34 PLANKTON OF WISCONSIN LAKES 



Tying two 15 cubic centimeter tubes and having a speed of about 4,000 

 revolutions per minute was used. The enumerations were made in 

 the usual manner and, in general, they were made in duplicate ; in most 

 instances it was found that the two counts checked very closely for most 

 of the organisms, but whenever the differences for the common forms 

 seemed too large, a third count was made. In the more abundant forms 

 also, the counts were frequently checked by counting them in the cen- 

 trifuged as well as in the uncentrifuged material. The mean of the 

 different counts has been regarded as representing the number of the 

 various organisms. 



Samples of water were also obtained at different depths of the lake 

 for a study of the vertical distribution of the various forms noted in 

 the nannoplankton, but the data secured in the enumerations will not be 

 discussed here. 



The results of the counts made on the samples of water taken from 

 the tank are shown graphically in figures 32 and 33. As already indi- 

 cated for the net plankton, the numbers of the various organisms differ 

 so widely that it has been necessary to use the spherical type of curve 

 for the nannoplanktonts also, in order to show all of them on the same 

 diagram. The curves in these two figures indicate the number of indi- 

 viduals in 10 liters of water. The number of individuals per unit 

 volume of water is so much larger in the nannoplankton forms than in 

 the net plankton forms, that a much smaller volume of water has been 

 used for the diagrams of the former than for those of the latter ; that is, 

 10 liters in the nannoplankton as compared with one cubic meter in the 

 net plankton. The organic matter of the nannoplankton has been 

 platted on the same basis as in the net plankton, namely, the number 

 of milligrams of dry material per 10,000 cubic meters of water. In 

 spite of being based on a much larger volume of water, this type of 

 curve does not show the results for the organic matter nearly as well 

 as the curves in figure 28. It will be best to compare the two types 

 of diagrams in this connection. 



The various forms of organisms appearing in the nannoplankton of 

 Lake Mendota are shown in figures 32 and 33. The rhizopods were 

 represented by four different forms. One of them was a small Amoeba 

 which was irregular in its seasonal distribution; large numbers were 

 found at all depths in some instances. An average of 255,000 indi- 

 viduals per liter of water w^as noted in a sample obtained in August, 

 1916 ; in most of the samples in which this form was found, the numbers 

 ranged from 1,000 to 6,000 per liter. Three unidentified rhizopods were 

 noted ; they were present much more frequently than Amoeba, but the 

 number rarely exceeded 1,000 to 2,000 per liter of water. 



