Having evaluated the data by Woodiwiss' method, stations with a 

 biotic index 7-9 were placed into the category of clean waters, 5-6 into 

 moderately polluted, 4 was considered polluted, and 2-3 indicative of 

 dirty waters. Extreme gradations, especially clean (Index 10) and very 

 dirty waters (Index 0-1) were not encountered. This evaluation of the 

 degree of pollution reflects rather objectively the actual situation in 

 parts of the rivers investigated. The calculated values of the biotic 

 index showed a good correlation with such chemical indices of pollution 

 as BOD5, and biochromate oxygen consumption (Figures 1 and 2). These fi- 

 gures show that the value of the index regularly decreases with an in- 

 crease in BOD5 and COD. 



It is especially important that such an essential factor for distribu- 

 tion of benthic animals as bottom type did not interfere with the evalua- 

 tion of the degree of pollution when using Woodiwiss' technique. Thus, 

 samples taken on the cleanest station on the Izhora from silt gave the 

 same high index value of 5 as those taken from stones on the same station. 

 Samples taken from the same bottom types, subject to different degrees of 

 pollution, had different values of this index. For example, samples 

 taken from stones in various parts of the stream had indices of 4 and 9; 

 from clean sands, 2.5, 7; from silted sands, 2-6; and from silts 2-5. 

 This provides an assurance that the values depended primarily on the de- 

 gree of pollution. The great advantage of the method of Woodiwiss is its 

 simplicity, it does not require identification of species of benthic ani- 

 mals. 



However, when using this method, one should remember that under the 

 conditions of sparse fauna, especially on pure sands, more samples must 

 be collected for more correct evaluation. Otherwise, unjustifiably low 

 values of the biotic index may be obtained. 



The method of Woodiwiss has been used on some English and French 

 rivers. Investigations associated with this study have shown that it may 

 be used on the water-bodies of the West, North-West and Center of the 

 European USSR. For wider applicability over all of the USSR, it is 

 necessary to perform special investigations of the fauna of different 

 area. In this way the peculiarities of the fauna in various zoogeogra- 

 phical regions will be considered. 



Recently, indices of species diversity are used to evaluate the 

 degree of water pollution. Among the most frequently used is the Shannon 

 calculation of the Wilm and Dorris index. This index was applied to the 

 present study on samples of phytoplankton, zooplankton, zoobenthos as a 

 group, and separately for the chironomid larvae. It was found that the 

 values calculated by this index are not solely a function of the degree 

 of pollution. The diversity index calculated by the composition of 

 zoobenthic samples exposed to similar levels of contaminants had lower 

 values on stations with uniform habitats. Further, when species of large 

 size ranges prevail, the index also becomes slower. Considerable 

 seasonal variation also occurs at a given station. The hatch of aquatic 

 insects and departure of the imago stage will have a substantial impact 

 on the seasonal dynamics of the zoobenthic community. Pollution is only 



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