Vertical Distribution of the Crustacea — Summer. 383 



resulting from the temperature conditions. In both years the pop- 

 ulation of the lower half of the lake in the latter part of June 

 is equal to or greater than that in tbe same region in the early 

 or even the latter part of April. The population of the 9-12 m. 

 level remained substantially stationary until the middle of June, 

 1895, and the same was true of this level and the level below 

 until the middle of July, 1896. In June, 1895, the population 

 of the bottom level was high, owing to the accumulation there 

 of large numbers of diseased and dying Daphnia hyalina; but 

 as soon as these had died, the numbers rapidly fell off, and the 

 population in the 15-18 m. level was very small in the first half 

 of July. 



In 1894, observations begun with the 1st of July, and at that 

 time the population below 9 m. was extremely small, far smaller 

 than in either of tbe succeeding years. At that time the 

 temperature conditions below the surface were not observed, 

 but it is fair to infer that the thermocline was established at a 

 comparatively early date in that year. A second fact which 

 influenced the distribution in 1894 is the unusual preponder- 

 ance of Diaptomus among the Crustacea in that year. A very 

 high percentage of this species is found at all times in the up- 

 per water, while Cyclops, whose per cent, in the lower water is 

 greater than that of any other species, was represented by very 

 small numbers. 



During July the population of the waters below 9 m. declines 

 very rapidly, as will be seen from the table which gives the 

 population of the lower water daring the months of June, July and 

 August. 



Table XXIX. — Population per cubic meter. 



June 1-15 . . . 

 June 16-30 . . 

 July 1-15 .. 

 July 16-31 ... 

 August 1-15 . 

 August 16-31 





1895. 







1896. 



9-12 m. 



12-15 m. 



15-18 m. 



9-12 m. 



12-15 m. 



45,000 



36,000 



46,600 



24,000 



12,600 



13,300 



9,200 



17,500 



24,200 



18,800 



14,200 



4,200 



2,200 



25,100 



20,900 



10,900 



2,100 



1,400 



9,700 



700 



27,500 



4,100 



600 



11,200 



1,200 



32,700 



3,500 



550 



49,600 



6,900 



15-18 m. 



30,600 



15,000 



2,600 



300 



20 



500 



