ANNUAL DISTRIBUTION OF THE ORGANISMS. 39 



In the first summer the greatest amount of plankton was about 

 the first of August and the middle of October. In the following 

 February and March, however, the total of plankton was much 

 larger than even in the summer months, while the greatest rec- 

 ord of the year was reached about the middle of June. From 

 this point there was a decrease followed by lesser maxima the 

 first of August and the first of September. There was then a 

 steady decrease to a minimum in the middle of January. From 

 this point there was an increase to a March maximum corre- 

 sponding to that of the preceding year, but the total of this 

 March maximum was much smaller than that of the March of 

 the preceding year. It then fell to a May minimum, which waa 

 almost as small as that of the winter. This was followed by a 

 slow increase to a single summer maximum in the latter part of 

 July, from which there was a slow fall through the remainder 

 of the summer and autumn months. 



CONSTITUENTS WHICH PRODUCE PLANKTON MAXIMA. 



It is a matter of great interest to determine what organisms 

 are responsible for the maxima, and what are lacking at the 

 minimum periods. Steuer has stated that the result of his work 

 indicates that the general plankton curve follows closely the 

 rotatoria curve. This is hardly true of either of the lakes un- 

 der consideration. But what organisms are responsible for 

 these maxima ? A comparison of the general plankton curves 

 with those of the individual animals and plants shows no close 

 relation between the total plankton and the species. There is 

 this general relation that most forms are produced in greatest 

 numbers in the months from May and June until October ; this 

 is especially true of the plants. But no one organism seems to 

 have a controlling influence on the total amount of plankton, ex- 

 cept in one or two particular cases. This is shown by a careful 

 analysis of the plankton constituents at the various maximum 

 periods. 



In the summer of 1899, in Lake Winnebago, there were two 



