60 PLANKTON OF WISCONSIN LAKES 



increase in the numbers of Ceratium and Melosira, and a second rise 

 during the first week of August corresponded to increases of Melosira 

 and Fragilaria. The September rise of organic matter accompanied 

 a marked increase in the diatoms as well as a distinct rise in the num- 

 ber of Coelosphaerium. 



The diagram for 1913 (fig. 24) shows that a large crop of Tabellaria 

 in May yielded a large amount of organic matter, while increases in 

 Ceratium and Melosira during the last week in July and the first week 

 in August correspond to an expansion in the curve for organic matter 

 at this time. Fragilaria was the chief organism responsible for the 

 autumnal rise. 



Increases in the numbers of diatoms and of Microcystis in May, 1915 

 (fig. 25), were accompanied by an increase in the organic matter, while 

 a large increase of Aphanizomenon was found in June, which helped to 

 keep the organic matter high when the other two forms declined in 

 numbers. The October maximum of this year corresponded to a rise 

 in Fragilaria. In 1916 (fig. 26) the peak during the last half of 

 May and the first half of June was accounted for largely by the in- 

 crease in the diatoms during this interval. The rise in the amount of 

 organic matter in the autumn of 1916 was correlated in time with 

 increases in the numbers of the various diatoms ; some of these forms 

 together with Aphanizomenon prolonged the increased quantity of 

 organic matter until the middle of December. 



Explanation of the Net Plankton Diagrams 



Figures 22 to 26 show the numerical results obtained for the samples 

 of net plankton; the spherical type of curve has been used in these 

 diagrams. The curves show the number of individuals or colonies per 

 cubic meter of water. In order to bring out the variations in the 

 quantity of organic matter more clearly by this type of diagram, that 

 curve was platted on a different scale; it shows the number of milli- 

 grams of dry organic matter in 10,000 cubic meters of water. 



The species of Cladocera and Rotifera were enumerated separately, 

 but it was not practicable to indicate each species in the diagrams. The 

 following abbreviations have been used for the different forms: 



DI = Diaptomus, CY = Cyclops, NA — nauplii, DA = Daphnia, 

 RO = Rotifera, CE = Ceratium, MI == Microcystis, CO = Coelosphae- 

 rium, AP = Aphanizomenon, AN = Anabaena, LY = Lyngbya, SM = 

 Staurastrum, ME = Melosira, TA = Tabellaria, FR = Fragilaria, AS 

 = Asterionella, ST = Stephanodiscus, OM= organic matter. 



