475 



MONTHLY MEANS OF VERNAL TEMPERATURES FOR ILLINOIS AND OF PLANKTON 



PRODUCTION — CM.^ PER M^ 



The most notable instances of correlation between deflec- 

 tions of temperature and plankton production are to be seen in 

 the early spring of 1896 and the late autumn of 1897. These 

 correlations have already been noted in connection with the 

 discussion of production in the river and the several back- 

 waters. The mean temperatures of the air here given corrob- 

 orate our conclusions based on the relatively scanty data of 

 water temperatures delineated in the thermographs of the plates 

 accompanying this paper. Thus, the spring of 1896 was 2° be- 

 low normal in the state as a whole in March, but was 7.2° 

 above normal in April, and 9.4° above the mean for 1898. Mean 

 plankton production is also higher in April in 1896 than in any 

 other year. Indeed, in this month the descending scale of mean 

 temperatures in 1896-1898 is accompanied by a similar scale 

 of decreasing mean production of plankton, and, as has been 

 noted in the discussions of the course of production in the 

 river and backwaters, the vernal pulse of 1896 is from 10-14 

 days earlier than in 1898, when, as this table of mean tempera- 

 tures shows, the April mean of air temperatures was 9°-10° 

 below that of 1896. 



