I068 FRESH-WATER BIOLOGY 



feed chiefly upon diatoms. The smaller Crustacea feed upon 

 bacteria and minute algae and protozoa, and are, in turn, devoured 

 by larger organisms. 



In the pubUc water supplies the appearance of algae and proto- 

 zoa in large numbers is the occasion of complaint by the water 

 consumers, for these organisms make the water unsightly and ill 

 smelling. They also clog filters and increase the cost of water 

 purification. 



Water as a Conveyor of Disease Germs. There are few if any 

 bacteria pathogenic to human beings which are naturally found in 

 fresh waters. Trouble comes only when waters become infected 

 with pathogenic bacteria derived from other human beings or from 

 animals. Such bacteria do not thrive and multiply in natural 

 waters so far as is now known but are merely mechanically trans- 

 ported by water. Furthermore, instead of multiplying in water 

 pathogenic bacteria tend to decrease in numbers after the time of 

 infection. This is an important practical matter as it greatly 

 affects the safety of all public water suppHes. For example, a 

 rapidly flowing stream may convey infection for long distances in 

 a short time, while, on the other hand, a lake or pubHc reservoir 

 may store the water for so long a time that there is opportunity for 

 dangerous bacteria to die. 



The longevity of pathogenic bacteria in water depends upon 

 many things among which may be included the original character 

 of the bacteria themselves, the temperature of the water, the oppor- 

 tunities for sedimentation and destruction by other organisms, the 

 effect of sunhght, etc. Using general figures it may be said that 

 under average conditions about 70 per cent of typhoid fever bac- 

 teria will disappear the first week, 90 per cent during the first two 

 weeks, and 99 per cent during six weeks. They will live longer 

 in cold water than in warm water. That is why most of the 

 water-borne typhoid fever epidemics have occurred during the cold 

 months of the year. The spirillum of Asiatic cholera is known to 

 behave in a similar manner but less is known in regard to the rate 

 at which it dies. There is some reason to believe that it has a 

 shorter Hfe in water than the typhoid fever bacillus. There is 

 a group of bacteria presumably of intestinal origin which give 



