474 



imum. In general, this is a period of declining plankton in 

 channel waters, where production in these two months falls 71 

 per cent, below that in the preceding four months of maximum 

 heat, and in Quiver Lake, where it falls 48 percent, below. On 

 the other hand, in the rest of the backwaters there is a slight in- 

 crease in these two months as compared with the production in 

 the period of maximum heat. In Dogfish, Flag, Thompson's 

 and Phelps lakes the October-November increase in percent- 

 ages over the average summer production in each of these sev- 

 eral localities is 33, 362, 26, and 11 per cent, respectively. In 

 view of these divergent tendencies in production under similar 

 temperature conditions it is evident that other factors are 

 operative, or at least more potent, in controlling autumnal pro- 

 duction. The October production is as a rule higher than that 

 of November, and suggests a tendency towards an autumnal 

 pulse comparable with the vernal pulse but of lesser amplitude. 

 The vernal pulse occurs in rising temperatures of 60°- 70°, and 

 this autumnal one in falling temperatures of 60°- 50°. 



The month of December does not on the average quite at- 

 tain the minimum winter temperature, though in some years, 

 as in 1897 and 1898, it approached closely to it. Neither does 

 the plankton production drop to so low a level on the average 

 or in individual years in channel waters as during the two 

 colder months which follow. In general the same relation ex- 

 ists in the backwaters, though exceptions occur — -principally in 

 Thompson's Lake. 



Thus, in a large way, temperature plays an important part 

 in controlling plankton production. Additional proof of its 

 potency is to be found in the correlations between production 

 and exceptional divergences from the normal course of tem- 

 perature changes, such, for example, as early or late vernal 

 rise or autumnal decline. 



The accompanying table (p. 475), kindly furnished by Mr. 

 W. G. Burns, Section Director for Illinois of the U. S. Weather 

 Bureau, gives the vernal air temperatures for 1896-1898, and 

 permits a comparison with the course of plankton production. 



