NATURAL LIGHT FIELDS 



735 



Similar sets of curves, showing a less rapid decline in intensity at the 

 red end of the spectrum, were given by Johnson and Kullenberg (1946) and 

 Levring (1947) for sea water at the West Coast of Sweden. Levring at- 

 tributed the absorption in violet and blue — which often converts the blue- 

 green color of pure water into a yellow-green — to the presence of suspended 

 particles. Some additional data can be found in Table 22.X. 



Algae are found to a depth of 120 m. These deep water species live in a 

 light field that contains almost exclusively green radiation (practically no 

 red light is available below 10-20 m.). The total intensity of light avail- 

 able to them is only a few per cent of the light enjoyed by the species living 

 close to the surface of the sea. 



400 



500 600 700 

 WAVE LENGTH, m/i 



500 600 700 

 WAVE LENGTH, mM 



Fig. 



22.56. Light absorption by algae in different depths (in meters) in per 

 cent of light incident on the surface (after Seybold 1934). 



We have discussed in chapter 15 (Vol. I) the ways in which plants adapt 

 themselves to the chromatic composition of the light fields. There is no 

 doubt that the brown algae, containing fucoxanthol, are capable of absorb- 

 ing more light in the middle of the visible spectrum than the green plants, 

 and that the presence of phycobilins in the Rhodophyceae and Ctjanophyceae 

 increases their absorbing capacity in this spectral region even more strongly. 

 Table 22.VIII showed what these changes in the pigment system mean 

 for the absorption capacity of the plants in different spectral regions. 

 However, this table referred to illumination with light whose intensity is 

 constant throughout the spectrum. The consequences of the same changes 

 for the absorption of natural light were discussed by Seybold (1934), Mont- 

 fort (1934) and Levring (1947), and we must refer to their papers for de- 

 tailed results. We merely mention, as an example, that, according to Sey- 

 bold, a Rhodophycea will absorb up to 95% of the light available in a 50 ra. 



