382 Birge—The Crustacea of the Plankton. 
was rapidly declining. In late June the population of all levels, 
declines altogether. 
This relation is even more conspicuous in the diagram for 
1896. The population below 9 meters did not increase at all 
until the end of April, while that of the upper levels increased 
several fold, the 0-3 m. level growing more rapidly than that 
below. In the first half of May the lower half of the lake- 
gained absolutely more than either of the levels above, its gains 
per cubic meter being about half as great as those of the upper 
water. In the last of May the levels below 12 meters continued 
to gain, while the 9-12 m. level was approximately stationary, 
and the upper strata fell off rapidly and about equally. At this 
time the lower half of the lake contained nearly 40 per cent, of 
the total number of Crustacea, nearly equally distributed, while 
the upper three meters contained only about 28 per cent. In 
early June all the strata below the 0-3 m. level lost heavily* 
owing to the disappearance of the spring broods of Cyclops 
and D. pulicaria ; while the 0-3 m. level remained approx¬ 
imately stationary, the new broods of Chydorus and Diaptomus y 
which appeared in that level, compensating for the decline in 
other species. The result of this decline in the population 
of the lower water serves to give the 0-3 m. stratum over 50 
per cent, of the whole population, and the number in this level 
continues between 45 and 50 per cent, during the remainder of 
the summer. 
Bummer — From the middle of June to the middle of September. 
The change from the late spring to the early summer has just, 
been spoken of. The most important fact influencing the ver¬ 
tical distribution at this time is the formation of the thermo- 
cline, and the accompanying exclusion of the Crustacea from the 
lower waters of the lake, and ultimately from the entire region 
below the thermocline. The thermocline was observed in each year 
about the middle of June — June 11th, 1895, June 13th, 1896 — 
and was present regularly afterward. The depopulation of the- 
lower waters does not coincide with these dates, as will be seen, 
from the tables. This would be expected since the exclusion of the 
Crustacea is due to the chemical condition of the lower water,. 
