THE LIFE OF DEVILS LAKE 43 



increasing from above downward, were nearly 50% fewer at the 

 4 than at the 3 m. level. 



Again, consider the Diaptomus charts of Marsh (1898, pi VII). 

 In August, 1896, there occurred, according to the chart, two well 

 marked maxima and minima, with numbers ranging from 1563 to 

 3803*, a difference of nearly 150%. Similar, tho less marked 

 irregularities are shown in the curves of Birge (I.e.)- 



Irregularities in vertical distribution, similar to those just 

 mentioned, have been discussed by Birge and Juday (1911, p. 116) 

 who consider them evidence of stratification of the organisms con- 

 cerned. They deny the likelihood of their causation thru errors in 

 collection or counting, but apparently overlook the possibility of 

 irregularity in horizontal distribution being responsible for an 

 apparent irregularity in vertical distribution, nor do these authors 

 attempt an explanation, merely contenting themselves with the 

 statement that "such results . . . should be expected . . ." 



These differences may, of course, be accentuated, or the reverse, 

 by the scale and time interval chosen, and the number of collections 

 averaged. Taken in the aggregate they are of little significance and 

 do not obscure the main features of the curves. Considered sepa- 

 rately, however, they do have significance, in all probability indi- 

 cating the irregularity of horizontal distribution already referred 

 to.** 



2. There appears to be no constant relation between the 

 environment and the periods of maximum and minimum production. 

 Reproduction is, in most cases at least, greater in summer than 

 winter, so that, obviously the factors of light and temperature play 

 the all important roles in determining abundance. But even this 

 rule has exceptions. Diatoms frequently develop in enormous num- 

 bers in winter, as noted by Marsh (1900, p. 176) in Green Lake, 

 Wisconsin, and others. The above statement obviously does not 

 mean that plankton production is independent of environment. It 

 is closely dependent thereon. But the factors are so many, and 

 so closely interwoven with one another, that it is difficult, if not 

 impossible, to determine any general laws governing their inter- 

 dependence. 



There is often a plancton maximum in spring associated with 

 increase of temperature and light, with overturn and consequent 

 mixing of the water and influx of food materials in the run-off from 

 rain and melting snow. In addition to this maximum there may 

 occur other less constant maxima in mid-summer and autumn. 

 This is the general type of plancton production presented by Devils 

 Lake, Lake Mendota*** and the lake of Zurich,**** etc. 



•Total catch. 

 **See pp. 36-S. 

 ***Fide' Birge (1. c.) 

 ****Fide' Lozeron (1902). 



