68 Bacteria in Relation to Country Life 



time, the presence of readily decomposable organic 

 matter may prove undesirable because it favors the 

 growth of other bacteria probably responsible for less 

 serious intestinal disturbances. 



The presence of larger amounts of organic matter is 

 also undesirable because its decomposition gives rise 

 to the formation of nitrates and of other inorganic 

 matter which serves as food for microscopic plants 

 known as algae. This is especially true of lakes and 

 sluggish streams in which the rapid increase of the algse 

 may result in the formation of a green scum on the 

 surface. Under such conditions, the water loses its 

 clear sparkhng appearance and acquires a fishy taste, 

 or other undesirable odors and flavors. Since algae, like 

 the green plants, will grow only in the presence of sun- 

 Hght, their development may be suppressed by keeping 

 the water in the dark. This applies particularly to the 

 storing of drinking-water in reservoirs where the algse 

 are more likely to develop. In a number of European 

 cities, covered reservoirs have been used successfully 

 for years, and the growth of algse has been thus elimi- 

 nated, even in waters rich in mineral salts. 



Temperature. — The temperature of the water is, 

 undoubtedly, an important factor in the increase and 

 decrease of its bacteria. In so far as the introduction 

 of organisms from outside sources is concerned, it has 

 already been shown that surface waters contain fewer 

 bacteria in the summer than in the winter, at least in 

 moderately temperate cHmates; and that the greater 

 number of organisms in the winter is due to the greater 

 amount of surface drainage. It may be assumed that 



