LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
375 
The vertical distribution of temperature in the river is more uniform. The 
difference between the surface and bottom temperatures was usually less than 1°. 
The swifter the current the more uniform the temperature in different levels. 
For example, at station 96 on August 29 between 2 and 2.30 p. m., at a depth of 
10 feet, when the velocity was 0.91 foot per second, the difference in surface and 
bottom temperatures was 2.4°, whereas at station 97 on August 29 between 2.30 
and 3 p. m., at a depth of 14 feet, when the velocity was 1.38 feet per second, the 
difference was only 0.9°. 
PLANKTON AND DETRITUS. 
VOLUME. 
As has been stated above, two methods were employed for volumetric deter- 
mination of plankton, and the data obtained by each method are given in the 
tables. It is very well known to all limnologists that the volume of plankton 
measured by the settling method (sometimes called the gravimetric method) 
depends not only on the amount of organisms in the sample but also on the kind 
of plankton. For example, the plankton rich in diatoms, or Dinoflagellata, after 
24 hours of settling, forms a compact mass on the bottom of the tube. If the 
catch contains many crustaceans, and especially if some big water fleas with long 
spines and appendices, such as Leptodora or Bytotrephes, are present, the mass 
settles on the bottom loosely. Therefore when one deals with different kinds of 
plankton, taken from different lakes for instance, comparable data can be obtained 
only by the centrifuge method, for, when comparing the data secured by the two 
methods (Table 29, p. 422), it will be noted that the figures representing volumes 
of plankton obtained by the settling method vary greatly and are not in accord 
with the figures secured by the centrifuge method. (See stations 13, 28, 30, 110, 
and others, Table 29, p. 422.) For example, when examining the amounts of 
plankton from different depths by the settling method, an increase would be found, 
where the centrifuge method would show a decrease, and vice versa. 
No definite ratio of the data obtained by the gravimetric method to the data 
of the centrifuge method could be established. In most cases, as one would expect, 
the volume of plankton read on a settling tube is greater than that obtained after 
two minutes of centrifuging, and this difference in volume varies widely. Kofoid 
(1903) found that it ranged from 8 to 76 per cent. In the present investigation 
there were instances where no difference at all between the data obtained by the 
two methods was found. This usually happened when the samples contained 
much silt and detritus or when the amount of plankton was very small. (See 
stations 156, 158, Table 29, p. 433.) In most cases, however, the volume of plankton 
measured by the centrifuge method was from 30 to 70 per cent less than when 
determined by the settling method. These results apparently are in accord with 
those obtained by Kofoid. It may be concluded, therefore, that the possible error 
of the gravimetric method varies from 0 to 70 per cent. We are unable to control 
the density of plankton material when it is left to settle on the bottom of the tube 
by gravity only. The results obtained are too inaccurate, and this method ought 
to be abandoned entirely. In this paper, therefore, we shall discuss only the 
results arrived at by the centrifuge method. 
