170 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



made of the total plankton it has, on the other hand, been found 1 23 that this is so 

 distributed that nearly the same volume of it occurs under each square yard of the 

 surface at equal depths. 



From these two principles Hensen concluded that a determination of the amount 

 of plankton under a unit of area of any part of the sea would afford a measure of the 

 productive capacity of that part. 



It remained to find some means of making such determination. After much 

 labor IJensen finally adopted the method of drawing a net vertically from the bottom 

 to tin- surface. Such a net strains out the plankton contained in a vertical column of 

 \\ah-r and catches the whole amount of plankton under an area of the surface equal 

 to the net opening. From the plankton so obtained the total plankton of the water 

 under consideration may be calculated and the results expressed in volumes or by 

 weight or by enumerating the contained individuals. The productive capacity of a 

 body of water, as expressed in its plankton production, may thus be compared to that 

 of other bodies of water and so may be made of practical use. 



The method which Hensen used in the sea was later extended by Apstein, his 

 pupil, to fresh water. Apstein's results were published in various special papers and 

 finally collected into a single very useful volume. 1 This method, with some slight 

 modifications, has since been used in this country by Reighard, 2 Ward, :i and others. 



The great advantage which this method enjoys over others is that the water from 

 which the net strains the plankton is a vertical column extending from bottom to sur- 

 nid is thus a representative sample of all the water from all depths in the lake 

 examined. This column of water bears the same relation to the whole body of water 

 that a sample removed from a sheet of metal by a punch bears to the whole sheet. 

 There is no other method applicable to all conditions which has been shown to have 

 tins advantage. 



There are. however, certain difficulties in the use of this method. These were 

 known to Hensen and he attempted to obviate them. The net does not, as a matter 

 of fact, filter the whole of a column of water through which it passes. A part of t lie 

 water is pushed aside and a part filtered. By an elaborate set of experiments 

 Hensen tried to determine what part of the water was pushed aside. This depends 

 upon the form of the net and upon the material of which it is made. If the net filters 

 half of the column of water, then in order to know the amount of plankton actually 

 in the column it is necessary to multiply the amount of plankton taken by two. The 

 number l.y which one must thus multiply is known as the coefficient of the net. The 

 coefficient of the net was assumed by Hensen to remain practically constant. There 

 are, however, two factors which may cause a change in the net coefficient clogging 

 nf the net by foreign particles and shrinkage of the net doth so as to diminish the 

 si/e of the openings in it. This change in net coefficient is the liist, difficulty in the 

 use of Hensen's method. If the pores of the cloth (No. L'O bolting-cloth) used for such 

 nets become rlog-ed the net will filter less water than before, i. e., its coefficient will 

 become greater. If the net coefficient thus changes, the results obtained with a 

 given net at different times, or b\ different observers with different nets, can not In- 

 accurately compared, and a large part of the advantage of the method is lost. It is 



1 Apstein, C. Das SHsswasserplankton. Kiel, 18JI6. 



Reigbard, Jacob. A biological examination nf L:.kc St. Clair. Hull. -tin of tin- Mirliiuan I i>h 



Commission, No. 4. 



'Ward, H. B. A biological examination of Lake Michigan. Bulletin of .Mii-biuan l-'isli Commis- 

 sion, *No. 6. 



