62 



AQUATIC CONDITIONS 



determined by the depth. Small lakes with incomplete circulation in 

 summer are cold at the bottom, being heated at the surface only (Fig. 1 1). 

 Lake Michigan is a deep lake and none of these fluctuations is felt 

 throughout (see Table III below and Table IX, p. 74). In summer the 

 water of the surface is warmed, but if the vertical circulation is what 

 we suppose it to be, all the heat in the waters flowing downward at the 

 leeward side (Fig. 10) must be absorbed above no meters. Table III 

 shows the temperatures recorded by Ward (75); these were evidently 

 taken at the bottom and do not therefore represent the temperatures 

 at the same level in the open water, especially those records made in 

 the shallower situations where the sun's rays can reach the bottom 

 essentially undiminished in intensity. 



TABLE III 



Temperature of Lake Michigan 



3. LIGHT (76) 



Light is an important factor in controlling the distribution and 

 activities of animals. The depth to which light penetrates water is 

 therefore of importance. Forel found that in Lake Geneva, Switzer- 

 land, during the period when the water was clearest, light diminished 

 gradually from 25 to 65 meters, and then decreased rapidly to 115 meters 

 where there was not sufficient light to affect the photographic plate. 

 No doubt future investigation with more accurate means of measuring 

 light will show that very faint light penetrates much farther. The 

 depth of light penetration in fresh water is usually determined by the 

 amount of sediment in the water. Forel found that in Lake Geneva 

 the depth of light penetration decreased with the melting of the mountain 



