24 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



We can say, in general, that Diaptomus oregonensis occurs in 

 its greatest abundance during the months from June to Novem- 

 ber, inclusive, that it is found only in very small numbers from 

 the latter part of November until the last of March; that it 

 has a constant fall maximum and one or more mid-summer 

 maxima which occur between the last of July and the first week 

 in September. So far as I know there are no preceding obser- 

 vations on the annual periodicity of D. oregonensis except 

 those recorded by Birge (Birge 7 9T). 



The curves of D. gracilis and D. graciloides as recorded by 

 Apstein, Steuer, and Seligo correspond in general with these 

 curves, but differ in details. Especially noticeable is the state- 

 ment of Apstein that the maximum of D. graciloides in Lake 

 Plon occurs in the winter. In comparing my results with 

 those of Birge, while there is a general resemblance in the 

 curves, there are certain marked differences. He states that 

 in Lake Mendota the smallest catches of the year were made in 

 the latter part of April, and he infers that the conditions of life 

 are harder for them after the going out of the ice. In Lake 

 Winnebago there was a distinct increase during the month of 

 April with a drop during the first half of May. I am unable to 

 explain this increase, but it does not appear that the going out of 

 the ice worked any hardship in the case of the Lake Winnebago 

 Diaptomus. 



In Lake Mendota there seems to have been no increase corre- 

 sponding to the November maximum of Lake Winnebago. It 

 is possible that this fact has some bearing upon the question 

 of the effect of temperature upon the species. Birge (Birge 

 '97, 326) states that the reproduction of D. oregonensis is more 

 promptly checked by a fall of temperature than is that of any 

 other species. Of course there is no question of the truth of 

 the general proposition that the reproduction of this Diaptomus 

 is dependent upon higher temperatures. The general charac- 

 ter of the curves shows that at once. If, however, it were par- 

 ticularly sensitive to a decline in temperature, should we not 

 expect fall reproduction to stop soonest in the lake that cools 

 off the most quickly? Lake Mendota has a maximum depth 



