lowermost three meters of water. Temporary thermal stratification 

 was the indirect cause of the change in chemical conditions. 

 Stratification was established not more than two weeks prior to the 

 time of sampling and probably was destroyed one or two days after 

 sampling. The area affected apparently was small, for there was no 

 thermal stratification farther west at that time. 



There is no reason to believe that the reduction of oxygen 

 resulted from other than natural causes. Nor is there reason to 

 believe that fishes of the region were harmed. The amount of oxygen 

 available was so small that it probably would have been fatal to any 

 thdt remained, but it is well known that fishes can and do avoid 

 waters low in oxygen. Thus, in Lake Mendota the yellow perch regularly 

 moves out of the hypolimmon during the summer period of stagnation, 

 although it may return for short periods (Pearse and Achtenberg, 1920). 



Only one station in the entire area studied showed low 

 oxygen and high free carbon dioxide on every date for which samples were 

 taken. This was Station 2U9, about five miles above the mouth of 

 Maumee River, On the five dates in 1930 for which data are available, 

 the oxygen content ranged from 2.9 to Ii.U parts per million and from 

 3li to U9 per cent of saturation. At the mouth of the river, oxygen 

 was sometimes low and sometimes high as a result of the reversals of 

 current in the river. In 1929 and 1930, it ranged from 1.0 to 10,5 

 parts per million and from 12 to 112 per cent of saturation. There 

 were no marked oxygen reductions at a distance of 2.25 miles from the 

 mouth of the river. At the mouth of River Raisin in 1930 oxygen ranged 

 from 0.0 to 9.0 parts per million and from to 98 per cent of 

 saturation. The lower one mile of the river in 1920 usually showed 

 less than 50 per cent of saturation, and sometimes much less than that 

 amount. No cases of marked oxygen reductions were noted at a distance 

 greater than one half mile out in the lake. 



In the two rivers mentioned, and in small areas near their 

 mouths, conditions with respect to oxygen and carbon dioxide were 

 commonly such as to be unfavorable for the normal existence of fishes. 

 It is not to be supposed that fishes were entirely excluded from such 

 regions. Professor Reighard, in his unpublished report on River 

 Raisin, noted the presence of considerable numbers near sewer outfalls. 

 However, they were principally species which appear to be tolerant to 

 polluted situations, such as the blunt-nosed minnow, the golden shiner, 

 and the gold-fish. The pure-water types were rare. It is not unlikely 

 that conditions in the lower part of Maumee River also exclude the 

 pure-water types but allow the more, tolerant ones to remain. There were 



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