Life of ike Waters 379 



to headquarters and call the culprit Crenothrix before the 

 bar of outran:ed ])rivate opinion. This is a mieroscopic fila- 

 meiitous plant allied to the bacteria, which sometimes devel- 

 ops extensively in water pipes and deposits iron in its sheaths, 

 and to which the stains on clothes are sometimes due. 



The work of the sanitary biologist in connection with waters 

 for drinking and other domestic uses has been to ascertain 

 the effect of various organisms present in the water, and 

 the means of control and removal of those which are in- 

 jurious. But it would take us too far afield to go into this 

 subject, which is largely a technical one of sanitary engineer- 

 ing. 



The work of the Bureau of Fisheries, being primarily of 

 an economic character, may perhaps best be discussed "else- 

 where, which leaves us the more purely scientific phases of 

 fresh water biology for consideration here. The problems 

 of the biologist who studies the life of inland waters, are 

 much the same as those of the biologist upon the sea. His 

 work is to ascertain the kinds of life inhabiting these waters 

 and their abundance and behavior in relation to their en- 

 vironment. His first undertaking then is to study the physi- 

 cal and chemical character of inland w^aters and to deter- 

 mine the species of their animal and plant iidiabitants, while 

 secondarily there opens up to him a vast field of questions 

 relative to the structure, nutrition, reproduction and move- 

 ment of these inhabitants, and the way in which their activi- 

 ties are related to the. various factors in their environment. 

 Most of these problems find a place as well in other fields of 

 biology to which reference has been made elsewhere. AVe 

 may here consider a few which belong especially in this dis- 

 tinctive field. 



In the yearly life of lake or river there occurs a cycle of 

 changes even more marked than those of the ocean. During 

 the warm bright months of summer the plants and animals 

 enjoy the heyday of their existence and may uudtiply so 

 rapidly that the water appears "soupy" from them. Espe- 

 cially is this true of the algae, which may form a thick green 

 scum on the surface of lake or pool. But with the advent 

 of the cold, when lakes and ponds pass into a period of win- 

 ter "sleep," the life which they contain seems almost to 

 vanish, so that where in summer one might find a thousand 

 individuals of animal or plant, in winter he may find one or 

 two or even none at all. At this time changes occur in the 

 water which may have a profound (even a life and death) 

 influence in the life of its inhabitants. AVhen a lake is frozen 

 over to a thickness of several feet, and when on top of the ice 

 sheet is laid a blanket of snow, several more feet thick, the 



