the normal carbonates to bicarbonates and thus make 

 the water neutral to phenophthalein. It is twice the 

 phenolphthalein alkalinity (in terms of calcium car- 

 bonate) expressed as parts per million of carbon 

 dioxide. 



In the summer of 1928 free carbon dioxide was 

 encountered in the bottom water of those relatively 

 deeper stations where there was a more or less pro- 

 nounced thermocline. In August, free carbon di- 

 oxide was found in bottom water from 10 different 

 stations. In September no free carbon dioxide was 

 found in samples from relatively shallow stations. 

 The 4 stations which showed free carbon dioxide at 

 the bottom in August had lost this and in addition 

 part of their half-bound carbon dioxide in September 

 of the same year. The probable reasons are in- 

 creased photosynthesis correlated with vertical 

 mixing. 



The free carbon dioxide progressively decreased 

 in the bottom water during theaimmer of 1929. Li 

 June there were 27 stations which showed a free car- 

 bon dioxide content at the bottom, and only 3 with 

 deficient or negative carbon dioxide. In July there 

 were almost as many stations with negative carbon 

 dioxide as with free carbon dioxide and in August 

 the trend was still more pronounced when the num- 

 ber of stations with negative carbon dioxide exceed- 

 ed by 4 the stations with positive carbon dioxide. In 

 September, 36 stations yielded negative carbon di- 

 oxide and only 10 showed positive carbon dioxide in 

 a total of 47 samples. 



The carbon dioxide situation in the lake during 

 July and September 1929 is shown in figures 27 and 

 28. Free carbon dioxide at the bottom, denoted by 

 the black circles, occurred in the offshore stations 

 throughout the lake in July, but in September posi- 

 tive carbon dioxide was almost without exception 

 confined to the bottom in the vicinity of the Deep 

 Hole off Long Point. The negative carbon dioxide 

 values increased at both the surface and bottom in 

 September; the bottom increase was particularly 

 striking. As shown in table 12, the entire two sets 

 of samples show an average increase of 1. 26 p. p. m. 

 of negative carbon dioxide at the bottom in Septem- 

 ber over that in July. 



There was an inverse relationship between the 

 dissolved oxygen and carbon dioxide content (figs. 



31-34). Where oxygen was uncommonly high the 

 free carbon dioxide was relatively low. This is 

 true for the general trend of the curves as well as 

 for the abrupt deviations from the average. The 

 siu:face oxygen saturation curve runs higher than the 

 bottom saturation curve, but the surface carbon di- 

 oxide curve runs lower than the bottom carbon di- 

 oxide curve. 



Striking correlations of conspicuous variations 

 of the carbon dioxide and oxygen curves for the bot- 

 tom may be pointed out in figures 33 and 34. In 

 August at stations 06.40, 06.41, and 06.42 there is 

 a notable decrease in the bottom oxygen saturation 

 and a correspondingly great increase in the free car- 

 bon dioxide content. The observations on these two 

 sets of values for the month of September show a 

 similar inverse ratio of carbon dioxide to oxygen in 

 the Deep Hole area. 



When the areas of low oxygen saturation and high 

 carbon dioxide content are plotted, as in figures 29 

 and 30, some significant general tendencies are made 

 obvious. 



In figure 29 is shown the areas where the bottom 

 oxygen saturation was found to be below 81 percent 

 for the 4 summer months. If a lower figure had been 

 plotted, the areas would have been much smaller, 

 and if a higher figure had been chosen, the contours 

 would have included a somewhat greater area. The 

 system is an arbitrary one. In June there were sev- 

 eral isolated regions where percentage saturation 

 fell below 81 percent. Oxygen content had decreased 

 in a considerable portion of the western lake area 

 and also in the Deep Hole during July. August shows 

 a similar situation in the west end and some increase 

 in the oxygen content of the deep waters off Long 

 Point. By September values for the entire lake had 

 risen above 81 percent, except in this Deep Hole region 

 where the oxygen saturation remained low. 



In June free carbon dioxide was present in the bot- 

 tom waters of practically the entire lake (fig. 30). A 

 steady decrease was evident with advance of the sum- 

 mer until in September only the bottom water of the 

 Deep Hole region off Long Point contained free car- 

 bon dioxide. 



From these facts the trend in the lake seems very 

 clear. After the spring overturn there was probably 



79 



