Vertical Distribution of the Crustacea — Summer. 385 
possible to indicate the real concentration of the Crustacea in 
the upper meter. During May the percentage lines all moved 
downward, owing to the downward movement of Cyclops during 
that month, as its numbers rose to their maximum. The move¬ 
ment extends into June, 1895; while in the early part of June, 
1896, the center of population moved upward more than 3 
meters, owing to the earlier death of the spring broods of Cy¬ 
clops in that year. The center of population then remains close 
to the three meter line until the middle of August. In late 
June and early July of both years there is a rapid decrease of 
numbers in the lower levels of the lake. The 90 and 95 per 
cent, lines reach the level of the thermocline early in July, and 
they remain there through July, August, and early September, 
closely following the thermocline as it moves downward through 
the water. The center of population, which remains for some 
time near the 3 m. level, moves downward rapidly in September, 
and reaches a depth between 7 and 8 meters in October. If the 
Crustacea were uniformly distributed throughout the lake it 
should lie at 9 meters. The 90 per cent, level was as high as 
8 m. in July and August, 1896; and between 9 and 10.m in 1895, 
but moves downward to about 16 m. in October. 
This practical exclusion of plant and animal life from the 
lower water during summer is a factor of great importance in 
the life of the lake, as the following considerations show: First, 
during this period the number of Crustacea and the quantity of 
the plankton is independent of the depth of the water below 
the level which the thermocline has reached. Second, the ex¬ 
clusion from the lower water of species unfavorably affected by 
warmth prevents their appearance in the plankton or causes 
them to decline during the summer, while in the other lakes in 
which the deeper water is inhabitable their numbers may go on 
multiplying. This is pre-eminently true of Daphniapulicaria, 
whose numbers are small in lake Mendota during the summer, 
while in many of the Ocohomowoc lakes it is abundant during 
the same period and inhabits the entire depth of the lakes below 
the thermocline. The summer decline of Cyclops brevispinosus 
may also be due to the same cause. Third, the total number of 
the Crustacea during the summer is far smaller than it would 
25 
