90 THE LIFE OF DEVILS LAKE 



its greater abundance in winter and early spring than in mid- 

 summer. What the factor is which determines the relatively large 

 number present at the former season is impossible to say. After 

 the lake becomes ice-bound there is almost a total absence of algae 

 so that food is very scarce. In Lake Mendota, on the contrary, 

 Birge (1. c. page 320) states that "the food supply is ample." In 

 Devils Lake I believe that the winter population consists of indi- 

 viduals which have developed in the preceding summer or fall ; that, 

 due to decrease of temperature and food, there is no active repro- 

 duction in winter, but this, on the other hand, reduces metabolism 

 and lowers the death rate and that consequently the numbers remain 

 fairly stationary. The diminution in spring, which in 1923 occur- 

 red before the lake opened, is probably due to the death of many 

 of the individuals which have lived thru the winter, possibly due to 

 lack of food. Rome of these survive, however, and give rise to a 

 large number of nauplii and adults in May or June. 



This genus is frequently colored a deep red, due, apparently, 

 to the presence of a highly colored oil derived from its food, prob- 

 ably diatoms. This was especially noticeable on one occasion 

 (7/13/16) when the water above the grade across Minnewaukan Bay 

 was literally colored red by great masses of Diaptomus driven on 

 shore by the wind. This material may be present in the nauplii 

 even before hatching. 



Diaptomus sicilis elsewhere is a cold water species, which might 

 explain its greater abundance in winter than in mid-summer in 

 Devils Lake, were it not for the August-September maxima of 1913 

 and 1914. In 1913 this occurred between 9/1 and 9/8 a period of 

 high and rising temperature, culminating in a surface temperature 

 of 23 C° at 1 p. m. on 9/6. In 1914, on the contrary, it occurred 

 a week earlier (8/24-8/31) when the temperature was falling* 

 (17° at the surface at 6 p. m. 8/26). It is impossible therefore, 

 from my experience to find any close correlation between tempera- 

 ture and the seasonal distribution of this species. There is probably 

 no one factor which determines it, as Robert (1. c.) has pointed out. 



Diaptomus sicilis is a widely distributed form, adapting 

 itself readily to a great variety of environments. Hence it is 

 not strange that it should occur even in so brackish an environ- 

 ment as Devils Lake. It occurs more or less commonly in all of the 

 bodies of the Devils-Stump Lake complex and is ubiquitous in its 

 distribution, being present in all parts of the lake, with the excep- 

 tion of the ooze. 



Dioptomvs shoshove has been reported in one collection 

 (7/27/20) from Main Lake and from Lake P and Spring Lake, and 

 a neighboring pond, not included in the complex. 



*See pi. 22. 



