The blue-green algae were represented by only a few important 

 genera. Coelosphaerium was more aliundant in May than in April or June, 

 but was rather consistently abundant after the middle of July, with an 

 average of 11 thousand units per liter. Oscillatoria also was abundant 

 in May. These tiro genera were responsible for the minor peak in the 

 curve for blue-greens in May (Fig. liO • Following a decline in June, 

 Oscillatoria again increased and reached a maximum of h5 thousand units 

 in late September. Mcrismopedia was rare or wanting in the early half 

 of the season, but increased rapidly in early August, and maintained a 

 level near 31 thousand units for the last four periods of the season. 

 Gomphosphaeria was very rare in 1930, only once exceeding 1 thousand 

 units per liter. Microcystis appeared in small numbers in late May and 

 remained rare until late July, In late August it increased greatly and 

 reached the season's maximum of 113 thousand units in late September, 

 During the last two periods it was the most abundant blue-green in the 

 plankton. Thus, Coelosphaerium and Oscillatoria were the only genera to 

 appear in considerable numbers in spring, and all genera were more 

 abundant in late summer and early autumn than earlier in the season. 



The genera belonging to groups other than diatoms, greens, or 

 blue-greens, appeared from time to time, but always in small numbers. 



Comparison of the seasons of 1929 and 1930 



It is evident from the foregoing discussions of seasonal dis- 

 tribution that the distribution in the two years was not exactly the 

 same. The differences are particularly striking for the genera, and less 

 so for the groups of phytoplankton organisms. It appears that dif- 

 ferences in the genera tended to compensate for each other, so that the 

 curves for groups were not as far different as one might expect. In the 

 following section, differences and siralarities in seasonal distribution 

 both of groups and genera will be considered. 



Diatoms . In order to facilitate comparison of the distribution 

 of diatoms in the two years. Fig. l5 was constructed. The curves are the 

 same as those for diatoms shown in Figs. 13 and lU, and are derived from 

 the data in Tables $1 and 53« Comparison of the curves for diatoms is 

 somewhat unsatisfactory. The periods of time covered were not exactly 

 the same; the season of 1929 began later and ended later than the season 

 of 1930. The curves agree very closely for the months of June, July, 

 August, and September, but not for the extreme ends of the seasons. In 

 the section on adequacy of the sampling program (p. 172) the curves are 



162 



