6o 



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 

 produced by green plants. The other gases 

 are produced chiefly by organisms as excretory 

 and decomposition products. 



III. PHYSICAL CONDITIONS 



I. CIRCULATION 



The distribution of dissolved salts and 

 gases is dependent upon the circulation of the 

 water, as their diffusion is too slow to keep 

 them evenly distributed. The circulation of 

 water in streams is probably such as to keep 

 all dissolved gases and salts about equally 

 distributed. The water of streams has been 

 found to be supersaturated with oxygen (74). 

 Oxygen is taken up by the water near the 

 surface. Nitrogen and carbon dioxide are 

 produced especially near the bottom, and if 

 the water did not circulate they would be too 

 abundant in some places and deficient in 

 others for animals to live. 



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. 10. Small lakes with little exposure to the wind 

 and with considerable depth frequently develop a summer circulation, 

 such as is shown hi Fig. n. 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. 



FIG. 9. A marine fish 

 affected with gas-bubble 

 disease causing protrusion 

 of the eyes, due to excess 

 of dissolved nitrogen in 

 aquarium water (after Gor- 

 ham). 



