NANNOPLANKTON OF LAKE MENDOTA 89 



The direct counts also show that about two-thirds of the total number 

 of bacteria in Lake Mendota are rod-shaped forms and one-third spher- 

 ical or coccus forms. The rods range in length from 1.2/* to 10.0//, and 

 their diameters vary from 0.2/x to 2.75/*. The shorter individuals are 

 more abundant than the longer ones, so that a large number of meas- 

 urements gave an average length of 2.5/* and a mean diameter of 0.9**. 

 The coccus forms vary from 0.22/* to 0.75/x in diameter, with a mean 

 of 0.44/*. 



These results, together with those obtained by other investigators, 

 constitute a basis for the computation of the live weight and the quan- 

 tity of dry organic matter in this crop of bacteria. The volume of a 

 rod-shaped individual of mean size, that is 2.5/*, long and 0.9/* in 

 diameter, is 1.5904 cubic micra, or 0.0000000015904 cubic millimeter. 

 The volume of a spherical individual with a diameter of 0.44/* is 0.0446 

 cubic micron, or 0.0000000000446 cubic millimeter. 



The maximum summer average in 1920 was 30,000 bacteria per cubic 

 centimeter, or 30 billion per cubic meter of water. Two-thirds of them, 

 or 20 billion, were rod-shaped individuals and one-third, or 10 billion, 

 were spherical forms. On the basis of the mean size the volume of the 

 rod-shaped bacteria in a cubic meter of water at that time was 31.808 

 cubic millimeters and of the spheres 0.446 cubic millimeter, making a 

 total of 32.254 cubic millimeters for the two groups. Rubner 5 found 

 that the specific gravity of water bacteria averaged about 1.05 so that 

 the live weight of the bacteria in a cubic meter of Mendota water dur- 

 ing the summer maximum of 1920 was approximately 33.9 milligrams. 



Rubner 6 and Nishimura 7 state that about 84.0 per cent of the live 

 weight of aquatic bacteria consists of water ; on this basis the maximum 

 crop of bacteria in Lake Mendota in 1920 represented 5.4 milligrams 

 of dry material per cubic meter of water. Nishimura also found 11.2 

 per cent of ash in a water bacillus which he analyzed; deducting this 

 percentage of ash from the Mendota material leaves 4.8 milligrams of 

 dry organic matter per cubic meter of water for the maximum summer 

 crop of bacteria in 1920. The average number from July to October, 



1919, was 3,000 bacteria per cubic centimeter, or one-tenth as many as 

 the summer maximum of 1920 ; the crop of the former year, therefore, 

 represented only about 0.48 milligram of dry organic matter per cubic 

 meter of water. The winter minimum of 1,500 bacteria per cubic 

 centimeter was only one-twentieth as large as the summer maximum of 



1920, so that it amounted to 0.24 milligram per cubic meter. 



5 Archiv fur Hygiene, Bd. 11, 1890, pp. 365-395. 

 8 Archiv fur Hygiene, Bd. 46, 1903, pp. 1-63. 

 7 Archiv fur Hygiene, Bd. 18, 1893, pp. 318-333. 



