lo8o FRESH-WATER BIOLOGY 



substances. Even the wastes containing inert suspended matter 

 may interfere with microscopic Hfe along the shores by smothering 

 the tiny vegetable and animal cells. Oily wastes, such as the 

 wastes from gas works, may produce films upon the surface of a 

 stream; they then interfere with the absorption of oxygen by the 

 water from the air and thus exert a prejudicial influence on the 

 natural agencies of purification. It is for these reasons that the 

 discharge of trade wastes into streams is a matter that is seriously 

 in need of regulation. The wastes from manufacturing estabhsh- 

 ments are often more objectionable even than domestic sewage. 

 Perhaps the worst conditions arise when streams are polluted both 

 with domestic sewage and with trade wastes. 



Identification of the Source of Water. Another practical applica- 

 tion of the microscopical examination of water is that of determin- 

 ing the origin of certain waters. One of the studies made in con- 

 nection with the celebrated Chicago drainage canal case was a 

 series of microscopical examinations of water from Lake Michigan 

 down the Ilhnois and Mississippi rivers to St. Louis. It was found 

 that certain varieties of organisms were present in the water of 

 Lake Michigan that could not be found in any of the tributary 

 streams, and the argument was made that as these same organisms 

 were found in the water supply of St. Louis taken from the Missis- 

 sippi river they must have been derived from Lake Michigan, 

 showing that some of the water supplied to St. Louis came from Lake 

 Michigan through the Chicago drainage canal and the rivers men- 

 tioned. 



The studies made at Rochester in 191 2 showed that the water 

 near the surface of Lake Ontario contained various microscopic 

 organisms that could be readily identified but that these were absent 

 from the lower strata. Serial studies made at the shore of the lake 

 sometimes showed the presence of these organisms but at certain 

 times they were absent. The inference was that on these days the 

 water at the shore was that which had been drawn shoreward from 

 the deep strata. This finding corroborated the temperature obser- 

 vations and the wind records, and proved that the effect of a strong 

 off-shore wind was to blow the surface water away from the shore 

 and draw in the cold deep water from the lake. 



