SOME PLANKTON STUDIES IN THE GREAT LAKES. 139 



by forcing air into it by means of a pipette held against the gauze at either end. 

 When the tube, emptied of its fluid, is placed in the next fluid it may be tilled by 

 immersing it in the fluid and removing with a pipette the air previously forced in. This 

 method prevents the loss of plankton in the manipulation, and it also saves much time 

 and enables one to pack many tubes of plankton in a single large vessel. 



When the plankton has been preserved in the tubes they must be taken to the 

 laboratory in order to carry out the rest of the work. Here the contents of each 

 plankton tube are measured in the following manner : The gauze is removed from 

 one end of a tube and carefully rinsed free from any adhering plankton. The alcohol 

 used in rinsing is placed in a beaker and the contents of the tube poured into the 

 beaker. The tube and the other gauze are thoroughly rinsed and the rinsings placed 

 in the beaker. The contents of the beaker are now turned into a tube graduated to 

 tenths of a cubic centimeter. This tube is allowed to stand twenty-four hours in order 

 that the lighter constituents of the plankton may settle, and the volume is then read 

 off and recorded. 



The volume thus obtained is the volume of plankton taken by the net in drawing 

 it vertically through, let us say, a distance of 200 cm. The area of the opening into 

 the net is about 1,250 sq. cm. Hence the net might be thought to have filtered a 

 volume of water equal to the area of its opening multiplied by the distance through 

 which it is drawn. On this assumption the net would have filtered, in this case, 

 1,250 by 200, which is 250,000 c. c. of water, and the volume of plankton taken (perhaps 

 1*5 c. c.) would be the volume contained in 250,000 c. c. of water. Since the gauze 

 net offers considerable resistance to the water it is not true that the net filters the 

 whole of the column of water through which it is drawn; on the contrary, a part of 

 this column is forced aside while another part passes through the net. The proportion 

 of water which passes through the net depends upon the velocity of the net. Let us 

 suppose that it is one-half of the whole column; then the 1*5 c. c. of plankton taken 

 represents not the amount in 250,000 c. c. of water, but the amount in 125,000 c. c. of 

 water. To get the volume of plankton in 250,000 c. c. of water, we must multiply the 

 volume taken by 2. Similarly, under whatever circumstances the net is drawn, if we 

 wish to know the amount of plankton contained in the column of water through which 

 it is drawn we must multiply the volume of plankton taken by some number which 

 represents the relation of the volume of water strained to the whole column of water 

 through which the net is drawn. This number is called the coefficient of the net. 



In the case of our net the volume taken when multiplied by the net coefficient 

 gives the total volume of plankton under an area of 1,250 sq. cm. of the lake surface. 

 In order to get the area under one square meter of lake surface this number is multi- 

 plied by 8 (1,250 c. c. is one-eighth of a square meter). The coefficient of the 

 net used is now being investigated, but has not yet been accurately determined, so 

 that the reduction of the volume taken to the actual volume and to the volume 

 under a square meter of surface can not be attempted at this time. 



The following table shows the results obtained by measurements of the volumes 

 taken at five of the stations. The volumes taken at the other stations, with the 



