DISCUSSION OF RESULTS. 59 



gation of some of the larger cladocera, such as Daphnia< or 

 Eurycercus. 



4. In winter the horizontal distribution of plankton in deep 

 water lakes will bear a more or less close relation to the depth. 

 This is explained by the fact that temperature is one of the 

 most important factors in the control of the distribution of the 

 plankton, and in winter the uniform temperature of the whole 

 depth of water makes possible a larger production of plankton 

 in deep water than in shallow. 



COMPARISON OF PLANKTON COLLECTIONS OVER MUDDY AND 

 STONY BOTTOMS. 



As intimated earlier in this paper, in the regular collections 

 on Lake Winnebago, one series was made over the muddy bot- 

 tom, well out in the lake, and another over the stony bottom 

 nearer shore. The average of the collections made in the 

 summer of 1899, when they were made daily through The 

 months of <Tuly and August, seems to show that the collections 

 over the muddy bottom were decidedly larger than those over 

 the stony bottom. This difference may be explained by the 

 different effect of the character of the bottom) on the plankton, 

 or by the effect of the slight difference in depth. I have shown 

 elsewhere that the depth has an influence on the amount of 

 plankton, and it is entirely conceivable that in the averages of 

 a large number of collections, a difference of a meter or so in 

 depth may have had a marked effect on the amount. Inas- 

 much as the greatest depth at which collections were taken was 

 only about five meters, a difference of a meter would mean a 

 difference of twenty per cent., which might have a decided in- 

 fluence on the amount of plankton. I am, therefore, inclined to 

 explain the difference between the muddy bottomi and the stony 

 bottom as due rather to the difference in depth than to the dif- 

 ference in the character of the bottom. 



