30 



FRESH-WATER BIOLOGY 



TABLE II 



Showing Depth of Light Penetration in Lake Geneva (Switzerland) and 



Conditions Affecting the Same in Both Lake Geneva (after 



Forel) and Lake Michigan 



In the eighth column the relative results are given in seconds, in terms 

 of the effect on the photographic plate, of exposures to the sun. 



Lake Michigan 



Rainfall 



Inches 



2.0 

 2-3 

 2-5 

 2.7 

 35 

 3-7 



Centi- 

 meters 



Velocity of wind 

 at noon 



Miles 

 per 

 hour 



Meters 



per 

 second 



5-1 

 5-2 

 6.4 

 6.9 



8.9 

 9-4 I 

 9.2! 



7.2 I 



7-7 I 

 6.6 I 

 6.6 1 

 5-3 ! 



Month 



Lake Geneva, Switzerland 



Rainfall and 

 light 



Prec. 

 in cm. 



January. . 

 , February. 

 .March. . . . 



, April 



. May 



.June 



•July 



.August. . . 

 . September 

 .October. . 

 . November 

 . December 



Light 

 limit at 

 depth in 

 meters 



Light and depth 



Intensity 

 of light 

 (March) 

 at depth 

 in next 

 column 



Depth 



500 sec. 

 500 sec. 

 500 sec. 

 400 sec. 

 360 sec. 

 120 sec. 



60 sec. 



25 sec. 



10 sec. 

 2 sec. 

 o sec. 



0.0 

 19.6 

 25.2 

 45-5 

 55-5 

 65.6 

 75-6 

 857 

 95-8 

 105.4 

 115.6 



Little work on the depth of light penetration has been under- 

 taken in the North American waters. In Table II the rainfall 

 and wind velocity over Lake Michigan are shown and the rainfall 

 for Lake Geneva (Switzerland). The greatest light penetration 

 in Lake Geneva comes when the rainfall is low and when the 

 mountains are still frozen. The Lake Michigan water commission 

 found in a brief period of study that the greatest turbidity fell in 

 January, February, March, and April. The table indicates that 

 this is in months with high wind velocity. The great rainfall of 

 the spring and early summer months tends to keep Lake Michigan 

 turbid, so the greatest light penetration may be predicted for Aug- 

 ust which has least rain and least wind. 



Various streams are normally so muddy that light cannot be ex- 

 pected to penetrate more than a few feet and the fauna accordingly 

 lives in very faint light. Others, as for example streams and lakes 



