THE IJFE OF DEVILS LAKE 69 



Of the 46 species, which are typically fresh water forms, "there 

 is nothing in their appearance to indicate that they have been in 

 any way modified I)y their changed environment." That many 

 species of diatoms are very insensitive to changes in their environ- 

 ment is show^n by a comparison of the flora of Minnewankan Bay 

 in 1916 w^ith that of the main hike. In that year by reason of heavy 

 snow fall the preceding w'inter, and of heavy rains in the spring the 

 Mauvaise Coulee, draining a wide extent of conntry north and 

 west of Devils Lake, was flowing for the first time in many years. 

 The construction of a highway grade across the mouth of the bay, 

 leaving it connected wnth the main lake by a culvert, thru which 

 water was flow-ing at the rate of about 7750 cu. m. daily, converted 

 the former into a lake with about 1/3 of the salt content of the 

 main lake ^4464 as compared with 12920 ])pm.). Of the fourteen 

 species found in the bay in this year thirteen were identical with 

 those in the main lake, while only one (Stephanodiscus niagrae) was 

 new% evidently brought down by the coulee. It is not unlikely that 

 a more thoro study would have revealed a larger number of species 

 common to both places, but the data suffice to show that many 

 species at least are very tolerant of great changes in their chemical 

 environment. 



The Ruppia zone is the chief source of the diatoms in tlie lake, 

 partly because it furnishes suitable attachment for the sessile species, 

 and probably because of the greater amount of dissolved food-stuffs 

 in this zone. They occur in considerable numbers in the pelagic 

 zone, however, and especially in the shore zone at certain times. 

 Thej^ may be carried into these zones from the Ruppia by currents, 

 or they may develop there independently of the latter. ProTably 

 both factors are involved in their presence in these zones. 



Their occurrence in the plancton .samples is very erratic. That 

 this is due to improper preservation, or to errors in sampling and 

 concentrating the material is unlikely. It is possibly to be explained 

 by the action of currents, just mentioned, especially in the case of 

 the shore samples, where the diatoms may be present in large num- 

 bers at one time, and soon after absent or verj' rare in the same 

 place. 



The follown'ng genera occur more or less frequently in the planc- 

 ton collections : Amphora, Amphiprora. Cyclotella, Cymbella, Cys- 

 topleura, (ryrosigma, Xavicula, Rhoicosphenia, S u r i r e 1 1 a and 

 Synedra, of which Cyclotella, Navicula, Surirella and Cymbella are 

 the commonest. 



The individual distribution of all of these has been plotted 

 as well as that of the diatoms collectively, but only the latter chart 

 and that of Chaetoceros are reproduced here.* Apart from the 



•Plato 16 and figs. IT, 21. 



