specific groups of microorganisms which may be encountered, judging from the 

 nature of the substances studied, e.g., cellulosolytic bacteria for the sew- 

 age of cellulose-paper plants, petroleum oxidizing bacteria when studying 

 petroleum-containing waste water or specific petroleum products, etc. Ex- 

 perimental durations are 21-30 days (Mosevich, 1973). These experiments 

 have been included in a large system of studies, mainly performed in labora- 

 tories of the GosNIORKh systems, though other water toxicology laboratories 

 do not always include them, since they do duplicate hydrochemical experi- 

 ments to some extent i 



It has been found that when water from natural bodies of water is placed 

 in aquaria, during the first four days a significant increase in the popula- 

 tion of microorganisms is observed, after which the number of organisms 

 varies within limits characteristic for the conditions in question. During 

 this time, the water from the natural body of water becomes aquarium water. 



The effects of pollutants may result in an increase in the total popula- 

 tion of microorganisms, or of certain specific groups, or may suppress bac- 

 teria processes. 



In the USA, based on the articles available to us, only one work 

 (Duthrie et aj_., 1974) is similar in methodology to works in the USSR: ex- 

 periments to determine the effect of diuron on microbial processes were per- 

 formed in experimental tanks. In the Laboratory for Study of Environmental 

 Pollutants at Gulf Breeze, Florida, a basically different system of studies 

 in "microcosms" (glass pipes containing water and soil) is used (Bourquin, 

 1977; Bourquin et al_. , 1977). The duration of these experiments is also 20- 

 30 days, but the results are basically different. Each experimental system 

 has its advantages and disadvantages; therefore, the comparability of re- 

 sults of these studies requires further checking. 



HYDROCHEMICAL EXPERIMENTS 



Studies are performed according to two main systems. 



1. Estimate of intensity and nature of breakdown of pollutants. 



2. Influence of pollutants on hydrochemical regime of bodies of 

 water, particularly processes of self-purification from sub- 

 stances other than the pollutant itself. 



Studies of the breakdown or the fate of the pollutant in the water 

 system have been undertaken in both the USSR and USA to varying degrees in 

 almost all experiments. In laboratories of the GosNIORKh system, chemical 

 determination of the eventual fate of the pollutant are always accompanied 

 by biological toxicologic tests, usually using Daphnia magna . Frequently, 

 the products of decomposition of the substance are more toxic than the sub- 

 stance itself. For example, experiments in our laboratory have determined 

 that in solutions of chlorophos (Dipterex) in natural water, during the 

 first 2-5 days, the mean survival time of Daphnia decreases to half; this 

 elevated toxicity is retained for 1.5 months in open vessels and up to 2 



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