56 PLANKTON OF WINNEBAGO AND GREEN LAKE 8. 



practically a uniform depth, and the collections from other 

 lakes were not numerous enough to have much bearing on the 

 question ; it is interesting to notice, however, that in collections 

 made in Birch lake in September, 1899, and August, 1900, 

 which were made at different depths, the amount of plankton 

 varied with the depth. It appears from the Green lake collec- 

 tions that there is a distinct relation between the depth and the 

 plankton, the amount of plankton increasing with the depth, 

 but this difference is much less marked in summer than in 

 winter. 



I do not remember to have seen any statements by other 

 authors of the effect of depth on plankton. It is known, otf 

 course, that there is a difference in amount between littoral and 

 limnetic plankton, but I think it is without doubt true, that in 

 the limnetic plankton, depth is a factor in distribution, the 

 variation in accordance with depth being most clearly marked 

 in the winter. It does not follow, of course, that the deeper 

 lakes necessarily have the greater amount of plankton — this 

 certainly is not true, but simply that depth, in any given lake, 

 is a factor to be considered in horizontal distribution. 



In the collections from Lake Winneconne, the uniform- 

 ity is very marked. In this case the collections were made at 

 different locations over the lake, and may be considered as 

 typical of the whole lake. This is true in this lake, however, 

 that the conditions vary little in different parts of the lake, and 

 we would expect greater uniformity. 



In Lake Poygan the conditions are much as in Lake Winne- 

 conne, both lakes being expansions of the Fox river: it is shal- 

 low, the depth varying but little, with low swampy shores. We 

 should expect, of course, a somewhat different fauna and flora 

 in the weeds alongshore from that existing in the limnetic re- 

 gion. But in the limnetic region, where the collections were 

 taken, the winds and the currents must produce all the changes 

 in environment. While the plankton differed more in Poygan 

 than in Winneconne, yet, if we throw out 99.169, the variation 

 is not very great. This collection was made in a place sur- 



