46 FRESH-WATER BIOLOGY 



amounts are not usually accompanied by lack of oxygen. Thus 

 the amount of carbon dioxide may be taken as an index of the 

 suitability of the water. Excessive acidity due to carbon dioxide 

 probably favors the germination of the Saprolegnias fungi which 

 are very destructive of fish eggs and fishes. 



FOOD AND BIOLOGICAL CONDITIONS 



Nitrates are necessary for the growth of aquatic plants and an 

 insufficient quantity is secured from mineral soil. Nitrogen can 

 be fixed only by nitrogen fixing bacteria, such as Clostridium, an 

 anaerobe, and Azotobacter, an aerobe. These bacteria occur on 

 plants and animals in the mud of the bottom of bodies of water. 

 Plants and animals provide carbon compounds for the bacteria; 

 bacteria provide nitrates or nitrites. 



Ammonia results from the decomposition of the dead bodies of 

 plants and animals. The bacteria (Nitrosomonas, Nitrobacter, Ni- 

 trococcus) oxidize it to nitrous acid; nitrous -acid, to nitric acid. 

 These acids unite with bases to form nitrates and nitrites. Work- 

 ing against these two sources of nitrate and nitrite are various 

 denitrifying bacteria (e.g., Bacterium actinopelte), which reduce 

 nitrogen compounds to free nitrogen. Their work is greatly influ- 

 enced by temperature. Baur placed nitrate inoculated with Bacte- 

 rium actinopelte at several temperatures with results as follows: 



a. Temperature, 25 C.: Denitrification initiated 24 hours after 

 inoculation; in 7 to n days later without nitrate. 



b. Temperature, i5C.: Denitrification initiated 4 days after 

 inoculation; in 27 days the solution was without nitrate. 



c. Temperature, 4 to 5 C.: Denitrification began 20 days after 

 inoculation; denitrification incomplete 112 days after. 



d. Temperature, o C.: Denitrification not initiated. 



The quantity of life in water is believed to be in proportion to 

 the available nitrogen compounds. The greatest quantity of plank- 

 ton in the sea is in the polar regions in the summer. It has been 

 suggested that the greater retarding effect of low temperature on 

 the denitrifying organisms as compared with the nitrate producers 

 is a cause of the greater quantity of life in the colder waters. Loeb 

 holds the theory that the greater quantity is due to the longer life 



