42 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



at points where natural recovery has not yet progressed sufiEiciently for the 

 livelihood of others. For this reason population peaks of various species 

 will be arranged in linear order downstream. Certain species, though often 

 occurring in clean water, are able to tolerate and frequently thrive under 

 severe pollutional conditions. Notable of these are tubificid worms and 

 chironomid larvae. Certain naiad worms, sphaeriid mussels, some snails 

 and the Isopod, Asellus communis, thrive in regions of low and improving 

 oxygen content. The presence of appreciable numbers of these, especially 

 in the absence of less tolerant species, is evidence of serious pollution. 

 Some of these biotic relationships and responses to pollution will be shown 

 in the following graphs. 



Total Oligochaete population, and especially the tubificid worms 

 (Graph 5), shows a positive response to the presence of pollutants. They 

 are notorious for this type of response, and their population trend follows 

 the B.O.D. trend quite closely. Even hard bottom populations, which are 

 normally low, increase considerably downstream. 



• 3ort tetUm 

 OHard totUn 



3 a 



S 



3 



Graph 6. Total Chironomidae, log. (no./sq. ft. x 100) 



Chironomid larvae (Graph 6) are absent from the immediate down- 

 stream vicinity of Park Falls but respond quickly to pollutional condi- 

 tions by producing an appreciable population within 2 miles. They reach a 

 population peak at about 11 miles and then decrease gradually downstream. 



The distribution of all other insect larvae (Graph 7) is in contrast 

 to the Chironomidae. A substantial population of these clean water forms 

 was present above Park Falls but dropped to zero following the introduc- 

 tion of pulp and paper mill effluents. The bottom was devoid of insect 

 larvae, other than chironomids, for a downstream distance of 8.6 miles, 

 and some habitats were still unsuitable beyond that point. 



