Methods in Plankton Investigations. 1!» 



planktons rich in diatoms nor to the silt-laden planktons from 

 Havana, for a large part of the silt is debris of organic origin, 

 and the method above described does not differentiate the 

 organic material of the plankton from that of the silt. For 

 the quantitative investigation of these silt-laden planktons we 

 are thus practically limited to the enumerative method with 

 such incidental help as may be derived from volumetric 

 determination. 



The volumetric determination has taken two forms, the 

 settling and the centrifugal methods. The former as used by 

 us is the same as that employed by Eeighard ('94) and Ward 

 ('96a). The plankton is transferred to graduated tubes and is 

 allowed to stand twenty-four hours, when the amount of the 

 plankton settled at the bottom of the fluid, is read by the 

 graduations upon the tube. The tubes used are the carbon 

 tubes employed by chemists in the Eggert color test for the 

 estimation of carbon in steel. Our tubes in most frequent use 

 contain 25 and 50 cubic centimeters respectively, are about 

 twelve millimeters in inside diameter, and are graduated to 

 tenths of a cubic centimeter. For very small planktons 

 another tube, containing only ten cubic centimeters and 

 measuring six millimeters in inside diameter, was used. After 

 a series of measurements in the tubes above described it 

 became evident that a considerable error was involved in the 

 method. Kepeated measurements of the same plankton in 

 the same tube, after standing twenty-four hours, revealed a 

 considerable variation in the volume, as high as 30% in some 

 instances. Furthermore, planktons do not settle to an equal 

 density. Those composed of Rotifera or small Cladoeera (as 

 Chydorns) pack closely, while others containing filamentous 

 forms, as Oscillaria or Fragillaria, and those in which the larger 

 Entomostraea are predominant, settle very loosely. Thus the 

 determination of the volume of the plankton by the settling- 

 method does not give a uniform test of the amount of plankton 

 present. Furthermore, the process is a tedious one, especially 

 when large numbers of catches are to be handled. 



The centrifugal machine (Plate VII.) was finally hit upon 

 as affording the best solution of the difficulties presented in 



