Extinction and Modification of Light 191 



reductions in solar intensity due to the greater angle of incidence. The 

 greatest northward development of this influence is illustrated in Fig. 

 6.4 in which the total solar radiation, determined by the combined ac- 

 tion of intensity and duration, is plotted for June 21 under conditions 

 of cloudless sky. At that period the amount of solar radiation received 

 each day at 50° N latitude is considerably greater than that received 

 at the equator, and the daily amount of light received in Scandinavia 

 is roughly the same as that falling on tropical Africa. If you are 

 amazed to find potatoes growing in Norway, or wheat being harvested 

 in Alaska, don't forget that once the plants have begun to develop in 

 the late spring, sufficient light exists in these latitudes for the plants 

 to continue growing all day and a good part of the "night." 



The foregoing review of the circumstances of light on the earth's 

 surface has shown that light is everywhere sufficient for life of some 

 type at least for part of the year. Plants and animals exist from pole 

 to pole. Mosses and lichens grow in abundance right up to the edge 

 of the ice in the polar regions. On the other hand, light is nowhere 

 too strong for animals and plants of some sort. Thus the whole range 

 of light on the earth's surface is generally compatible with life. Of 

 course, light may sometimes be too strong or too weak for individual 

 species, and in these instances it controls growth and distribution. 



Light in Water 



The sunlight available for plants and animals in the aquatic en- 

 vironment has entered the water from the air and hence has first been 

 subjected to all the changes imposed upon it by the conditions above 

 the surface. Ten per cent or more of the light is lost by reflection at 

 the surface or in the special conditions just beneath the surface 

 (Clarke, 1939). In addition, in passing downward, the light is fur- 

 ther modified by the water medium in respect to intensity, spectral 

 composition, angular distribution, and time distribution. 



Extinction and Modification of Light. The light factor in the water 

 environment is subject to a number of variable influences that modify 

 it profoundly. In order to consider the situation first in its simplest 

 terms, imagine a lake from which we have pumped out all the natural 

 water refilled with distilled water. Our lake then contains water of 

 uniform and maximum transparency from top to bottom. Light is 

 reduced in intensity both by absorption and by scattering, and the 

 rate of reduction is measured as the extinction rate, although the term 

 absorption rate is sometimes used loosely for the combined effect. 

 Pure water causes the extinction of light at different rates in different 



