ANNUAL DISTRIBUTION OF THE ORGANISMS. 41 



1899-1900, however, is the great increase in the total plankton 

 in February and March when the lake was covered with nearly 

 two feet of ice. The entomostraca figuring in this maximum 

 were Diaptomus sicilis and Bosmina. With these were asso- 

 ciated many rotatoria. But it was to Asterionella that the 

 great increase was mainly due. This organism was present in 

 countless numbers. " In the maximum in June Diaptomus sicilis 

 and coj>e]>od larvae were prominent, but here again Asterionella 

 w r as perhaps the most important. 



The maximum of the first of August, 1900, was due to several 

 entomostraca, as in the case of the preceding year. Daphnia 

 was less abundant than in 1899 but formed perhaps the most 

 important factor in the larger maximum of Sept. 1. In March 

 again occurs a winter maximum, as in 1900, but the total is 

 much less. This was caused, as the year before, mainly by the 

 enormous increase in Asterionella, associated with great num- 

 bers of Synedra pulchella and Synedm acus var. delicatissima. 



In the summer of 1901 there was a single maximum in the 

 latter part of July in which, as in the former years, Diaptomus 

 minutus played a very prominent part; at this time, too, Cera- 

 tium and Dinobryon were very numerous. 



In the winter of 1902 there was again an increase as in the 

 two preceding years caused in part by the numbers of Diapto- 

 mus sicilis, but more by the enormous numbers of Asterionella 

 and Synedra pulchella. 



COMPARISON OF AMOUNT OF PLANKTON IN DIFFERENT YEARS. 



It will be seen, by a glance at the curves, that the total amount 

 of plankton varies quite widely in the years under considera- 

 tion, and the question arises whether there may be plankton 

 poor years and plankton rich years. Three summers would 

 hardly give material to decide such a question, but it may be 

 noticed that the summer production in 1901 was distinctly less 

 in both lakes than in the preceding years. It is interesting to 

 notice in this connection, the plankton collections made in a 



