253 



Quantitative Investigation of the Plankton. 



general considerations. 



The purpose of this investigation was the determination, 

 by measurement, of the quantity of minute organisms develop- 

 ing in the water at intervals throughout the year, and by this 

 means to trace the seasonal fluctuations in production, and 

 the relation of quantitative changes to constant and fluctuat- 

 ing factors of the environment, to flood and drouth, to chemi- 

 cal conditions, to the ice blockade, and to vegetation; and to 

 contrast production in waters of the main and tributary 

 streams, in impounding l)ackwaters and the channel, and in 

 bottom-land lakes and the main stream. 



METHOD OP COLLECTION. 



The method used in determining the quantity of plankton 

 was based upon that devised by Hensen ( '87 ) and modified by 

 Apstein ('96) for use in fresh water. The changes and modifi- 

 cations which were made to adapt the method to use in our 

 situation and to correct some of its errors have been described 

 in detail by me elsewhere ('97); I shall, therefore, only briefly 

 refer to a few phases of the subject of special pertinence or 

 interest in this connection. 



The changes in method during the progress of the work 

 are indicated in Tables III.-IX. From June, 1894, to May 20, 

 1896, the plankton was collected by means of the silk net, 

 made after Apstein's smaller model (see Apstein '96) of No. 

 20 silk bolting-cloth of Keller's manufacture. This net was 

 drawn through the water at a uniform rate of one half meter 

 per second for a distance of thirty meters. As shown in the 

 tables referred to, most of the hauls were made by the ob- 

 lique-haul method devised by Prof. Frank Smith, in which the 

 net was drawn along an oblique rope from bottom to surface 

 across channel in the river, and across the current, where cur- 

 rent existed, at our other stations of collection, except in 



