STREAM POLLUTION 
399 
bottoms, it is very difficult to define all of the conditions at the bottom of a stream 
which may or may not be suitable for bottom inhabitants and therefore affect the 
fish fauna; for over this already complicated association of rock, soil, debris, and 
living organisms pollution may spread a blanket of silt, sludges, pulps, and poisonous 
fluids. 
Studies of the bottom faunae of polluted streams, particularly those carrying 
quantities of organic pollution, have shown that many of the bottom species of 
unpolluted streams are very sensitive to pollution conditions, so that as pollution 
progresses the normal bottom fauna changes giving way to certain more tolerant 
species of tubificid worms, chironomid midges, sphaeriid mollusks, and leeches. Con- 
sequently, various investigators (Marsson, 1911; Suter and Moore, 1922; Turner, 
1927; Richardson, 1928; Wiebe, 1928), in connection with other data have correlated 
the presence of certain bottom forms with different degrees of stream pollution, and a 
few such species of bottom organisms have come to be used to some extent as indices 
of pollution. Richardson (1928) however, in discussing 49 bottom species definitely 
associated with pollutional or subpollutional conditions in the Illinois River points 
out the difficulties attending the use as indices of pollution, of even such species as the 
tubificid worm, Tubijex tubijex, and the midge, Chironomus plumosus, which are com- 
monly regarded as characteristic of pollutional conditions bordering on septic. The 
problem of index species is greatly complicated by the fact that the number of individ- 
uals of any particular index species may vary from zero over a wide range in adjacent 
parts of any polluted stream as the result of conditions other than those produced by 
pollution. 
In the present studies, bottom samples were taken throughout the field work as 
a part of the regidar routine. Reviewing the results of the examinations of many 
hundreds of bottom samples, there is no doubt that bottom samples are a necessary 
part of any pollution study, and that the findings concerning the bottom organisms 
are very valuable when considered with the chemical and physical data. However, 
the writer agrees with the statement of Richardson (1928, p. 410), in connection with 
his Illinois River studies, that index species are of service in pollution studies only 
when used with the greatest caution, and when checking with other indicators. 
With due regard to the limitation just discussed, three organisms — namely, the 
water mold, Sphaerotilus natans, the rattail maggot, Eristalis sp., and the sludge fly, 
Psychoda sp. — were often found helpful in delimiting the septic zone near the source of 
extensive organic pollution; and the tubificid worms, Tubijex sp. and Limnodrilus sp., 
and the larvae of the red midge, Chironomus sp. (particularly Chironomus plumosus ), 
if their presence or absence were supported by the chemical and physical analyses and 
by the data concerning the biological complex as determined from repeated samplings, 
were frequently used in defining the extent of bad to moderate organic pollution. In 
the lesser degrees of organic pollution and in most cases of chemical pollution index 
species as such were of little value. 
On the other hand, the abundance of the various species comprising the bottom 
fauna and particularly the species composition of the bottom fauna at various stations 
in the polluted area w r ere usually of large value, regardless of the sort of pollution, when 
correlated with the physical and chemical data. Gross pollution of several kinds was 
quickly detected from bottom samples both by the physical and chemical conditions 
