involved. Methyl orange alkalinity showed a distinct seasonal change; it was 

 low in spring and faLl, and high in siimmer. It may be seen that on three 

 dates (AiJgust 1 and 19, and September 5) samples were taken both at Station 8F 

 and Station 1^8. In each case a higher surface temperature was observed at 

 Station l58, and there was a greater difference in temperature between top and 

 botton at l58 than at 8F. In each case, also, there was more oxygen, less free 

 carbon dioxide, and higher pH, at the surface of Station 158 than at Station 8f. 

 These differences are believed to be due, in part at least, to the fact that 

 Station 8F was visited in the morning and Station 158 in the afternoon. 

 Longer exposure to the sun at Station l58 would account for the higher surface 

 temperature, greater temnerature gradient, and greater activity of photo- 

 synthetic organisms. In each case the bottom waters at the two stations were 

 much the same in temperature, and in chemical ccnstituents, as far as the 

 latter were observed. 



General discussion 



The data on dissolved oxygen, carbon dioxide, and pH presented in the 

 preceding pages are believed to be representative of the Island Section for 

 the period of time covered, that is, from early April to late October. Addi- 

 tional data from other stations for the same period are at hand, but they show 

 essentially the same features as those already given. 



According to the figures presented in Tables lli-20, and in the accompany- 

 ing text, the oxygen content of the surface water ranged from 7.1 to 13.0 

 parts per million. The per cent of saturation with oxygen ranged from 83 to 

 133, but almost all of the sanples showed a saturation between 90 and ^^ per 

 cent, 'with a few exceptions the lower oxygen values, that is, those below 90 

 percent of saturation, were correlated with a decline of plankton. There is 

 no reason to believe that low oxygen content of the surface water was ever 

 the result of pollution. 



Oxygen content of the bottom water ranged finm 0.78 to 12.6 parts per 

 million, and from 8.6 to 105 per cent of saturation. The very low value 

 indicated was found at Station 59A on August 9, 1930, near the end of a 

 period of thermal stratification. It is not known exactly when the strati- 

 fication was established, but it certainly was after July 2U, so that the 

 lower water had been isolated from the air not more than l6 days. This may 

 appear to be a very short time in which to bring about nearly complete 

 exhaustion of oxygen. However, in Lake Mendota in 1906, almost all of the 

 oxygen at the bottom was removed two w eeks after stratification, even though 

 the temperature was less than 12* (Birge and Juday, 1911, Plates I and II). 

 At Station $9A the temperature at the bottom was 20.7", and decomposition 

 would proceed much more rapidly at that temperature than at 12". Thus the 

 low oxygen at Station 59A (and at Station 37A at about the same time) can be 

 explained on natural grounds, and it is not necessary to assume the presence 

 of polluting materials. The case cited is the only one of marked depletion of 



78 



