384 



Birge — The Crustacea of the Plankton. 



While the absolute population of the lake during the summer 

 months has varied very greatly in the three years of my obser- 

 vation, the vertical distribution of the animals has been almost 

 exactly the same, as may be seen from the following table: 



Table XXX.— Average percentile distribution of Crustacea June 15-Sept. 

 15. {In 1894, July 7-Aug. 23.) 



Average No. 



Pee cent, in each 3 m. level. 



0-3m. 



3-6. 



6-9. 



9-13. 



la-is.; 



15-18. 



1894 406,000 



1895 707,000 

 1896 1,116,000 



45.5 

 44.0 

 45.1 



30.2 

 24.6 

 27.5 



16.0 

 18.4 

 14.9 



6.7 

 8.9 



7.7 



1.3 



2.2 

 3.4 



0.4 

 1.9 

 1.2 



From this it appears that from 44 to 45.5 per cent, of the 

 Crustacea were present in the upper three meters of the lake 

 from the middle of June to the middle of September, and from 

 25 to 30 per cent, more between 3 and 6 meters, from 15 to 18 

 between 6 and 9 meters, leaving from 8.5 to 13 percent, for the 

 lower half of the lake. 



The percentile distribution of the Crustacea during the summer 

 and its relation to the thermocline are shown in Figs. 26 and 27. In 

 each diagram the depth is computed above which were found in each 

 half month, respectively 25, 50, 75, 90, and 95 percent, of the 

 Crustacea, on the assumption that the Crustacea in each of the 

 3m. levels were equally distributed through it. The points repre- 

 senting the depths for the corresponding percentages were 

 platted on the diagram and then connected by lines. There is 

 added in each diagram the position of the isotherm of 20° 

 which lay in the thermocline in both years, although in 1896 

 the lake cooled below 20° before the thermocline disappeared. 

 In Fig. 26, the temperature for each date was computed from 

 the average of the week preceding and that following the date. 

 The temperature-line of Fig. 27 is taken from Fig. 4. 



The diagrams show that 25 per cent, of the Crustacea are al- 

 most always found in the upper two meters of the lake. No 

 doubt the position of this line would be higher if it had been 



