The value a = N+10, where N is relative abundance of individuals of 

 each of the subfamilies in percent of the total abundance of the chirono- 

 mid larvae. The value 10 is introduced to set limits for changes in the 

 value of the index, K. For example, an increase of this number leads to 

 decrease in the range of possible values of K, and simultaneously to de- 

 crease in its sensitivity. At 10, an optimum relation of the gradation 

 of the index and its sensitivity is attained. Since in clean waters the 

 relative abundance of the Orthocladinae larvae is close to 100 percent, 

 and in the most polluted waters the abundance of Tanypodinae larvae 

 approaches 100 percent and the larvae of the subfamily Chironomidae in- 

 habit both clean and polluted waters, the indicator value of chironomids 

 (a cn ) for the evaluation of the index K is reduced to one half. 



Possible changes in the value of this index in natural waters lie 

 within the limits of 0.09 and 21. Determination of the value of this 

 index of subfamily composition of the chironomid fauna in the rivers 

 studied in this investigation, and in other reports (Gromov, 1950), have 

 shown regular increases with water pollution. In the cleanest waters, K 

 values varied from 0.136 to 1.08, and in the most polluted, from 0.9 to 

 11.5. Identification of chironomid larvae to subfamily is not difficult. 

 Estimation of the index value K is relatively simple, and it appears to 

 accurately reflect the degree of pollution of a river. 



A critical review of methods using oligochaets for evaluation of 

 water quality has shown that the most suitable index is that of Goodnight 

 and Whitley (1961). In the opinion of N.P. Finogenova and A.F. Alimov, 

 an index characterizing the role of oligochaets in the total biomass, but 

 not in the total abundance of animals may be developed in additon to 

 Goodnight and Whitley's. The value of this index increases with an in- 

 crease in pollution. 



The littoral zooplankton community of polluted waters is characterized 

 by a decrease in the total number of crustaceans with an increase in 

 pollution. Simultaneously, as has been shown by M.B. Ivanova in this 

 study, a regular decrease in species composition and abundance of clado- 

 cerans occurs, and copepods dominate over cladocerans. In the most 

 polluted areas, the crustacean zooplankton is represented only by cyclo- 

 poids. The least sensitive to pollution appears to be Eucijclcpi iet- 

 fuitatm . 



All the indices suggested by this study have the distinct advantage 

 of less rigorous taxonomic requirements. These indices also use broader 

 taxomic categories, which naturally increases their wider applicability. 

 It is probable, however, that evaluation of the degree of pollution can 

 not be based solely on these indices. They should be considered as 

 supplemental, since the validity of each may be different in various 

 situations. 



Further investigations are required to make the methods of hydro- 

 biological analysis more exact, to determine the most substantial systems 

 of analysis, and to clarify both the lists and indicator value of separate 

 species under various conditions, and in various geographical regions. 



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