49 



oratories and in this case plankton is obtained from an exceedingly restricted 

 area. The advantages of the plankton pump for certain conditions should not be 

 overlooked. In certain regions where the phytoplankton is very thick no plank- 

 ton net can give a reliable picture of the zooplankton because of the almost im- 

 mediate clogging of the pores of the net by the diatonris. In such cases the most 

 reliable measurement of the zooplankton is obtained by means of the plankton 

 pump as described by Wiborg (1948). In situations where excessive clogging 

 occurs if a plankton pump is not available, the plankton sampler must be used 

 with very short hauls only. When clogging is excessive, it is indicated by the 

 counter not turning or turning backwards. Clogging can be nainimized by using 

 the coarsest net which will still retain the species of zooplankton to be studied. 



A new device for the continuous sampling of plankton at five levels has 

 been described by Tonolli (1951). It is called the plankton bar and has been 

 used on Italian lakes. It is doubtful whether this device could be used in the 

 ocean but it might be serviceable in very restricted, quite waters, such as 

 fjords, estuaries, and salt ponds. The plankton bar consists essentially of five 

 plankton nets towed at five levels simultaneously from one cable. To the tail of 

 each net is attached a rubber tube which leads to the surface where it discharges 

 into a vacuum chamber and there the plankton is collected continuously. This 

 device appears rather complicated but it is perhaps too early to judge the possi- 

 bilities of the ingenius new developnnent. 



High speed Samplers - The "hi-speed" sampler developed at Scripps Institution 

 is obviously a great step forward in zooplankton research. The many good fea- 

 tures of the instrument have been described in Ahlstrom's manuscript. Every 

 effort should be made to publish a description of this sampler and an account of 

 its performance. 



SAMPLING OF PHYTOPLANKTON AND BACTERIA 



In the opinion of many workers the only feasible method of obtaining 

 phytoplankton on a quantitatively exact basis is through the use of water samples. 

 This is spot sampling in the extreme and criticized in the preceding section. 

 The Hardy continuous plankton recorder has been used for phytoplankton as well 

 as zooplankton for years and its possible more extensive use in other parts of 

 the world should not be overlooked. 



A great deal of Harvey's excellent work was done with a net for the col- 

 lection of phytoplankton. Any net fine enough to retain the phytoplankton will 

 clog very rapidly. Hence, this is perhaps the most likely place to use the plank- 

 ton pump; where the sea conditions and the depth of the water are such as to 

 make this feasible. In every situation in which individual water samples can be 

 shown to be adequate for determining the population, they are obviously the 

 simplest method to use. 



I would like to comment further on methods of estimation of phytoplank- 

 ton catches in the laboratory. There is much to be said in regard to chloro- 

 phyll measurements. A great deal of work has been done by means of centri- 

 fuging the water sainples and although subject to possible criticism, much of this 

 work seems to be fundamentally reliable. The continuous centrifuge is a possi- 

 ble method for certain purposes. This type of centrifuge has been used chiefly 

 in fresh water, but there is no reason why it should not be more extensively 

 employed for marine investigations. 



A settling procedure is sometimes used to concentrate plankton as a pre- 

 liminary to a final centrifuging. In the Scandanavian laboratories, however, 



