50 PLANKTON OF WINNEBAGO AND TZREEN LAKES. 



face in Green lake ordinarily comes in the early part or middle 

 of August, while the annual mean of this part of the state has 

 its maximum about the middle of July. Lake Winnebago, be- 

 cause of its slight depth, warms up more quickly and cools with 

 corresponding quickness. 



In comparing the temperature curves with the total plankton 

 curves we find only a general resemblance. The greatest 

 amount of plankton, generally speaking, is found in the hottest 

 months. Remembering what I have said before, that the plank- 

 ton is largely dependent for its amount on the plants, this is 

 what would be expected, inasmuch as a high temperature is fa- 

 vorable to plant growth. In some cases a maximum seems to 

 follow the period of highest temperature, but this is by no 

 means always the case. Inasmuch as plants are more impor- 

 tant in the plankton of the shallow lakes, it follows that the 

 plankton of such lakes follows the temperature curve more 

 closely than does that of the deep lakes. The deep lakes have 

 annual conditions more closely approaching uniformity, hence 

 the variations during the year are less marked, and as they 

 never reach the high summer temperature of the shallow lakes 1 , 

 they never have such a large production of plants like Gloio- 

 trichia, Anabaena, and Lingbya. 



The fact, too, that the Crustacea form so much greater pro- 

 portion of the plankton in the deep lakes, makes the correspond- 

 ence with the temperature curve less, for some of the Crustacea 

 have winter maxima. 



In comparing the annual curves of the Crustacea, it appears 

 that the summer maxima of most of them came somewhat 

 earlier in 1899 than in 1900. This does not seem to> be true 

 of the general plankton. I have been interested to know what 

 made the difference, but I am not sure that I have detected the 

 real reason. The maxima of the general plankton, as has been 

 said, are largely dependent on the plants rather than the ani- 

 mals. It would seem then, that the cause which made the 1899 

 summer maxima of Crustacea greater than those of 1900 must 

 have been something that would affect Crustacea but would not 



