55 PLANKTON OF WISCONSIN LAKES 



increase in number at this season ; but in the other two years the rise 

 was due chiefly to a single form, such as Tabellaria in 1913 and Aste- 

 rionella in 1914, while the other forms showed only a relatively small 

 increase at this time. 



Following the vernal period the green and blue-green algae become 

 the predominant phytoplanktonts and they usually hold this position 

 until autumn. In 1913, however, the marked rise in the organic mat- 

 ter in late July and early August (fig. 7) was due to two organisms, 

 one of which was a diatom; both Ceratium and Melosira began to in- 

 crease in numbers during the first week in July. The former reached 

 a maximum of 25,800,000 per cubic meter of water on July 21 and the 

 latter rose to 29,900,000 filaments per cubic meter on August 4. After 

 the latter date both declined rapidly in numbers, corresponding to the 

 decline in the organic matter. 



In the curve for 1912, Ceratium was responsible for the peak which 

 appeared in late July (fig. 7). Its average number in +he upper 10 

 meters was 15,665,000 individuals per cubic meter of water on July 30. 

 The peak shown for the first week in September was produced by an 

 increase in the diatoms, Melosira being the chief form while Fragilaria 

 was second in importance. The former rose from 1,077,000 per cubic 

 meter on July 30 to 17,639,000 on September 2 and the latter increased 

 from 74,000 to 4,548,000 filaments per cubic meter durmg the same 

 period. The secondary peak shown in the first week of September, 

 1915, was produced by an increase of two diatoms, namely, Fragilaria 

 and Tabellaria, the former being the more important factor. 



The five autumnal maxima shown in figures 7 and 8 were due chiefly 

 or wholly to marked increases of the diatoms. In 1911 there was a 

 large increase of Melosira in late September and of Fragilaria in Oc- 

 tober corresponding to the rise in the quantity of organic matter. 

 Among the blue-green algae also there was a distinct rise in the number 

 of colonies of Microcystis from the last of September to the middle of 

 October. The Cyclops population was also larger during the latter 

 half of October. 



In 1912 increases in Melosira, Tabellaria, and Fragilaria were corre- 

 lated with the autumn rise of the curve for organic matter. Coelo- 

 sphaerium also increased in numbers during the second half of Sep- 

 tember and the number remained fairly high until the last of October. 

 The first peak in the autumnal increase of organic matter in 1913 was 

 correlated with an increase in the numbers of Fragilaria and Melosira, 

 but the second peak came at a time when these two forms were declining 

 in numbers. In fact, it is difficult to account for the second peak in 

 October by means of the numerical data because the only increases in 

 numbers at that time were relatively small ones in Microcystis, Coelo- 



