durinp the warm months and reaopeared in small numbers in early October. 

 Amphora was not observed in 1930, 



In spring, a number of diatoms were abundant and some of them 

 were abundant for a long time. At the maximum for the group (late May), 

 Rhizosolenia was dominant. The next most numerous form at that time 

 (Melosira) had a count less than one third as great. In autumn, 

 Stephanodiscus was far more abundant than any other form, and it de- 

 termined the time of maximum abundance for the group. The only other 

 genera which made important contributions were Asterionella and Melosira. 

 Stephanodiscus wa? the most consistently abundant form in summer also. 



None of the green algae was extremely abundant in the season 

 of 1930, but the group m.ade a creditable showing owing to the large 

 number of genera which made important contributions to the plankton. 

 Nine genera were present in considerable numbers at some time during the 

 season. Oocystis was rare or wanting in the early part of the season; 

 in July and after it was nresent in small but fairly constant numbers. 

 Scenedesmus was present in every period but one, but was most abundant in 

 May (8 and 9 thousand units). This form was largely r esponsible for the 

 slight upward bend in May in the curve shown in Fig. lU. Dictyosphaerium 

 was most abundant near the end of the season, with a maximum of 22 

 thousand units in late September. Coelastrum was one of the more abundant 

 greens. Rare or absent in the first half of the season, it reached a 

 high count of 21 thousand in early August, The count in late August was 

 lower, but it increased again in September, At the end of the season 

 Coelastrum was on the decline. Sphaerocystis was present from early May 

 to late September, and was most abundant (8 thousand units) in late 

 August and early September. 



Pediastrum was encountered in all periods except the first and 

 last, but was usually very rare. The high count of 6 thousand units was 

 recorded for late September. Tetrastrum first appeared in July and never 

 exceeded h thousand units. Ankistrodesmus was absent prior to June, In 

 the last half of the season it showed two distinct periods of abundance; 

 one in late July (1? thousand units) and another in the last two periods 

 (13 thousand units), Westella appeared in late May, was rather abundant 

 in late sunmer, and reached the peak of 22 thousand units in late 

 September, It should be noted that Scenedesmus was the only green alga 

 to reach its maximum in the first half of the season. All of the others 

 showed their greatest abundance after the middle of July, The greens 

 not listed in the table were unimportant constituents of the plankton, 

 Cosmarium appeared frequently but in small numbers. Closterium was 

 absent from most of the samples, but in late September had an average 

 count of 3 thousand units. The remaining forms were too rare to require 

 mention. 



161 



