12 PLANTS IN PONDFISH CULTURE. 



A remarkable illustration of such an association was presented dur- 

 ing the present season in the studies connected with pond 9D, in 

 which Daphnia pulex^ one of the largest of the daphnids, was pro- 

 duced in enormous quantity by natural means. The blue-green alga, 

 A'phanizomeiion fios-aqua'^ contributed directly to its support and 

 proA^ed to be the most desirable food of a heavy, natural culture of 

 Daphnia pulex. 



The author's first observation of this association in the pond oc- 

 curred on June 21, 1918. At this time a "bloom" of the Aphani- 

 zomenon was approaching its maximum. It was so abundant that 

 the water appeared blue-green and oily. A few hand strokes of the 

 dip net would bring up a quart of it in concentrated form. The 

 individual colonies are ordinarily microscopic, but during the acceler- 

 ation of growth in " bloom " production the colonies are considerably 

 augmented and become readily distingiiishable. They appear as 

 flocculent masses, 7 to 16 mm. long and less than half as wide. 

 They are delicate and tissuelike and assume the form of flattened 

 spindle-shaped masses lightly suspended in the water at all depths, 

 from the surface to the bottom of the pond. Practically a pure stand 

 had developed, for there were few representatives of other floating 

 alga3 present. The station records noted the presence of the algae in 

 May. From that time until June 21, the date of the author's first 

 observation, multiplication must have taken place with great rapidity, 

 and it continued to do so until the maximum was reached in the 

 interval of July 24 to 30, when the algae could be rolled up from the 

 bottom like mush. 



This conspicuous growth of algae in the pond was attended by one 

 equally remarkable in the number of Daphnia produced. The 

 Daphnia swarmed among the algae, forming practically a pure cul- 

 ture of Daphnia pulex. A rough estimate by volume, obtained by 

 the gravity method, showed approximately 75 cc. per cubic meter. 

 Accurate determinatior^s were difficult because of the accompanying 

 algal debris in the concentrations. When the algal maximum was 

 attained the Daphnia pulse had already begun to decline. Cypris, 

 copepods, and rotifers increased in the zooplankton and Clathro- 

 cystis, Volvox, and Pleodorina in the phytoplankton. 



The daphnids fed continuously upon the Aphanizomenon. The 

 entire process of feeding could be observed easily by placing them 

 in a watch glass or in a hollow glass slide and observing them with 

 the compound microscope. The flocculent masses of the alga were 

 wafted into the open walls of the body and fed forward toward the 

 mouth by the action of the legs in the manner so aptly described by 

 Birge (1918). During the feeding operations the Daphnia were 

 offered a mixed plankton, but invariably they retained only the 

 Aphanizomenon in the food current streaming toward the mouth. 

 Observations on the feeding habits were continued from time to time 

 during a period of five weeks, and it was easily seen that the abun- 

 dant natural forage provided in Aphanizomenon was one of the de- 

 termining factors in the high productivity of Daphnia pulex. 



By reference to Tables 4 and 5 it is seen that Daphnia formed a 

 high percentage of the food content in the orange-spotted sunfish and 

 catfish with which the pond was stocked. Data are not at hand for 

 the very young catfish since it was desirable not to disturb the ponds 



