THE LIFK OF DKVILS LAKE 41 



elapsing between the time of collection and tliat of counting was 

 different for different years, with the result that some of the collec- 

 tions deteriorated more than others. Especially is this true of the 

 1914 series, which for the most part was not counted until 1919 

 and in which considerable disintegration of some of the algae and 

 bacteria occurred. No record has been made of Dictyosphaerium for 

 this year, since it appears in the counts only sporadically and has 

 evidently deteriorated. Deterioration of the closer colonies, such 

 as Coelosphaerium, occurred to u much less degree and probably 

 did not materially affect the records, while the diatoms were unaf- 

 fected. In general the counts of any one season are closely com- 

 parable with one another since they were made for the most part 

 at nearly the same time. 



A study of the charts in general reveals several interesting 

 facts. 



1. There is, in general, no evident relation between depth and 

 plancton abundance, conditions at the surface and at lower levels 

 not usually differing greatly, except in respect to light, which factor 

 is considered at greater length later.* The absence of a thermo- 

 cline in Devils Lake renders conditions here very different from 

 those in deeper lakes elsewhere. Oxygen may occasionally be 

 greatly reduced or absent at the bottom, but the layer of water 

 in which this occurs is thin, and it is quite possible that oxygen, 

 in the dissolved condition at least, is of less importance than has 

 generally been believed. This ([uestion is discussed at greater 

 length elsewhere, in connection with the organisms inhabiting the 

 ooze.** 



2. There are great variations in the collections at all points, 

 tho most conspicuous in the case of those taken near shore. They 

 may be due in part, in the case of some of the colonial algae, to 

 imperfect preservation of material, altho this is unlikely, since the 

 various samples must in most cases at least, have been influenced 

 equally by this factor. In the case of the animals, swarms no doubt 

 are very important, tho the extent of their influence is still a 

 problem.*** Even among the algae, Moberg, in his study of hori- 

 zontal distribution, found great variations, too great to be attributed 

 to errors in collecting or in counting the samples, and apparently 

 due to chance. In any event the plancton, both plants and ani- 

 mals are 7wt distributed uniformly, as is generally assumed. In 

 the shore collections these variations are probably due largely to 

 temperature. "Wind may also play an important part, especially 

 with the filamentous algae, but other, less evident factors, appear 

 to enter. 



♦See p. 46. 



**See p. 106. 



••♦In this connection, see Moberg (1918). 



