Vertical Distribution of Individual Species. 



393 



Table XXXII — Percentile distribution. Summer— Diaptomus. 



1894 

 1895 



1896 



1894 

 1895 

 1896 



1894 

 1895 

 1896 



Average 

 No. 



226,000 

 172,000 

 188,000 



Per cent, in each 3 m. level. 



0-3 m. 



49.2 

 42.7 



52.6 



3-6 



29.3 

 29.0 

 27 4 



6-9 



16.6 

 20.9 

 12.4 



9-13 



4.1 



6.1 

 5.9 



13-15 15-18 



0.5 

 0.7 

 1.9 



0.3 

 0.6 

 0.5 



Cyclops. 



138,000 

 183,000 

 290,000 



40.7 

 39.3 

 40.2 



28.4 

 25.2 

 27.1 



20.1 

 19.0 

 15.6 



9.4 



10.0 

 10.1 



1.7 

 3.1 



4.8 



0.3 

 3.4 

 2.3 



Daphnia hyalina. 



27,000 



41.9 



23.8 



21.4 



6.7 



r 

 1.0 



210,000 



52.3 



20.8 



17.6 



6.6 



1.3 



145,000 



44.7 



22.2 



16.1 



11.7 



4.7 



0.3 

 1.2 

 1.3 



The variations in the distribution of Diaptomus are greater, 

 although its numbers were more nearly constant, but in each 

 year the same characteristics are shown. The percentage of the 

 population found below the middle of the lake is 7.5 or less, 

 while in the case of Cyclops the number ranges from 11.5 to 

 more than 17 per cent. Daphnia hyalina also varies more in 

 the upper strata, but is in general intermediate in its distribu- 

 tion between the other two genera. The older individuals of 

 Daphnia hyalina are much more apt to accumulate in the lower 

 part of the water accessible to them than is the case with Diap- 

 tomus, and consequently the lower levels are apt to contain a. 

 larger percentage of this species, On the other hand the spe- 

 cies does not extend to the thermocline in numbers anything 

 like as great proportionately as does Cyclops, so that the lower 

 part of the inhabited water always contains a larger proportion 

 of Cyclops than of any other species. 



The vertical distribution of Daphnia, hyalina, therefore, dif- 

 fers very considerably in different years. If the species is pres- 

 ent in large numbers and the young are constantly appearing, a 



