PLANKTON OF LAKE MONONA 119 



per liter on July 30 and also on August 31. The largest numbers of 

 Aphanocapsa were found on June 23 and on August 31, namely, 645,100 

 and 716,800 individuals per liter, respectively. A minute Oscillatoria 

 rose to a maximum of 409,600 filaments per liter on September 24. 

 Fragments of the strands of Anabaena and filaments of Melosira were 

 most abundant in September and October. 



The minimum amounts of material obtained in the two catches of 

 July corresponded to relatively small numbers of the various forms 

 with the exception of the monads. In addition to this a much smaller 

 variety of organisms was noted in June and July than in the later 

 months. The marked rise in the quantity of organic matter in August 

 was due chiefly to an increase in the variety of organisms rather than 

 to any unusual rise in the number of any particular form. On June 

 23 but four forms of organisms were noted; this number rose to six 

 on July 13, to nine on July 30, to seventeen on August 31, and to 

 nineteen on September 15. The latter represented the greatest variety 

 for this year. The small colonies of Coelosphaerium and the diatoms 

 showed an increase in numbers corresponding to a rise of the organic 

 matter to a maximum on October 15. (Table 46.) 



In 1916 Schroederia and Aphanocapsa were the most abundant forms 

 in the material obtained on May 12 ; the former numbered 2,867,000 per 

 liter and the latter 409,600. By June 14 these two forms had decreased 

 very much in numbers, the latter, in fact, was not found after this date. 

 The monads had increased somewhat in numbers but not sufficiently to 

 counterbalance the decrease in the other forms, so that the minimum 

 amount of organic matter obtained in 1916 was noted on June 14. 



In 1916, also, a smaller variety of organisms was found early in the 

 season than later; the largest number of forms, namely, twenty-one, 

 was obtained on August 29, while up to the first of August the number 

 ranged from seven to ten. The distinct rise in the organic matter in 

 the latter half of July was correlated in time with an increase in the 

 variety of the organisms. (See figure 38, curve A.) The maximum on 

 October 12 corresponded to a rise in the monads, in Aphanocapsa and 

 in the diatoms, as well as a larger number of fragments of Anabaena. 

 After this date there was a decrease in the number of most forms and 

 also a decrease in the number of forms present, corresponding to a 

 marked decline in the amount of organic matter. 



The Total Plankton of Lake Monona 



Since the ash of the centrifuge material contains inorganic substances 

 derived from other sources than the nannoplankton organisms, it is best 

 to consider the total plankton (the net plankton plus the nannoplank- 

 ton) only on an ash free basis. In this connection also, it is necessary 



