Thus the blue-greens had only vne period of abundance, and this fell 

 in the month of September, 



Algae other than diatoms, greens, and blue-greens were too 

 rare at all times to require discussion. 



A glance at Fig. 13 is sufficient to show that the plankton 

 was dominated by diatoms for most of the season. From late May to 

 early August the plankton was almost exclusively composed of diatoms. 

 From late May to late July, greens and blue-greens were both rare, but 

 greens were consistently the more abundant. In August the blue-greens 

 outnumbered the greens, and equalled the diatoms during late August 

 and September. Greens and blue-greens each reached their maxima in 

 late September, at which time the blue-greens were much the more 

 abundant. In October, both greens and blue-greens declined, while the 

 diatoms continued to increase and r eached their maximum when the season 

 closed. Except for the brief period in late summer and early autumn, 

 the diatoms were distinctly the most abundant of the three groups. 



The phytoplankton as a whole showed two periods of abundance, 

 a minor maximum composed essentially of diatoms in spring, and a 

 tremendous maximum in early autumn, which was contributed to by all 

 of the three groups. 



Seasonal distribution of genera . Table $2 shows the seasonal 

 distribution of the various genera of algae which made important con- 

 tributions to plankton in the season of 1929. The figures for abundance 

 are average of counts made for all s tations visited in each period of 

 two weeks. 



In addition to being the most important group numerically, the 

 diatoms were represented by more genera of importance than the greens 

 or blue-greens. Eight genera of diatoms appeared in considerable 

 abundance at some time during the season. Of these, Asterionella, 

 Fragilaria, Melosira, Synedra, and Stephanodiscus were particularly 

 conspicuous. Tabellaria, Navicula, and Amphora were relatively rare, 

 Asterionella had two periods of great abundance. In the spring 

 maximum there were 29 thousand vnits per liter; it nearly disappeared 

 in the summer but increased to a second maximum of 100 thousand units 

 in late October, Fragilaria showed one period of abundance; it was 

 rare until September, when it increased markedly, reaching a maximum of 

 33 thousand units in the last half of that month. Melosira, like 

 Asterionella, showed tvro maxima of about the same magnitude, one in 



152 



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