A LIMNOLOGICAL RECONNAISSANCE 45 



meshes of the finest bolting- cloth. The former constitutes 

 what is known as the net plankton and the latter the nanno- 

 plankton. Strictly speaking the latter term has been applied 

 only to organisms whose maximum diameter does not ex- 

 ceed 25 IX., but it is such a convenient term that it is pro- 

 posed to extend its meaning to include all of the forms that 

 escape through the meshes of the net. 



Methods. The net plankton was obtained by means of a 

 closing net whose coefficient has been determined as 1.2; 

 that is, about 837c of the column of water through which it 

 is drawn passes through the net and is strained. The ma- 

 terial was transferred from the plankton bucket to vials 

 and preserved in alcohol. In the subsequent enumeration 

 the volume of the catch and preservative was reduced to 

 10 cc. ; after shaking thoroughly 2 cc. were removed with 

 a piston pipette and the Crustacea and rotifers contained 

 therein were counted. The number thus obtained multiplied 

 by 5 gives the total number of the various organisms in the 

 catch. When only a few individuals of the larger Crustacea 

 were present the total was ascertained by direct count. 



The Protozoa and algae were enumerated by placing a 

 cubic centimeter of the material in a Sedgwick-Rafter cell 

 and then counting the various forms in the usual manner 

 with a compound microscope. 



The dry weight and organic matter of the net plankton 

 were also obtained. For these determinations a net haul 

 was made from surface to bottom (0-32 m.). This catch 

 was concentrated on a small piece of bolting cloth ; the ma- 

 terial was then carefully removed from the cloth with a 

 knife blade and transferred to a small platinum crucible. 

 Considerable care is necessary at this stage to prevent loss 

 of material. A drop or two of chloroform kills the organ- 

 isms and prevents putrefaction while the material is dry- 

 ing. The air dried material is then placed in a drying oven 

 at 60° for 24 hours and then in a desiccator for 24 hours. 

 The dry weight of the catch is obtained by means of a sensi- 

 tive assayer's balance. The material is then ignited in an 

 electric furnace until the ash appears vvhite, after which 

 the weight of the ash is ascertained. 



