78 FRESH-WATER BIOLOGY 



pushed aside not only is greater as the net moves faster but is 

 increased as the net becomes clogged and is therefore greater 

 toward the end of the haul than at its beginning. The filtering 

 capacity of the net gauze is further liable to change with age, as 

 its pores clog and its threads loosen and tend to obstruct the 

 openings. Although elaborate methods have been devised for 

 determining the errors of the plankton net, no one of them is satis- 

 factory. 



2. The Plankton Pump. The difficulties encountered in the use 

 of the plankton net for accurate quantitative work have led to the 

 development of the plankton pump, which is now largely used in 

 conjunction with the ordinary plankton net and which, used in that 

 connection, has nearly displaced the closable plankton net (Birge, 

 1895; Marsh, 1897) in fresh water. This may be any pump which 

 delivers at each stroke a known and constant volume of water. 

 The water is drawn through a hose which extends from the pump 

 to any desired depth and may terminate in a metal cone, closed 

 by very coarse wire netting, which serves to exclude foreign bodies 

 from the hose. From the pump the water may be conveniently 

 delivered through a shorter hose to some device for filtering the 

 plankton from it. For this purpose a plankton net is used. The 

 net may be suspended in air and the water pumped into it, but 

 some small organisms are thus forced through the net gauze and 

 lost, and others are doubtless injured by the impact of the stream 

 of water and the weight of the water in the net. This is avoided 

 if the net be held under water with only the canvas cone above 

 the surface. The whole operation may be readily carried out by 

 one person if the net be supported in the water by a wooden 

 float surrounding the cone (Fig. 23) and the delivery hose be 

 attached to the net (Kofoid, 1897). When sufficient water has 

 been pumped, the net is taken up and the catch removed and 

 treated in the usual way. 



The end of the suction hose may be allowed to remain at any 

 desired depth during the pumping. The pump is calibrated so 

 that the volume of water delivered at each stroke is known. The 

 number of strokes made during any haul is counted, so that a 

 simple calculation gives the total volume of water pumped. 



