blue-greens. The diatoms recovered in September and were again more 

 abundant than greens or blue-greens. In early October, diatoms were 

 ] "ss abundant than blue-greens, but more abundant than greens. In 

 spring and early summer, greens were slightly more abundant than blue- 

 greens, but this relation was lost after July, when blue-greens greatly 

 outnumbered greens. 



The phytoplankton as a whole showed two periods of great 

 abundance. The first came in May, and was made up almost exclusively of 

 diatoms. The second came in September, and was composed of all three 

 groups, but diatoms and blue-greens were especially abundant. 



Seasonal distribution of genera . Table ^h shows the seasonal 

 distribution of the most important genera encountered in the plankton 

 during the season of 1930, As in Table 52, the figures for abundance are 

 averages based on counts from all stations visited in each period of two 

 weeks . 



Of the eight genera of diatoms listed, only Asterionella, 

 Melosira, and Stephanodiscus showed two pronounced maxima. The remain- 

 ing five showed the usual spring maximum, but in autumn they were only 

 slightly more abundant than diiring the -summer period of decline. All 

 genera except Stephanodiscus were more abundant in spring than in autumn, 

 Asterionella reached its high count of 23 thousand units per liter in 

 early June. It declined in surmer, but recovered to l6 thousand units 

 in early October, Fragilaria was less abundant than Asterionella, and 

 reached its maximum several weeks earlier. For some unknown reason it 

 became abundant in late July (12 thousand units), at which time it was 

 the dominant diatom. Melosira was most numerous in late May (31 

 thousand units); in late Septentjer and early October the count was 

 about one half as great, Synedra was a conspicuous form in April and 

 early May, almost disappeared in summer, and increased only slightly in 

 autumn, Tabellaria reached a maximiwi of 31 thousand units in early 

 June, The counts of late September and early October were lower than 

 some of the mid-summer counts, but higher than the minimum of early 

 September, The spring maximum of Stephanodiscus came in early April and 

 the minimum in late June, During the summer it gradually increased and 

 reached the season's maximum of 208 thousand units in late September, 

 This figure is probably too high as an average, due to the tremendous 

 count at Station 59A on September l8. However, there is little doubt 

 that Stephanodiscus was the dominant diatom during September and early 

 October, Navicula was rare at all times, but reached its highest counts 

 in May. Rhizosolenia was the most abundant diatom during the spring, 

 with a maximum of 100 thousand imits in late May. It was absent or rare 



159 



