98 FRESH-WATER BIOLOGY 



not end with the production of such a relatively simple compound as 

 ammonia. Further oxidation of the ammonia to nitrites takes 

 place and the nitrites in turn are oxidized to nitrates. The for- 

 mation of nitrites and nitrates, like the formation of ammonia, is 

 due to bacterial activity; this process is known as nitrification. 

 Special and peculiar varieties of bacteria are concerned in the proc- 

 ess of nitrification. One species is able to oxidize ammonia to 

 nitrite, but is unable to carry the process of oxidation any further. 

 At this stage of decomposition a second species takes up the work 

 and completes the process by oxidizing the nitrites to nitrates. 



If we follow the fate of the nitrogen introduced into a sewage- 

 polluted river, we find that there occurs first a breaking down of 

 the albuminous compounds and a consequent increase in the 

 amount of "free ammonia" in the water; further down, nitrites 

 begin to appear and eventually nitrates are found. A river water 

 in which the process of nitrification has occurred and which is 

 therefore rich in nitrates affords a peculiarly favorable medium 

 for the growth of plant life and often "blooms" with a myriad of 

 microscopic algae. The presence of a multitude of algae in- 

 fluences in its turn the life conditions of aquatic protozoa and of 

 higher animal organisms. At times when through the advent of 

 low temperature or other unfavorable conditions the algae die off, 

 the albuminous compounds constituting their dead bodies undergo 

 decomposition; ammonia, nitrites, and then nitrates are again 

 formed, and the nitrogen cycle begins anew. The food supply of 

 the whole plankton of fresh-water streams and ponds is therefore 

 dependent upon the activity of bacteria, and the share of these 

 organisms in producing or modifying the conditions under which 

 all aquatic life is possible can never be ignored. 



