NET PLANKTON OF LAKE MENDOTA • 25 



24, 1913; regular trips were usually continued until the first or the 

 second week in December. 



Table 5 (p. 186) shows the general distribution of the net plankton 

 during the different months of the year. The first column under each 

 month indicates the number of runs or catches made during the month 

 and the second column gives the average amount of dry net plankton 

 per cubic meter of water that was secured in these catches; the latter 

 shows, therefore, the monthly average of this matrial. The results for 

 1916 and 1917 are based on material obtained from much smaller quan- 

 tities of water than in the previous years because net plankton was 

 taken only from the water which was centrif uged for the smaller organ- 

 isms. In these instances the quantity of water averaged about 1,200 

 liters per catch. 



The 415 catches from Lake Mendota were combined in such a way as 

 to make 184 samples for chemical analyses. Thirty-eight of these sam- 

 ples consist of single catches while the others are combinations of two 

 to five catches each. In general it was the plan to make two or more 

 catches each week during the open season in order to obtain a better 

 average of material as well as a sufficient amount for analysis ; usually 

 all of the material obtained during a week was combined into one 

 sample. Two of the samples from Lake Monona consist of two catches 

 each while the others from this lake as well as all of those from Lake 

 Waubesa contain but a single catch each. 



Table 5 serves to show only the more general variations in the quan- 

 tity of net plankton during the different months of the year ; the time 

 period is too long a unit to bring out the details. This general table 

 indicates that there are marked differences in the amount of net plank- 

 ton in the different years and also that the maximum and minimum 

 amounts are not found in the same months from year to year. The 

 results on the various lakes will now be taken up in greater detail. 



LAKE MENDOTA 



Organic Matter in Net Plankton 



The organic matter of the net plankton consists of that portion of 

 the material which is consumed in the ashing process. In general the 

 amount of magnesium carbonate in the ash is so small that it is not 

 necessary to make a correction for the carbon dioxide removed from it 

 during the ashing process. During the greater part of the year from 

 70 per cent to 90 per cent or more, of the dry weight of the net plankton 

 consists of organic matter; but during periods in which the diatoms 

 predominate the organic and inorganic constituents may be almost 

 equal in amount. 



