194 



Light 



the seacoast and by submerged or floating vascular plants along the 

 shores of inland waters in much the same way that light is cut down 

 by the larger vegetation on land. In the free water of ponds, lakes, 

 and the oceans the phytoplankton is sometimes sufficiently abundant 

 to produce a noticeable reduction of light. Plankton populations may 

 cause an additional extinction of light indirectly by adding detritus or 

 stains to the water after the organisms have died and disintegrated. 

 A thick "bloom" of algae in a pond may thus reduce the light supply 

 to such an extent as to curtail its own growth and that of other plants 

 in the water layers beneath. 



G Y 



Wavelength 

 Fig. 6.7. Reduction in intensity and shift in spectral composition of light in 

 heavily stained Rudolph Lake, Wisconsin. (Clarke, 1939, AAAS Ptibl. No. 10.) 



In temperate and coastal seas and in the majority of clear inland 

 lakes fine particles or stains are present that tend to absorb or scatter 

 the blue component of light more strongly than occurs in pure water. 

 As a result the green component of sunlight is usually the most pene- 

 trating in these situations and gives the water its characteristic emerald 

 color. The organic stains occurring in some ponds and rivers absorb 

 the shorter wavelengths so strongly that the red or orange components 

 of sunlight become the most penetrating. In Rudolph Lake, Wiscon- 

 sin, for example, the combined absorption of the water and of stains 

 in the water causes not only a very rapid reduction in the light with 



