these same stations showed a marked decrease indic- 

 ative of the summer decline. In fact mere traces 

 were obtained at Cleveland, Fairport, and Ashtabula. 

 At this time, however, the number of diatoms in- 

 creased eastward from Long Point where the decline 

 had not begun. Diatoms were relatively scarce 

 throughout the lake in August (fig. 45), but a conspic- 

 uous rise occurred in the western area in mid-Septem- 

 ber. Of the 4 months, June and September showed 

 the greatest diatom productivity (fig. 46). In order to 

 determine accurately the cycle it would, of course, 

 be desirable to extend investigations over a much 

 longer period of time. Recent observations on the 

 city water supply at Erie, Pennsylvania, have shown 

 a diatom maximum in November with a decrease in 

 January (Gottschall 1930). 



During the period of observations, the prevalent 

 species were Asterionella formo ji, Fragilaria cro- 

 tonensis, Melosira granulata , and to a lesser degree 

 Tabellaria fenestrata . The other species, Stephan - 

 odiscus niagarae , Rhizosolenia eriensis , etc., were 

 not common. The author has, however, found 

 Stephanodiscus very plentiful in the Buffalo city 

 water supply during the winter months. These obser- 

 vations appear to agree with those made by Vorce 50 

 years ago. Tabellaria did not, however, attain the 

 the prominence in the summer community of diatoms 

 in 1928 and 1929 that was claimed for it in 1880. 



The green algae were scarce in June, but showed 

 some increase in July and August at a few stations. 

 A rapid rise occurred in this group throughout the 

 lake in September. The common genera were Coel - 

 astrum, Dictyosphaerium , Pediastrum, Sphaerocystis , 

 and Staurastrum. During the autumnal pulse, dia- 

 toms were more numerous than the green algae in 

 the western area, but in the eastern part of the lake 

 the green algae were more abundant than diatoms 

 (fig. 52). 



The blue -green algae were almost entirely ab- 

 sent from the eastern lake waters in early June. West- 

 ward from Long Point, however, the group was very 

 abundant, Aphanizomenon being the common organ- 

 ism. In July this pulse of organisms declined but 

 they became more abundant in the eastern waters. 

 Anabaena continued to grow in this latter area until 

 its abundance in August rivalled the conditions 

 found for blue -green algae in the western section in 

 June. There was a general decline in September ex- 



cept at some of the western stations where consider- 

 able numbers of Coelosphaerium and Merismopedia 

 flourished. 



Protozoa were found in comparatively small 

 numbers throughout the lake. Probably the methods 

 used in catching and preserving the plankton were not 

 adequate for the apprehension of small and fragile 

 members of this group. During August Vorticella 

 was fairly common along with Anabaena to which the 

 species was attached. This epiphytic protozoan 

 reached 500 per liter at several stations. Difflugi a 

 was widespread in small numbers during June and 

 July, declined in August, and reappeared in the Sep- 

 tember samples. 



Rotifers were abundant in June and July at several 

 eastern stations. Anapus ovalis reached 200 per liter 

 at station 24. Anuraea cochlear is and Polyarthra 

 platyptera were widely spread throughout the lake and 

 were most numerous during July when their numbers 

 varied from 10 to 40 per liter at the various stations. 



MICROPLANKTON COMMUNITIES 



The distribution of diatoms, green algae, and 

 blue -green algae obtained in the surface samples has 

 been plotted for the 4 months in figures 51 and 52. 

 These graphs are intended to clarify the relationship 

 of the more important groups in the phytoplankton 

 communities. 



Diatoms were the most abundant group in the 

 communities of phytoplankton throughout the lake in 

 June (fig. 51). Green algae occuned in relatively 

 smaller numbers. The blue -green algae, however, 

 did not parallel the fluctuations from station to sta- 

 tion of the other two groups. In fact, where the 

 blue -green algae were abundant, the diatoms were 

 relatively few. This situation is particularly striking 

 in the western part of the lake where there were great 

 variations in the horizontal distribution of the diatoms, 

 Asterionella and Melosira, and blue -green algae, 

 chiefly Aphanizomenon . The data indicate consider- 

 able changes in the ratio of the component groups in 

 the communities from one station to another at this 

 time. 



In July (fig. 51) there was again this parallelism 

 in the fluctuating curves for diatoms and green algae. 

 The blue -green algae had decreased in the western 



137 



