306 Aquatic Societies 



slowly, but even such large forms as Daphne have an 

 extraordinary rate of increase, as we have already 

 indicated on pages 186 and 187. The rises in produc- 

 tion grow out of : 



1. Proper conditions of temperature, light, etc. 



2. Abundant food 



3. Rapid increase 



Declines follow upon failure of any of these, and 

 upon the attack of enemies. So swift are the changes 

 during the growing season that those who systematically 

 engage in the study of a lake's population take plancton 

 samples at intervals of not more than fourteen days, 

 and preferably, at intervals of seven days. 



Local Abundance Plancton organisms tend to be 

 uniformly distributed in a horizontal direction. Al- 

 though many of them can swim, their swimming, as 

 we have noted in the preceding chapter, is directed far 

 more toward maintenance of level, than toward change 

 of location. There are, however, for many plancton 

 organisms, well authenticated cases of irregular hori- 

 zontal distribution, one of which, for Carteria, we 

 quoted on pages 103 and 104. Alongside that record 

 for a Mttle flagellate, let us place Birge's ('96) record for 

 the water-flea, Daphne pulicaria, in Mendota Lake. 



"The Daphnias occurred in patches of irregular extent 

 and shape, perhaps 10 by 50 meters, and these patches 

 extended in a long belt parallel to the shore. The 

 surface waters were crowded by the Daphnias, and 

 great numbers of perch were feeding on them. The 

 swarm was watched for more than an hour. The water 

 could be seen disturbed by the perch along the shore 

 as far as the eye could reach. * * * * On this 

 occasion the number was shown to be 1,170,000 per 

 cubic meter of water in the densest part of the swarm." 



