88 PLANKTON OF WISCONSIN LAKES 



panied by a decrease in Aphanocapsa and Coelosphaerium, the two 

 changes just about balancing each other according to the results for 

 organic matter. 



In 1916 the rise in organic matter in late June and in early July 

 corresponded to increases in the numbers of monads, Coelosphaerium, 

 Aphanocapsa, and Oocystis. The prominent September peak in the 

 curve for organic matter in 1916 was correlated in time with a marked 

 rise in Cryptomonas, Stephanodiscus, and the fragments of diatom 

 colonies. 



Role of Bacteria in the Nannoplankton 



No quantitative observations were made on the bacteria of Lake 

 Mendota during this plankton investigation, except to ascertain by 

 means of several series of plate cultures that the centrifuge removed 

 about one-third of the bacteria normally present in the lake water. A 

 quantitative study was begun in July, 1919, and the work is still being 

 continued, March 1, 1922. The results obtained during this interval 

 of time furnish enough data for a fair estimate of the role of the 

 bacteria in the plankton complex of the lake. 



In this quantitative study the number of bacteria has been deter- 

 mined both by direct counts and by plate cultures. For the plate 

 method Nahrstoff-Heyden agar was selected for the culture medium 

 because it gave the highest counts and apparently the largest number 

 of different types of colonies. Several series of direct counts have 

 yielded an average of ten times as many bacteria as the plate cultures, 

 so that it is necessary to multiply the plate counts by the factor ten in 

 order to ascertain approximately the number of bacteria per cubic 

 centimeter of water by the plate method. 



On the basis of the direct counts, the total number of bacteria in 

 Lake Mendota from July to October, 1919, averaged about 3,000 per 

 cubic centimeter of water from surface to bottom in 23.5 meters of 

 water. In late autumn and early winter the number decreased to a 

 minimum average of 1,500 per cubic centimeter. In the spring and 

 early summer of 1920 the number rose steadily to a maximum average 

 of 30,000 per cubic centimeter; the number remained near the maxi- 

 mum until the latter part of August, after which there was a gradual 

 decline to a winter minimum of 2,000 bacteria per cubic centimeter of 

 water in January, 1921. In the following spring and summer, the 

 number averaged from 3,000 to 5,000 per cubic centimeter, but in Sep- 

 tember the number rose rapidly. A maximum of 60,000 individuals 

 per cubic centimeter was obtained at a depth of 10 meters on September 

 22, 1921. Following this the number declined to a winter minimum 

 of about 3,000. 



