60 AQUATIC CONDITIONS 
turn away when they encounter as small an increase as 5 c.c. per liter 
of carbon dioxide. Since a large amount of dissolved carbon dioxide 
is commonly accompanied by a low oxygen content as well as other 
important factors, the carbon dioxide content of water (strongly alkaline 
waters excepted) is probably the best single index of the suitability of 
the water for fishes. 
Fishes do not turn away from ammonia. Ammonia is rarely present 
in any great amount in nature. The effect of dissolved methane is 
unknown. Oxygen and nitrogen go into solu- 
tion from the atmosphere and oxygen is also 
Yh produced by green plants. The other gases 
are produced chiefly by organisms as excretory 
and decomposition products. 
Ill. Puystcat CONDITIONS 
I. CIRCULATION 
The distribution of dissolved salts and 
Ye, ~ gases is dependent upon the circulation of the 
Us, Wy f/ water, as their diffusion is too slow to keep 
NN Gi’ \e / them evenly distributed. The circulation of 
SSS eo water in streams is probably such as to keep 
ZI . ‘ all dissolved gases and salts about equally 
Nig ae distributed. The water of streams has been 
9 Bole found to be supersaturated with oxygen (74). 
2 Oxygen is taken up by the water near the 
Fic. 9—A marine fish Surface. Nitrogen and carbon dioxide are 
affected with gas-bubble produced especially near the bottom, and if 
disease causing protrusion the water did not circulate they would be too 
of the eyes, due to excess abundant in some places and deficient in 
of dissolved nitrogen in 5 “i 
a eathtiL water (alter Gor others for animals to live. ; 
ham). In lakes, during strong winds (74), there is 
a piling-up of water on the leeward side and a 
lowering of the level on the windward side. This is usually com- 
pensated for by a downward flow of the waters along the bottom, 
as shown in Fig. ro. Small lakes with little exposure to the wind 
and with considerable depth frequently develop a summer circulation, 
such as is shown in Fig. 11. Such lakes are without oxygen in the 
deeper water in summer (74), and will not support the fishes which are 
known to inhabit the deeper water of Lake Michigan; hence we con- 
clude that Lake Michigan must have a deep circulation at all times. 
