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. 



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 



