At present, the influence of more than 30 different substances on 

 Vaphnia populations have been investigated, in only two cases were excep- 

 tions observed. These exceptions were noted: 1) when studying the in- 

 fluence of sodium chloride on populations of V. magna (adaptation limits 

 of the Vaphnia to salination exceeded the increase in sensitivity), and 2) 

 in the case of the influence of sulphate sewage from paper mills on V. 

 magna (adaptation to organic substances). In the both instances, the ex- 

 periments with V. long-lspina yielded typical results. This latter species 

 adapts to a lesser degree to increased salinity, and is more sensitive to 

 saprobic pollution. In the case of an increase of sensitivity of popula- 

 tion to pollutants, it is necessary to deal with the increase of sensitiv- 

 ity of individuals in the moment when the saturation biomass in reached 

 (Lesnikov, 1970). Four main types of effects of substances on the pro- 

 ductive properties of population can be discerned: 



Type 1. The substance increases mortality of individuals with- 

 out disturbing the functions of growth and reproduc- 

 tion in individuals. This type is analoguous to the 

 effect of predation and fishing on the population. To 

 some extent, the death of some of the individuals is 

 compensated by intensification of growth and repro- 

 duction in others. Thus, even the death of a part 

 of the community may not lead to a notable decrease 

 in the rate of growth of the biomass. 



Type 2. The substance influences the rate of metabolism and, 

 thus the rate of growth in weight of individuals, but 

 the reproductive function remains undisturbed. Usu- 

 ally, the biomass of the test population is close to 

 that of the control, but is attained at a later time. 



Type 3. Reproductive function of the population is disturbed. 

 The maximum biomass in the test population usually 

 does not reach that of the control . In Vaphnia, no 

 cases of formation of ephippia during the period of 

 high biomass are observed, although these processes 

 take place in the control. This is the most dangerous 

 type of the effect. 



Type 4. Under the influence of substances, the biomass is 



higher than in the control. For the Vaphnia popula- 

 tions, this is usually the influence of sewage which 

 passed through biological treatment (discharge of a 

 part of activated sludge increases the nutrient base 

 for Vaphnia). However, if the sewage is derived from 

 industrial enterprises, the stimulation may be accom- 

 panied by evident intoxication of a part of the indivi- 

 duals. In addition, with this type of effect, it is 

 necessary to account for the possibility of a change in 

 the water quality which may render it unsuitable for 

 valuable fish, e.g., the whitefishes and salmonids, which 

 can be displaced by less valuable fish. 



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