early June and a second in late September, Synedra was rare in spring 

 and summer (except in late July), increased explosively in September 

 and was most abundant in late October (9li thousand units per liter), 



Tabellaria was never very abundant, but was definitely more 

 numerous in spring and autumn than in summer. The maxima reached 



6 thousand units each. Stephanodiscus was unique among the algae in 

 being abundant at all times and in showing relatively' slight changes 

 in abundance. Tt was least numerous at the beginning and end of the 

 season; during June-July the mean count was 23 thousand and in August- 

 September the mean was 39 thousand units, Stephanodiscus was the most 

 consistently abundant of the phytoplankton organisms, Navicula was 

 rare in spring and surnner, increased somewhat in September and reached 

 a maximum of 13 thousand units in late October, Amphora was present 

 only in traces prior to Septentoer, and was never more numerous than 



7 thousand units per liter, Rhizosolenia, which is not listed in the 

 table, was present only in spring and autumn and never exceeded 



2 thousand units per liter, Surirella and Gyrosigma were rather con- 

 sistently present but never in large numbers. 



The spring maximum of diatoms was composed largely of 

 Asterionella, Melosira, and Stephanodiscus. The autumn maximum was 

 more varied in composition; with large numbers of Asterionella, 

 Fragilaria, Melosira, Synedra, and Stephanodiscus, During the months 

 of July and August, the diatoms were almost exclusively represented by 

 Stephanodiscus, except in late July, when Synedra was also abundant. 



Only four genera of green algae appeared in the plankton in 

 large numbers during the season of 1929. They were Oocystic, Scenedesmus, 

 Dictyosphaerium, and Coelastrum, All were rare in spring and summer, 

 and abundant in autumn. Oocystic began to increase in early September, 

 reached the maximum of 78 thousand units per liter in early October, and 

 declined to 8 thousand units in late October, Scenedesmus was the most 

 consistently abundant green alga during the spring and summer, but was 

 less conspicuous in autumn. The peak of 17 thousand units was attained 

 in late September, Dictyosphaerium was absent in late August and very 

 rare prior to that time. It suddenly became abundant in early September 

 (ii2 thousand units) and was about equally abundant in late September, 

 In October it became relatively rare again. Coelastrum was rare in 

 spring and early summer, increased somewhat in late summer, and reached 

 a maximum of 38 thousand units in late September. Like Dictyosphaerium, 

 Coelastrum was rare in October. Thus, each of the important green 

 algae had only one period of abundance and this fell in September and 

 October. Of the genera not listed, Crucigenia, Sphaerocystis, 

 Eudorina and Pandorina were present in many of the spring and summer 



15U 



