46 IOWA STUDIES IN NATURAL HISTORY 



Samples of water for a study of the nannoplankton were 

 obtained from the various depths by means of a water 

 bottle. These minute organisms were removed from the 

 water with an electric centrifuge which has a speed of 

 3,600 revolutions per minute. The sedimentation was con- 

 tinued for about six minutes and the material from the 

 bottom of the tube with one cubic centimeter of water was 

 transferred to a Sedgwick-Rafter counting cell and the var- 

 ious organisms were enumerated. These organisms should 

 be counted in the living state since experiments have shown 

 that some of them are destroyed by the various preserving 

 agents. It is necessary, therefore, to count this material 

 as soon as possible after securing the samples. 



It has been found practical to obtain the dry weight and 

 organic matter in the nannoplankton also. A sufficient 

 amount for this purpose is usually obtained from 90 cc. to 

 120 cc. of water. With the standard centrifuge 30 cc. of 

 water are centrifuged at each run and the material from 

 three or four runs is concentrated in one tube from which 

 it can be removed after a final centrifuging with one cubic 

 centimeter of water and transferred to a small platinum 

 crucible. A drop of chloroform is added and the water is 

 allowed to evaporate. Beyond this point the procedure is 

 the same as that described above for the net plankton. A 

 certain amount of organic matter is dissolved in the water 

 and this is not removed with the centrifuge ; it is necessary, 

 therefore, to run a blank for this by determining the amount 

 of loss on ignition for one cubic centimeter of centrifuged 

 water. 



THE NET PLANKTON 



In order to ascertain the vertical distribution of the var- 

 ious planktonts, catches were made with the closing net 

 from each 5 m. stratum of the lake. The organisms in the 

 different strata have been enumerated and the results of 

 these enumerations are shown in table 23. The figures 

 given in this table indicate the number of individuals or 

 colonies per cubic meter of water. 



Phytoplankton. As might be expected the phytoplankton 

 was confined chiefly to the upper 10 m., or the epilimnion. 



