Vertical Distribution of the Crustacea — Autumn. 389 
hardly shared in the rise to the early summer maximum until 
two weeks after the 0-3 m. level, while in the lower part of the 
lake the population declined, or remained stationary throughout 
July. In August the Crustacea of the 9-12 m. level increased in 
number as the thermocline moved downward into that level, 
while no increase was perceptible in the population of the lake 
below 12 m. until after the middle of September; after which 
date the numbers rapidly increased. 
No increase of population was seen in the upper levels of the 
lake after the month of July; and if this diagram is compared 
with Fig. 6 which shows the changes in the total population of 
the lake, it will be seen that the autumnal maximum, which is 
clearly indicated, comes entirely from the increase of population 
in the lower water of the lake. 
The same general facts appear in the diagram for 1896, but, 
if possible, in a form even more striking. The 0-3 m. and 3-6 m. 
levels follow each other closely, while the spring increase in 
population comes later in the lower levels of the lake. In the 
9-12 m. level the population remains stationary during May, 
when that of the upper levels is rapidly falling, and at the 
same time the Crustacea in the water below 12 m. are increas¬ 
ing in number; more rapidly in proportion to increased depth. 
In the 0-3 m. level at the first of June the population was sub¬ 
stantially stationary, while that in the water below was falling 
rapidly. This condition was brought about by the new broods 
of Chydorus, which nearly made up for the loss in numbers of 
other species. 
In 1896 the thermocline moved downward much more rapidly 
than in the preceding year and as a result of this movement, 
the Crustacea in the lower water began to increase in numbers 
at an earlier date. (See Figs. 3, 4, 26, 27.) A marked increase 
occurs in August in the 9-12 m. level and begins about two 
weeks later in the levels below. As in 1895, so also in 1896, the 
fall maximum is caused by the increase in the population of the 
lower water, with the exception that in late October of 1896 
there was a great increase in the number of the Crustacea in the 
0-3 m. level, due to the appearance of great broods of D. hyalina 
at this time. These soon disappeared, so that the Crustacea in 
