NET PLANKTON OF LAKE MENDOTA 



59 



Fig. 21. — Diagram illustrating the construction of the spherical type of curve. 



The curves in figures 22 to 26 represent the number of individuals 

 or colonies of the various organisms in the net plankton per cubic 

 meter of water ; this is a rather large unit of volume, but a smaller one 

 would make it difficult to show some of the forms which are present 

 in relatively smaller numbers, such as Diaptomus and Daphnia, for 

 example. The scale used in these curves is the same as that employed 

 by Lohmann, namely, 1 = 0.25 mm., and his table has been used to 

 ascertain the lengths of the radii. For purposes of comparison with 

 the enumerations of the net planktonts, the results obtained for the 

 organic matter are also shown by this type of curve ; it was necessary 

 to make use of a larger volume of water for the organic matter in order 

 to bring out more clearly the various changes in quantity. These 

 curves for the organic matter, therefore, show the number of milli- 

 grams of dry organic matter per 10,000 cubic meters. Figures 7 and 

 8 indicate the variations in the quantity of the organic matter much 

 better and these diagrams should be considered along with figures 22 

 to 26. The printed figures showing the numerical results for the net 

 plankton are approximately one-fourth as large as the original dia- 

 grams. 



In 1911 (fig. 22) numerical work was begun on the net plankton in 

 April but the gravimetric and chemical study did not begin until the 

 first of June. The increase of organic matter in the second week of 

 June was correlated with an increase in Ceratium and slight increases 

 in the number of diatoms, while the August rise of the organic matter 

 corresponded to marked increases in Ceratium and Melosira. In the 

 autumn of 1911, there was a marked rise in the number of diatoms, 

 especially Melosira and Fragilaria, with an increase in the organic 

 matter at this time. 



In 1912 (fig. 23) there was no marked rise in any one form corre- 

 lated with the increase in organic matter during the month of April, 

 but several forms were present in considerable abundance at this time. 

 A rise in the organic matter about the first of June accompanied an 



