LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
385 
Table 17. — Vertical distribution of ■plankton at stations 112, 113, 114. 
Depth, meters. 
Station 112. 
Station 113. 
Station 114. 
Plankton, 
cubic cen- 
timeters 
per cubic 
meter. 
c°. 
Plankton, 
cubic cen- 
timeters 
per cubic 
meter. 
c°. 
Plankton, 
cubic cen- 
timeters 
per cubic 
meter. 
C°. 
0 
35 
23.3 
32 
23.3 
43 
23.9 
1.5’...”.! . . . . . . . . . 1 . . . . . . . . . . . . . . . ...A..!..”!!! 
37 
23.2 
34 
23.2 
22 
23.6 
3 
31 
23.2 
43 
23.2 
C 1 ) 
4.6 
37 
23.1 
36 
23.1 
6.1 
28 
23.1 
34 
22.7 
7.6 
36 
23.0 
24 
21.4 
9.1 
13 
22.0 
19 
21.2 
10.6 
9 
21.0 
9 
20.6 
n 
20.3 
11 
19.8 
1 Bottom. 
PUMP AND NET COLLECTIONS. 
At every station on the lakes the plankton samples were obtained both with 
the pump and with the vertical plankton net. Although in the present investiga- 
tion only the results of the pumping method are taken into consideration, it is inter- 
esting to compare the two methods. Up to the present time the plankton net 
remains the chief instrument of limnological investigation, and the results obtained 
by this means are used for quantitative investigations and for estimates of the 
productiveness of basins. The source of error in this method lies in the determina- 
tion of the so-called coefficient of the net. The meaning of the coefficient of the 
net and how it is found can be seen from the following: 
The volume of water (M) filtered through the net drawn straight from the 
bottom to the surface of the lake is usually less than that computed by the formula 
M= CH, where G is the area of the net opening and H is the depth of water. 
For more accurate results the formula M=q Ctv is applied, where t is the time 
in seconds required to lift the net from bottom to surface, v is the velocity of ver- 
tical movement of the net in meters per second, q is the reciprocal of the coefficient 
of the net ( K ), and C, as before, is the area of the net opening. The resistance 
of the net causes a certain quantity of water to be pushed aside and q is the factor 
of correction, which varies, depending on several conditions. The coefficient of 
the net ( K ) can be computed, using Hensen’s formula which has been found on the 
basis of experiments made with filtered water, and it can be applied to a net of 
known silk of definite dimensions and drawn at given velocities. The other methods 
consist in comparing the quantities of organisms or Lobelia seeds added to water 
(Reighard, 1894) caught in the net with the quantities obtained by filtering an exact 
volume of the same water. According to Amberg (1900) the filtering capacity of 
the net, and therefore the coefficient of th£ net, depends on the size of the meshes, 
the area of the net opening, the area of the filtering cone, the form of the net, the 
velocity of the lifting, the depth to which the net has been lowered, and the com- 
position and the amount of plankton. Burckhardt (1900) pointed out that the 
net coefficient depends also on the length of time the net has been used. Kofoid 
(1903) came to the same conclusions, to wit: 
A uniform coefficient, and, moreover, one founded on the operation of the net in filtered water, 
would not adequately correct the error, since it takes no account of the seasonal changes in the quantity 
and kind of plankton, and does not recognize the effect of the progressive clogging of the net by the 
catch, or the change of the net with use. 
