88 Bacteria in Relation to Country Life 



The waters of lakes and ponds. — These are subject to 

 the same phenomena of purification already noted in 

 the preceding pages in connection with the water of 

 rivers. In lakes, as in rivers, the action of dilution, 

 sunlight, sedimentation, and the decomposition of the 

 organic food bring about the decrease in the numbers 

 of bacteria. In some directions, however, there are 

 important differences, if not in kind, at least in degree. 

 The influence of sedimentation is more strikingly ap- 

 parent in lakes than it is in rivers, for the velocity of 

 the current, so important a factor in the latter, plays 

 but a subordinate part in them. 



The water of small lakes and ponds is more liable to 

 be affected than that of larger lakes by the growth of 

 algae, and by the drainage from adjacent land. Another 

 important influence that is of more consequence in this 

 connection is that of sunlight and circulation. The 

 slighter depth of the water allows a more thorough 

 aeration of the bottom layers, and the more frequent 

 interchange between the surface and bottom water un- 

 doubtedly exerts a direct effect on the bacteria. 



The development of plant life in shallow lakes and 

 ponds is, at times, so great as to modify, to a marked 

 degree, the numbers and kinds of bacteria, and the gen- 

 eral character of the water becomes such as to produce 

 .diarrhea in most persons who drink it. In smaller ponds, 

 animals frequently come to modify the bacteriological 

 character of the water ;to their own injury. Comparisons 

 made between ponds to which animals had free access 

 and other similar ponds from which they were excluded 

 showed ver^ marked differences in the bacteriological 



