INTRODUCTORY 19 



many marine animals have developed luminescent organs for use at night 

 or in the ocean depths. 



When light passes through sea water it suffers diminution in intensity 

 owing to the absorptive power of water and solutes and the scattering 

 effects of water molecules and suspended particles. Pure water allows 

 maximal penetration of radiant energy in the visible portion of the spec- 

 trum from 400 m// to 580 m t u (violet to yellow), less of light waves from 

 580 mil to 700 m/< (orange and red), while far ultra-violet and infra-red 

 are heavily absorbed. In oceanographical work the rate of decrease of 

 light with depth is given by the extinction coefficient 



jti v = 2-30 (log 10 p 1 - log 10 p 2 ) d 



where p x and p 2 are the percentage illuminations at two points differing 

 in depth by d metres. The extinction coefficient is a measure of the true 

 absorption by sea water, absorption by coloured substances in solution if 

 any, and scattering of light by suspended particles (34). 



The amount of light which penetrates into the sea depends on several 

 factors, namely surface intensity and the transparency of the water. There 

 are obvious diurnal, seasonal and latitudinal changes in the intensity of 

 incident light. A variable amount of the light that falls on the surface of 

 the sea is reflected back, the amount being minimal when the sun is at the 

 zenith. Oblique rays, on entering the water, must travel farther than verti- 

 cal rays to reach the same depth, and are quickly absorbed before they 

 penetrate far beneath the surface (37). 



Studies in different regions have shown that the transmission of light 

 in the sea varies widely with locality. Light penetration is maximal in the 

 open ocean in the tropics, where turbidity and plankton density are low 

 and the sun's rays at noon fall vertically on the surface, and is reduced at 

 higher latitudes. In general, absorption is much greater in coastal than in 

 oceanic waters because of the greater turbidity of the former (Fig. 1.6). 

 Oceanic waters off the coast of Washington, for example, show minimal 

 extinction coefficients twice that of pure water, and maximal values up to 

 ten times as great. In coastal waters (Strait of Juan de Fuca), minimal and 

 maximal values are sixteen times and thirty-four times as great as those 

 for pure water. 



Absorption changes across the visible spectrum according to the charac- 

 ter of the water. In clear oceanic water, with a minimal content of sus- 

 pended matter and organisms, penetration is greatest in the blue and least 

 in the red region of the spectrum, whereas in coastal waters containing 

 more suspended material, maximal penetration shifts to the green. This is 

 due to the differential scattering effects of particulate matter on light of 

 different wave-lengths, the blue end of the spectrum being affected most. 



Utterback (38), who has investigated light penetration off the coast of 

 Washington, found that clear oceanic water had maximal transparency 

 at wave-length 480 m//, at which 97-5 % of the radiation penetrated 1 metre; 

 and in coastal waters, maximal transparency occurred at 530 m/ti, with 



