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. 11). 
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 110 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 
Hour P.M. Tempera- | Tempera- Temperature at 
Date Unless Sky ture of ture at Depth in Depth 
Stated Air Surface Next Column 
Meters Feet 
Aug. 16 | 4:05 Clear | 16.9°C.) 18.3°C.! 18.3°'C.| 64-0, F | *<'5.66n)') rono 
Aug. 18 |.9:004.m,| Cloudy | 18.9°C.| 17.2°C.| 16.7° C.| 62.0 F.| ‘11.32 || voyuer 
Aug. 18 |12:25 Clear- | 16.7° C.| 17.5°€.) 7.2°C:) 44-0 F:) 220625 aes 
ing 
Aug. 16 | 5:10 Clear | 16.7°C.) 18.3°C.) °7.5°C.) 45.5° F. 32.00) mone 
Aug. 25 | 3:25 aa 20.0°C.| 19.4°C.| 7.2°C.| 44.00 F) °43.38.| agg 
Aug. 16 |12:05 Clear 15.6" C.| 18.3°C,| 52°C.) ange B55. 03cieaeeos 
Aug. rt (10:30A.M.| Hazy een 18.9°C.| 5.2°C.| 4u.3° FB.) 30s, can agaro 
Aug.16 | 1:50 Clear 16.7°C.) 18.3°C.| 4.2° C. 390.5° B.| 112.0oqng0ges 
Aug. 18 | 4:30 Scat- 18.9 C.| 18.3°C.| 4.2°C,) 30.5° BF.) 132.66. 4gGre 
tered 
clouds! 
3. LicHT (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 
