36 



LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



sanitary recovery. At the same time, their linear distribution is influenced 

 by the intensity of pollution, while population magnitude is determined 

 by degree of suitability of the habitat. Some organisms find pollutional 

 conditions ideal for their livelihood and characterize these conditions by 

 their presence. They are sometimes called index organisms. Others are 

 inhibited by these same conditions, and their absence may be significant. 



Fig. 3. 



Such biotic responses are the basis for biological criteria of stream condi- 

 tion as summarized in Figure 3 modified from Hentschel.^ Some of these 

 are as follows: 



(a) Population density generally is in direct proportion to degree of 

 pollution except at the two extremes. 



^ Hentschel, E. 1923. Abwasserbiologie. Handbuch der biologischen Arbeitsmethoden 

 by Emil Abderhalden. Sec. IX, Part 2, Book 1. Berlin-Vienna: Urban und Schwarzen- 

 berg. 



