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 thatt 

 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. ln< 

 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,, 

 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. 



Summer — 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 seem 

 from the tables. This would be expected since the exclusion of the 

 Crustacea is due to the chemical condition of the lower water,. 



