354 



Birge — The Crustacea of the Plankton. 



Table XXIV. — Number of limnetic Crustacea during July, 1894-1896, 

 stated in thousands per sq. m. of surface. 



July. 



Diaptomus 



Cyclops 



Daphnia hyalina. 

 Chydorus 



1894. 



260.5 

 95.4 

 15.5 



1895. 



202.2 

 227.8 

 207.6 

 160.1 



1896. 



177.5 

 244.2 

 192.2 

 313.5 



Daphnia retrocurva was entirely absent in 1894, while begin- 

 ning its regular development in the two latter years. 



It seems quite evident that the presence of Lyngbya in the 

 lake was the determining factor in causing the numbers of all 

 species except Diaptomus to be so exceptionally small. The 

 influence of this alga is not by any means confined to the adults. 

 It is even more important in its action upon the young. In all 

 the species of Crustacea the immature forms are found near the 

 surface, and during the day the upper one-half meter, or there- 

 abouts, is occupied by immature Crustacea. This is the same 

 region as that in which the Lyngbya is most abundant, and 

 since Lyngbya is wholly unmanageable as food for the immature 

 Crustacea, its presence in the upper water exerts a very unfa- 

 vorable influence upon the development of the new broods which 

 may be hatched while it is the predominant alga. It is note- 

 worthy that Diaptomus, which maintained its numbers through 

 the Lyngbya period, is the species of Crustacea which combines 

 great locomotive powers with effective means of collecting food. 

 Daphnia has the most effective food collector, but is inferior in 

 locomotive powers. Cyclops is inferior to both species in both 

 ways, but ordinarily has an advantage in its omnivorous habits 

 and its greater adaptability to different conditions of life. 



In late July Lyngbya began to decline, and Aphanizomenon 

 and Melosira began to develop. Parallel with this change in 

 the character of the algae, Cyclops and Daphnia hyalina in- 

 creased rapidly, and in late August, when Melosira was the 

 predominant alga, Cyclops and Daphnia were the predominant 

 Crustacea. Chydorus had fairly entered upon its period of rapid 

 multiplication at this time but its numbers only became large as 

 Aphanizomenon multiplied in September. 



