NANNOPLANKTON OF LAKE MENDOTA 87 



the form was still present in considerable numbers when the observa- 

 tions were discontinued on the first of June. S. astraea yielded a 

 larger number of individuals per liter of water than any other organism 

 that was found in the nannoplankton. The mean of two counts made 

 on a sample taken from the tank on April 18, 1916, was approximately 

 35 million individuals per liter of water. In 1917 the maximum num- 

 ber of S. astraea was a little more than 27 million per liter, while in 

 1915 it was only about 8.5 million. 



Another species of Stephanodiscus, a distinctly larger form than the 

 preceding, was found each year in the spring and in the autumn. It 

 was never present in very large numbers, however ; the largest number 

 was observed in the first week of October, 1916. 



Fragments of the colonial diatoms, such as Asterionella, Fragilaria, 

 and Tabellaria, were found in the centrifuge material and they were 

 enumerated. As indicated in the diagrams, these fragments were most 

 abundant in spring and in autumn. 



Filaments of the blue-green alga Aphanizomenon were found in most 

 of the samples, but usually relatively small numbers of them were 

 noted. 



The vernal increases in the organic matter of the nannoplankton 

 were correlated in time with the increase in the numbers of certain 

 forms; in 1915, for example, the maximum for organic matter was 

 found at the same time that Aphanocapsa reached its maximum point 

 for the entire series of observations. Stephanodiscus astraea was also 

 increasing in numbers at this time, but its increase was not sufficient 

 to counterbalance the decline of Aphanocapsa in the last week of April 

 and the first week of May. Thus, there was a decrease in the organic 

 matter at this time corresponding to an increase of this diatom, but its 

 maximum number this year was only about a quarter as large as in 

 1917. 



The vernal maxima of organic matter in 1916 and in 1917 were both 

 correlated in time with the maximum number of Stephanodiscus 

 astraea. There was also a marked increase in Cyclotella and in Apha- 

 nocapsa at this period in the former year and a distinct rise in the 

 number of monads was noted at this time in the spring of 1917. 



The marked rise in the organic matter in the latter part of July and 

 during the first week of August, 1915, was correlated in time with in- 

 creases in the numbers of monads and of Coelosphaerium. In Septem- 

 ber there was an increase in Aphanocapsa and in Coelosphaerium at 

 the time of the rise in the organic matter, but there was no well marked 

 maximum of the latter during this autumn and neither was there any 

 unusual rise in the number of organisms. A fairly well defined in- 

 crease in the monads in late October and in November was accom- 



