PLANKTON OF LAKE MONONA HI 



Half a dozen kinds of rotifers were noted, but only one, Anuraea coch- 

 learis, appeared regularly in the catches. The flagellates were repre- 

 sented by Ceratium and Yolvox. 



The green and blue-green algae consisted of representatives of Micro- 

 cystis, Coelosphaerium, Aphanizomenon, Anabaena, Lyngbya, Staura- 

 strum, and Pediastrum, while the diatoms included forms belonging to 

 Melosira, Tabellaria, Fragilaria, Asterionella, Synedra, and Stephano- 

 discus. 



The largest net catch of the entire series of observations was obtained 

 on October 15, 1912. (See sample No. 238, table 45). The chief consti- 

 tuents of this material were two algal forms, namely, Microcystis and 

 Melosira. The former yielded nearly three million colonies per cubic 

 meter of water and the latter two and a half million filaments per cubic 

 meter. Fragilaria was also fairly abundant at this time, the number 

 being approximately three-quarters of a million per cubic meter. 



The samples collected in 1913 yielded two maxima of organic matter, 

 one on July 24 (sample No. 326) and another on October 18 (No. 360). 

 The former was due mainly to a rise in the amount of Lyngbya and 

 Microcystis; a cladoceran, Chydorus, also showed a sixfold increase in 

 number between July 15 and July 24. The October maximum was 

 produced by a very large crop of the diatom Melosira. (See figure 37, 

 curve A.) 



Two maxima of organic matter were also found in 1915, one on June 

 23 (sample No. 554, table 45) and another on October 15 (sample No. 

 5167). The catch obtained on June 23 consisted mainly of Aphanizo- 

 menon, an enormous crop of which was present in the upper water at 

 this time. The chief constituents of the net plankton during the Octo- 

 ber maximum were Microcystis and Melosira. 



In 1916 there was almost a fivefold increase in the organic matter 

 of the net plankton between June 14 and June 23 (sample No. 649 

 and No. 659) ; this was due to a marked increase in the number of 

 Daphnia pulex. By far the greater portion of the material in sample 

 No. 659 consisted of this cladoceran, while only a minor part was fur- 

 nished by the algae; this is the only instance in any of the lakes 

 wherein a marked rise of the organic matter of the net plankton could 

 be attributed directly and solely to any organism other than an alga. 

 The scarcity of the algae in the net plankton at this time was due, un- 

 doubtedly, to the fact that the large Daphnia population was feeding 

 extensively on them. (See figure 37, curve B.) 



Following the Daphnia maximum a very large crop of Volvox was 

 found in the upper water of Lake Monona on July 6, 1916. Colonies 

 of this flagellate were present in enormous numbers at a depth of three 

 meters on this date so that a substantially pure catch, amounting to a 



