Vertical Distribution of Individual Species. 



397 



Table XXXIII.— Cyclops, 1896. Number per cubic meter stated in thou- 

 sands. 



Depth, meters. 



April 1-15 . 

 April 16-30 

 May 1-15.. 

 May 16-31 . 

 June 1-15 . 

 June 16-30 



0-3. 



3-6. 



6-9. 



9-13. 



12-15. 



17.2 



11.7 



18.9 



20.3 



12.8 



109.4 



84.1 



52.5 



28.8 



18.8 



190.2 



124.9 



117.4 



84.5 



52.9 



37.0 



37.3 



34.3 



35.2 



42.1 



20.5 



13.7 



7.6 



6.7 



5.7 



59.2 



32.4 



17.9 



13.4 



6.7 



15-18. 



15.0 



9.6 

 42.7 

 64.8 

 14.1 



9.5 



Marsh ('97, p. 204) finds that Cyclops fluviatilis is present in 

 great numbers near the surface. Its distribution, therefore, 

 agrees more nearly with that of Diaptomus than it does with 

 C. brevispinosus. The latter species is present in Green lake 

 in very small numbers apparently in and below the thermocline 

 in summer. 



Daphnia hyalina. 

 Figure 29. — Table F, Appendix. 



There are two facts which give the peculiarities of vertical 

 distribution of Daphnia hyalina and the allied species D. retro- 

 curva. These are: First, a decided tendency of the young 

 animals to accumulate in the superficial strata of the water, 

 frequently in the upper meter. Second, a tendency on the part 

 of the older animals to settle toward the bottom. These species, 

 therefore, show a very high percentage in the upper levels of 

 the lake in periods when they are increasing, and especially at 

 those times when the broods of young appear. On the other 

 hand, when the species is declining in numbers, and in the in- 

 tervals between the appearance of broods, the distribution may 

 be comparatively equal throughout that part of the lake inhab- 

 ited by the species. As examples, compare the table on page 

 398, and the detailed figures of Table F, Appendix. 



The percentage in the upper level rarely falls below 25, even 

 in the winter. In May, when the spring broods appear, the 

 average number in the 0-3m. level ranges from 45 to 55 per 

 cent., and the same ratio is found during the summer when the 

 species is increasing in numbers. On the other hand, when the 



