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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 Hensen finally adopted the method of drawing a net vertically from the bottom 
to the surface. Such a net strains out the plankton contained in a vertical column of 
water 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. Tbe 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 Beighard, 2 Ward, 3 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- 
face, and 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 
this 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 the 
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 by 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 
of the net by foreign particles and shrinkage of the net cloth so as to diminish the 
size of the openings in it. This change in net coefficient is the first difficulty in the 
use of Hensen’s method. If the pores of the cloth (Ho. 20 bolting cloth) used for such 
nets become clogged 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 by different observers with different nets, can not be 
accurately compared, and a large part of the advantage of the method is lost. It is 
1 Apstein, C. Das Siisswasserplankton. Kiel, 1896. 
2 Reighard, Jacob. A biological examination of Lake St. Clair. Bulletin of tbe Michigan Fish 
Commission, No. 4. 
3 Ward, H. B. A biological examination of Lake Michigan. Bulletin of Michigan Fish Commis- 
sion, hlo. 6. 
