82 PLANKTON OF WISCONSIN LAKES 



forms are more plentiful in the centrifuge material so that it is impos- 

 sible to give a fair estimate of the percentage of ash in the nannoplank- 

 ton organisms during these seasons. 



While the water of Lake Mendota does not show any turbidity due to 

 silt, except after unusually heavy rains, yet it contains regularly a 

 larger or smaller amount of such material in suspension; without ex- 

 ception, silt was noted in all of the centrifuge samples that were used 

 for the enumeration of the organisms. No attempt was made to meas- 

 ure the quantity of this silt directly, but the amount removed from the 

 lake water by the large centrifuge is represented roughly by the quan- 

 tity of inorganic matter left after the ash of the bowl water and that 

 of the organisms are deducted from the total ash of the sample. The 

 results obtained in this way are indicated for a number of samples in 

 table 23, p. 194. The samples represented in this table include only 

 those in which the diatoms constitute but a relatively small part of the 

 material as shown by the enumerations. Where these organisms are 

 abundant in the centrifuge material the ash of the nannoplankton would 

 exceed 10.0 per cent of the dry weight and the samples given in the 

 table have been selected with a view of keeping the ash content within 

 the 10.0 per cent limit. 



No bowl ash is shown for the samples of 1915 because the bowl water 

 was not added to the samples of that year. The sixth column of table 

 23, shows the amount of dry silt in milligrams per cubic meter of water. 

 During the summer months, the quantity of silt varies from about 370.0 

 milligrams (sample No. 5124-26) to 1,349.0 milligrams (sample No. 

 579-81) per cubic meter of water, representing almost a fourfold varia- 

 tion. Expressed in other terms, these quantities are 0.37 and 1.35 parts 

 per million, respectively. The particles of silt have not been meas- 

 ured, but they are so minute that even these surprisingly small amounts 

 represent an enormous number of individuals. 



The specific gravity of the more common rock-forming minerals aver- 

 ages about 2.6, so that this may be assumed as approximately the spe- 

 cific gravity of the silt also. On this basis the volume of 370.0 milli- 

 grams of silt is about 140 cubic millimeters and that of 1,349.0 milli- 

 grams equals 520 cubic millimeters, thus giving a range of 0.14 cubic 

 millimeters to 0.52 cubic millimeters of this material per liter of water. 



The last two samples given in the table show the results for the Jan- 

 uary and March catches of 1917. In both of these samples the amount 

 of silt obtained by difference, namely, 235.5 milligrams and 112.5 milli- 

 grams, is well below the summer minimum; the March catch, in fact, 

 shows only about a third as much as the summer minimum. The Janu- 

 ary sample was taken about a month after the lake became covered with 

 ice and the comparatively small amount of silt found on that date seems 



