 

 of the different colours is taken into account the colour curve lies further towards 

 shorter wavelengths. As far as the colour is concerned the most important point on this 

 curve approaches that corresponding to a 50 m thick layer where the colour value 

 is 485 m/Li. This value agrees fairly well with the colour of the Sargasso Sea, especially 

 if the higher order scattering which would give a further slight displacement towards 

 shorter wavelengths is taken into account. The absorption and the scattering of light 

 are thus responsible for the blue colour of the tropical and subtropical areas of the 

 ocean and they are reinforced by the greater brightness of the sunlight and of the diffuse 

 light from the sky and by the almost completely pure sea-water of these areas. 



For water masses that are not so pure and contain large numbers of suspended 

 particles (mostly plankton), as is usually the case in higher latitudes, the depth from 

 which the selective scattering is reflected is less, and the colour gradually reverts to a 

 value of 495 m/x. This would be more or less the longest wavelength for the colour 

 of the sea if only absorption and scattering were involved during its formation. Other 

 causes are, however, required to explain the greenish colours of longer wavelength 

 than 495 m/i that are also of frequent occurrence in the open ocean. Investigation has 

 shown that these are due to coloration caused by yellowish substances dissolved in the 

 water. These substances appear to be related to humus and are apparently to be re- 

 garded as products of phytoplankton metabolism. They displace the colour of the 

 water towards the green especially in water masses such as in the English Channel 

 and in the North Sea where values of 498 m/x to 505 m^u may occur. In coastal regions 

 further humus material carried by fresh water flowing into the sea from rivers is 

 added to the more oceanic yellow material and causes a further displacement towards 

 yellow-brown colours. In addition to these yellow substances there may also be 

 fluorescence phenomena in the seas as Ramanathan, and later Kalle, believed; these 

 would give a further displacement towards the green but the extent to which such fac- 

 tors are present is not yet certain. 



A qualitative survey of the contribution of each single factor to the colour of the 

 sea has been given by Kalle in Fig. 36. In the clearest water and with a depth of visi- 

 bility of 50-60 m, selective scattering plays to a very large extent the principal part. 

 If cloudiness due to the presence of plankton occurs, the depth of visibility gradually 

 decreases and the natural absorptive colour of water which tends towards a greenish 

 shade begins to predominate. At the same time small amounts of yellowish substances 

 may be formed as the colour tends more and more towards green. With the increasing 

 turbidity the yellow material becomes more and more important until finally, at very 

 small depths of visibility, the discoloration is due to the natural colour of the material 

 causing the turbidity. Very close to the coast the natural colour of the bottom begins 

 to show through the shallow water, and the colour of the water is clearly altered to- 

 wards this. 



