Vertical Distribution of the Crustacea — Spring. 381 



production of the species, while the presence of a larger number 

 in the bottom water of the lake than in the surface water indi- 

 cates that the species is past its maximum and is already be- 

 ginning to decline in numbers. 



In both years the numbers of Crustacea in the upper water 

 show an increase during April, due to the multiplication of 

 Cyclops. This increase went on, as was shown in the early part 

 of this paper, much more rapidly in 1896 than in 1895. As a 

 result, the population both of the surface water and of the 

 lower levels increased much more rapidly in 1896, and the latter 

 part of April, 1896, represents about the same condition of the 

 development of the Crustacea, as does the first half of May in 

 1895. In each case more than 40 per cent, of the Crustacea 

 were present in the upper stratum, while the 15-18 m. level 

 had not increased greatly in numbers above its condition in 

 winter. In the latter part of April, 1896, the 15-18 m. level 

 contained less than 3 per cent, of the whole number of Crustacea 

 present; and in the first part of May, 1895, it contained less than 

 "7 per cent. As the number of Cyclops and Daphnia pulicaria be- 

 came greater, they moved downward into the deeper water, so that 

 it became relatively more populous. In the latter part of May, 

 1895, the 15-18 m. level contained 10 per cent, of the Crustacea, 

 while in 1896 it contained over 40 per cent. This increase in 

 the population of the lower strata goes on after a considerable 

 decline has come in that of the upper strata. The lower 

 water lags behind the upper both in the increase and decrease 

 of its population, and the maximum population of the lower 

 strata comes from two to three weeks after the maximum popu- 

 lation of the lake has passed. 



These relations become more obvious if we divide the lake some- 

 what arbitrarily into three levels, 0-3 m., 3-9 m., 9-18 m. 

 The distribution of the Crustacea among these three regions is 

 shown in Figs. 24 and 25. By reference to these it will be 

 seen that in 1895 the two upper levels increased much more 

 rapidly than did the lower half of the lake from the latter part 

 of April to the middle of May. In the latter part of May the 

 reverse is true; and in early June the population of the lower 

 Water was stationary, while that of the upper half of the lake 



