content has been increased by pollution. In that sense the Island Section 

 should be regarded as nolluted, but the comoarative figures in Table 23 show 

 that it is not heavily polluted. 



The data in Table 23 show that the Island Section contained on the average 

 almost the same amount of nitrogen as the pure waters reported by Mason, 

 although the proportions of the different forms of nitrogen were different in 

 the two waters. Aside from any consideration of potability, there seems to be 

 little choice between the two with regard to sujtability for aquatic organisms. 

 Certainly the resemblance is closer than between the Island Section and the 

 polluted waters reported by Mason. On the basis of .these comparisons, the 

 vjater of the Island Section may be regarded as relatively pure. Conparisons 

 with other parts of the Great Lakes as t^iven in Table 23 do not point to heavy 

 pollution in the Island Section. 



The data in Table 22 show some points of interest on the question of 

 oollution. Of the six stations, Station 59A had the largest amount of albu- 

 miDoid amiiionia. This station is located near Sandusky Bay and probably is 

 affected by -polluted water from that source at times. Station 8F had the 

 smallest amount of albuminoid ammonia. Station 8F has the most northerly 

 location of those in the Island Section, and probably is affected by water 

 from Detroit River more than the other stations. This is suggested by the 

 fact that Detroit River is quite consistently low in albuminoid ammonia and 

 in chloride (Table U 6) . There is no evident relationship between the amount 

 of nitrogen at the remaining stations and their location with respect to 

 sources of pollution. 



Conclusions Regarding Pollution 



Pollution by domestic sewage may change a number of thp normal character- 

 istics of a lake. Perhaps the most obvious chemical change is the reduction 

 of the oxygen content, which results from the mineralization of nitrogen com- 

 pounds contained in the sewage, kith sufficiently heavy pollution, all of 

 the dissolved oxygen may be mthdrawn from the water. Pronounced reduction 

 of the oxygen supply is undesirable because most of the organisms normally 

 present in a lake require a large supply, and they disappear when it is not 

 available. For that reason pollution may be regarded as harmful to a lake. 

 However, the water may be so lightly polluted that the oxygen withdrawals 

 cannot be distinguished from those of natural origin.. Under such conditions 

 it is safe to conclude that the pollution has no harmful effect on the water, 

 provided, of course, that poisonous chemicals are not present. 



This seems to be the case in the Island Section. In discussing the 

 data on oxygen, it was pointed out there was no evidence of oxygen reductions 

 due to pollution. The data on nitrogen tend to confirm this conclusion. 

 While it is probable that the nitrogen content has been increased by pollu- 

 tion, it is equally probable that the additional demand uoon the dissolved 



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