54 



The Sea-water and its Physical and Chemical Properties 



Wove length, // 



Fig. 28. Energy distribution in solar radiation after passing through water layers of different 

 thickness (according to Schmidt). A-B, at the water surface; A-C, after passing through 

 1 cm of water; A-D, after passing through 1 m of water; A-E, after passing through 



100 m of water. 



Table 20. Extinction values for sunlight passing through sea-water 



Down to a depth of j 00 1 mm 

 Extinction in per cent' 0-6 



01 mm ] 1 mm 

 4-8 i 14-1 



1 cm 

 270 



10 cm 



45-1 



1 m 



64-1 



10m 



81-8 



100 m 



98-6 



The extinction coefficients in Table 1 8 are valid only for pure sea-water. The water 

 of the sea is, however, not optically pure, and always contains more or less large 

 amounts of suspended organic and inorganic particles. The intensity of the light 

 passing through the water is still further reduced by scattering on these particles as 

 well as by the ordinary extinction. It may be so strong that the actual absorption, 

 especially in the presence of very small particles Rayleigh's law applies, but for larger 

 particles the scattering is almost independent on the wavelength. It depends primarily 

 on that part of the total surface influenced by the sun radiation of all the individual 

 particles present in a unit volume. Scattering by large particles is then no longer 

 colour selective (Pernter, 1901). 



The reduction in the intensity of radiation in the sea under natural conditions has, 

 for the first time, recently been subjected to more accurate investigation, because of 

 its special biological interest (see especially Jerlov, 1951; Joseph, 1952). These 

 measurements have been made principally with photo-electric cells which have a 

 sensitivity extending over a considerable range of wavelengths, while the extinction 

 coefficients mentioned above were measured by spectrobolometric methods. The re- 

 sults are thus only comparable after appropriate corrections. The most detailed 

 measurements have been made on lakes (Sauberer and Ruttner, 1 941) ; measurements 

 in the sea which are of greater interest in the present connection are rather few in 

 number. The extinction coefficient applies to the solar radiation and the diff'use sky 

 radiation taken together. When radiation passes through water it undergoes a pro- 

 gressive alteration both qualitatively and quantitatively. The long wave and short 

 wave parts of the spectrum are filtered out almost at once so that the light soon takes 

 on a bluish-green or blue colour. With a greater degree of optical impurity the effect 

 of the scattering is less colour selective; the remaining light is more greenish, or with 

 strong turbidity even yellowish green (Pettersson, 1936). At the same time the light 



