area, but had extended themselves into the Buffalo 

 area, so that now their horizontal distribution was ap- 

 proaching uniformity with Anabaena growing in the 

 west and Coelosphaerium in the east. The green algae 

 were somewhat more numerous than the blue -green, 

 and diatoms most plentiful of all. In general, the 

 three curves show the same trend although at stations 

 6, 17, and 53 the diatoms were conspicuously promi- 

 nent in the community. 



Diatoms were at a minimum and the blue -green 

 algae, especially Anabaena , had taken the lead at 

 many stations in August (fig. 52). It is interesting 

 to note that Anabaena fluctuated from none, or mere 

 traces at several stations, up to 3, 000 per liter at sta- 

 tions 6, 17, 47, and 53. In the west Melosira still 

 persisted in large numbers at a few stations. In gen- 

 eral, the trends of the groups were similar, but there 

 were also notable exceptions where odd ratios of the 

 constituents were observed in the communities. 



During the first week of September (fig. 52) sam- 

 ples obtained from the eastern waters showed consider- 

 able quantities of green algae. In fact, this group 

 outnumbered the diatoms east from Long Point. The 

 blue -green algae had decreased from a maximum 

 abundance in August to minimum occurrence in the 

 September communities. In the western area during 

 the second week of September there occurred such a 

 tremendous diatom maximum that the other com- 

 ponents were rendered relatively insignificant. Even 

 though Merismopedia and Coelosphaerium were numer- 

 ous at several of the shallow western stations, the 

 abundant diatoms, Asterionella , Fragilaria, and 

 Melosira, almost obscured their presence. 



The photomicrographs (figs. 53-56) show repre- 

 sentative types of phytoplankton communities taken 

 in the surface No. 20 net tows. The community found 

 in the waters off Erie, Pennsylvania, on August 13, 

 1929 (fig. 53A) was composed chiefly of the blue- 

 green algae, Aphanizomenon, Anabaena , and Micro- 

 cystis together with a few colonies of the green alga, 

 Sphaerocystis , and the diatom Melosira. Several days 

 later the surface waters off Cleveland, Ohio (fig. 53B), 

 supported a community dominated by the diatoms, 

 Asterionella , Mel osira, and Stephanodiscus. The 

 green algae, Pediastrum and Staurastrum were also 

 present in fair numbers, but blue -green algae were in- 

 conspicuous. These two communities illustrate a typ- 

 ical case of horizontal variation. 



Seasonal variation in the communities is pic- 

 tured for 2 stations in figures 54-56. The plankton 

 gatherings from station 13, near the Pennsylvania- 

 New York State line, show remarkable differences 

 in the constituent species during June (fig. 54) and 

 during August (fig. 55). The community in June con- 

 sisted of Asterionella, Fragilaria , Melosira , and 

 Tabellaria_, an almost pure stand of diatoms. In 

 August the diatoms had decreased and were largely 

 replaced by the dinoflagellate, Ceratium, the blue- 

 green alga, Anabaena, and green algae, Oocystis and 

 Staurastrum , and the rotifer, Polyarthra . 



Another interesting series (fig. 56) is shown by 

 the 4 monthly tows from station 3 near Buffalo. Dia- 

 toms formed an almost pure standing crop in June. 

 Fragilaria was still present in July but the other dia- 

 toms had declined and Ceratium, Dinobryon ,Stauras- 

 trum, etc. , were conspicuous in the community. By 

 August the blue -green Anabaena had been introduced 

 into the community and Fragilaria had decreased. 

 In September diatoms regained their prominence again 

 but a great many green and blue -green individuals are 

 evident in the community. 



From this discussion of communities it is obvious 

 that diatoms are the prevalent group in the lake, but 

 it is equally apparent that blue -green algae and green 

 algae attain a very prominent position in the plankton 

 at different times and places. In June and July dia- 

 toms were the dominant group. In August they de- 

 creased and the blue -green algae grew into promi- 

 nence as the typical warm -water group. In the lat- 

 ter part of August the blue -green algae declined and 

 the green algae came into prominence. In mid- 

 September diatoms again developed so rapidly as to 

 completely overshadow aU other groups. 



I 



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