32 



PROBLEMS OF LAKE BIOLOGY 



found to be about 5 per cent of the down- 

 ward component in coastal waters and 

 about 2.5 per cent in the deep basin of the 

 Gulf of Maine (Clarke 1936a). In the 

 case of 16 Wisconsin lakes investigated by 

 Whitney (1938b) this ratio ranged from 

 0.5 per cent to 4.8 per cent according to the 

 lake. Within each lake, however, the light 

 passing upwards remained a constant per- 

 centage of the downward light at all depths 



that the total light from all directions 

 reaching a point 5 to 10 meters below the 

 surface was 1.5 times the illumination 

 which would have been recorded by an ordi- 

 nary plane horizontal photometer. How- 

 ever, a great deal more work is needed on 

 this subject (ef. Johnson and Liljequist 

 1938). 



The distribution in time of the illumina- 

 tion at an.v point is controlled by changes 



PERCENTAGE OF SURFACE LIGHT 



001 



Fig. 5. Comparison of rates of penetration of four regions of the speetrum into A^ineyard Sound 

 (solid lines) and Tront Lake (broken lines). The curves represent the average values of the extinc- 

 tion coefficients as measured by Oster and Clarke (1935) for Vinevard Sound and bv Birge and 



Juday (1930) for Trout Lake. 



and the same is probably true of all oceanic 

 areas. This finding agrees with the theo- 

 retical calculations of Whitney and is of 

 great significance biologically because it 

 means that the directional character of the 

 light is not lost. Furthermore, since plank- 

 tonic organisms can receive light from any 

 direction we should have information not 

 only on the amount of light passing ver- 

 tically downward or upward but also on 

 the radiation traversing the water at other 

 angles. An attack on this problem has been 

 begun by Pettersson (1938), who has found 



in the length of day and by seasonal varia- 

 tion in transparency. The latter is of vital 

 importance since it has been shown to be 

 chiefly responsible for the tremendous dif- 

 ferences in amount of light received daily 

 by organisms at only moderate depths — as 

 much as 10,000 x at 30 meters (Clarke 1938, 

 1939). The agents causing changes in trans- 

 parency, liowcA'er, are not adequately 

 known and present another opportunity 

 for research. 



In dealing with the utilization of the 

 light energy by aquatic animals and plants, 



