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 Daphnia pulicaria, 

 whose numbers are small in lake Mendota during the summer, 

 while in many of the Oconomowoc 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 



