THE LIFE OF DEVIDS LAKE 65 



there was a marked increase about September 1, which apparently 

 corresponds to the mid-August maximum of 1913. 



In 1911 the records are too few to warrant any definite con- 

 clusions. 



In 1923 there are only a few scattering records of Lyngbya. 

 Apparently it is much less numerous now than formerly, correlated 

 possibly with the increasing concentration of the lake. It appears 

 to be absent from Stump and Lamoreau Lakes and very scarce in 

 East Lake. It was abundant in Main Lake in 1916 and present in 

 some quantity in 1918. altho the records for this year are too few 

 to warrant any conclusions as to its relative abundance at this time. 

 It is of interest that in 1916, when still abundant in most of Main 

 Lake it was apparently absent from Minnewaukan Bay which was 

 nearly fresh at this time*. Its absence or scarcity in Stump and 

 East Lakes supports the hypothesis of its dependence on a certain 

 degree of concentration (10000-15000 ppm.) ; but its apparent ab- 

 sence from Lamoreau Lake, which has about the same chemical 

 character as Main Lake, negatives this conclusion and renders the 

 cause of its erratic distribution in the complex uncertain. It was, 

 moreover, common in two collections from Mission Lake in 1916, 

 altho the concentration of the latter was 32640 ppm. at this time. 



Lyngbya martensiana has been reported from Lake 0. 



Nodularia spumigena. Excepting Coelosphaerium and Lyngbya 

 this is the most common alga in the lake. It is frequently driven 

 by the wind into extensive masses along the lee shore, while other 

 regions may be comparatively free from it. It develops in consider- 

 able quantity rather later than most of the other algae, appearing 

 sparingly during April and May and showing a rapid increase in 

 Jvine, with a maximum about the end of July and a more or less 

 distinct minimum about the middle of August. This was particu- 

 larly striking in 1913 in which it fell from an average for the sev- 

 eral depths of collection of 1720 on August 1 and 2 to an average 

 of 285 on August 20-26. But it occurred also in 1911 (August 8- 

 22) and 1914 (August 17). In 1912 the data are too few at this 

 season to warrant any conclusions. 



In 1914 a marked minimum occurred July 20-22, but the sud- 

 denness of the previous drop and the succeeding rise renders this 

 minimum open to question. Prior to July 20 Nodularia developed 

 mainly along shore and at the surface, so it is possible that a shift in 

 the wind, driving it into other parts of the lake, may have caused 

 this apparent drop (pi. 15). 



Nodularia appeared to be much more abundant in East than 

 in Main Lake, in 1913 and 1914 (fig. 20), but the records are too 

 few to permit of any definite conclusions*. It was abundant in 



*See table .1. 

 ♦See p. 54. 



