74 



FRESH-WATER BIOLOGY 



D. Quantitative Methods in Open Water 



I . The Quantitative Plankton Net. The plankton net and pump 

 are intended for the collection of plankton for quantitative inves- 

 tigations. The plankton net differs from the townet described 

 in that its rim extends upward into a truncated cone of canvas 

 (Fig. 17), and that it is provided with a removable bucket. 



The canvas cone hinders bottom ooze from entering the net and 

 also hinders the slopping out of the contents as the net is drawn 

 above the surface. It serves further to lessen the diameter of the 

 net opening, so that a larger fraction of the column of water above 

 the net opening is filtered and less of it is pushed aside by the 

 resistance of the filtering gauze. 



The plankton net (Fig. 1 7) in use at the University of Wisconsin 

 Is here first described with the permission of Professor Birge. The 



ring which supports the net is about 

 seven inches in diameter and from this 

 measurement the other dimensions of 

 the apparatus may be roughly measured 

 on the figure. The canvas cone stretches 

 from the net ring to an upper ring and 

 both rings are of one-eighth-inch spring 

 brass wire. Three eight-shaped pieces 

 of lighter wire are stnmg on each ring 

 through one opening, while the other 

 opening receives the eyes on the ends 

 of three connecting rods which hold the 

 two rings together. The upper support- 

 ing ring has three brass rings soldered to 

 it for the attachment of the draw lines. 

 The canvas cone and the band, which 

 is ordinarily sewn to the top of the net, 

 are in this case cut from one piece of 

 shrunken canvas. This is sewn around 

 the upper supporting ring and is attached 

 to the inside of the lower ring by means of a tape sewn to its out- 

 side. The bolting cloth net (No. 16 or No. 20 cloth) is sewn to 



Fig. 17. W'lsconsin plankton net. 

 (Original photograph from appa- 

 ratus loaned by Professor Birge.) 



