for late culmination of the greens in this lake is not evident. Whipple 

 (1927, p. 232) stated that late growths of green algae are usually 

 associated with stagnation, but, obviously, this explanation cannot 

 apply to Western Lake Erie, 



Taken as a group, the greens were distributed similarly in the 

 two years, but there were some marked differences in the composiion of 

 the group. In 1929, only four genera (Oocystis, Scenedesmus, Dictyo- 

 sphaerium, and Coelastrum) were prominent in the plankton. Of these 

 only Coelastrum and Dictyosphaerium were abundant in 1930, while 

 Oocystis and Scenedesmus were comparatively rare. Rarity of these forms 

 was compensated for by the addition of five genera which were rare or 

 wanting in 1929. None of these five was extremely abundant, but com- 

 bined with the four genera mentioned above, they were able to maintain 

 the same average abundance as in 1929 during comparable periods of time. 

 The four leading genera in 1930 were Coelastrum, Dictyosphaerium, 

 Ankistrodesmus, and Westella, 



One notable point concerning the composition of the 

 phytoplankton is the rarity of desmids, both in species and in abundance 

 of those present. West and West (1912) called attention to the fact 

 that the most important factor in the distribution and abundance of 

 plankton algae is the amount of dissolved salts in the water. They 

 found that desnlds predominate in regions having Precambrian and early 

 Paleozoic rocks, which have small amounts of dissolved salts, particu- 

 larly calcium, in the surface waters. Such waters are poor in diatoms 

 and blue-greens, while these two groups are conspicuous in waters with 

 a high content of dissolved salts, G. M. Smith (192li, p. 113) dis- 

 cussed this question at length and stated that his findings in North 

 A.nerican lakes supported the theory of West and West. Pearsall (1922) 

 stated the theory more precisely: desrdds dominate in waters with a high 

 basic ratio, that is, waters in v^ch the ratio ILJLilL is trcre than 



Ca+ Mg 

 1,5. Such waters are poor in nitrates, carbonates, and silica. Waters 

 with a basic ratio of less than 1,5 are rich in nitrates, carbonates, 

 and silica, and have diatoms dominant. They also support numbers of 

 blue-greens, and greens other than desmids. 



The known facts concerning the phytoplankton of the Great Lakes 

 are in accord with this theory. In Lake Erie the ratio ^ "*" ^^ is 0.17 

 and the mean for the five Great Lakes is 0;l6 (see Table 13) • Along with 

 this low basic ratio, we find that the phytoplankto n is dominated by 

 diatoms, and that desmids play a minor part (see especially Eddy, 1927j 

 Gottschall, 19 30 J and Burkholder, 1929a). 



168 



