THE LTFE OF DEVILS EAKE 37 



water is representative of any other sample collected at a different 

 point, provided environmental conditions at the two points are 

 identical. This question has recently been carefully investigated 

 at Devils Lake by Moberg (1918), who found an average variation 

 ranging from 70% for some of the algae Coelosphaerium, Chro- 

 (icoccus, etc.) to 185% for the crustecean Moina.* 



Maximum variations for the entire series are of course much 

 larger. Thus in one series Diaptomus showed a range of 400% of 

 variation from the mean and Monia 297%, and in another series 

 Brachionus satanicus showed one of 876%. 



While Birge (1897) maintains the general uniformity in hori- 

 zontal distribution of plancton, he finds nevertheless in a series of 

 collections made at ditl'erent points on the same day* a range of 

 variation from the mean from 49%. for Cyclops** to 180% for 

 Daphnia pulicaria and Ergasilus. These variations, while not 

 necessarily proving the existence of swarms, as Birge points out, 

 do nevertheless show an irregularity of horizontal distribution so 

 great as to invalidate any general conclusions regarding plankton 

 distribution unless based on a large number of counts. 



"My observations show so much variation in catches made at 

 the same place and in succession that I have little confidence in 

 the differential method of determining vertical distribution ; unless 

 a very large number of observations is made and averaged, so as 

 to eliminate the chance of variation in the single observation.'" 

 (Birge 1897, p. 376.) 



Marsh (1898) in a series of collections made at the same place 

 on Green Lake, Wisconsin on two successive days, found that Diap- 

 tomus ranged from 291 to 2966, and similarly large variations were 

 found by him in other genera. 



Birge himself (I.e.) and other writers have described the 

 occurrence of swarms, chiefly among the Cladocera and I have 

 observed them in both Moina and Diaptomus.*** 



Similar results have been obtained by other investigators 

 (Reighard, 1894, Apstein, 1896, Gantl()lti-lIorn3-old and Aimeroth, 

 1915, et al.), altho, in general, with smaller variations. Moberg's 

 work was done, chiefly on 500 cc samples, but in two series 19 litre 

 collections were taken. His results warranted the following con- 

 clusions. "1) The zooplankton in Devils Lake shows a great 

 irregularity in horizontal distribution, and this irregularity'' can- 

 not be correlated with any variations in amount of phytoplankton 

 or in the chemical and physical environment. It is more likely due 



*Asplanchna showed a range of 219%. The number here is too bTuall, however, 

 to have much significance. 



♦Time required for the series not stated. Presumably they were made under 

 as nearly as possible similar conditions. 



•'Including two species. 



•••See Moberg (1918. p. 266, f. n. 8). 



