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 



