42 PLANKTON OF WINNEBAGO AND GREEN LAKES. 



series of northern lakes about the middle of August in 1900 

 and 1901. In all the lakes the collections of 1901 were much 

 smaller than in 1900. It would seem probable then that the 

 summer of 1901 was a plankton poor summer. I do not know 

 what reason can be assigned for this fact. It is a somewhat 

 significant fact, however, that the curve of mean temperature 

 for 1901 differs distinctly from the mean in that it rises sharply 

 to a maximum in the middle of July and then declines rapidly, 

 instead of continuing at practically the same height through 

 most of August as is commonly the case. It is possible that the 

 short duration of the extreme hot weather of summer may have 

 had some effect in reducing the amount of plankton. 



COMPARISON OF PLANKTON IN GREEN LAKE AND LAKE 

 WINNEBAGO. 



In comparing the curves of the two lakes, perhaps the most 

 noticeable' difference is the enormously greater summer produc- 

 tion in Lake Winnebago. This is due to the very much greater 

 production of plants in the shallower lake. Green lake varies 

 from the mean much less during the year than does Lake Win- 

 nebago. The winter production in Green lake is absolutely 

 considerably greater than in Lake Winnebago. This, as has 

 been pointed out before is due both to the greater number of 

 entomostraca, and to the enormous production of Asterionella 

 in Green lake, accompanied sometimes by a corresponding in- 

 crease in the species of Synedra. I can only conjecture as to 

 the cause of the greater winter production in Green lake, but I 

 think it is possibly connected with the fact that it is very late 

 before it is frozen over. Lake Winnebago', because of its slight 

 depth, is cooled off early in the fall and is covered with ice while 

 Green lake is still open and exposed to aeration. The actual 

 amount of water in Lake Winnebago is small as compared with 

 its superficial dimensions, and its supply of oxygen must be 

 much smaller than that of a body of deep water like Green 

 lake: thus it seems to me that the conditions for animal life 



