LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
377 
most of the observations have been made at low stages of the river. Fortunately 
there was no considerable change in hydrographic conditions during the course 
of the investigation. This stability permits us to make a comparison of the pro- 
ductive capacity of the different sections of the river. Only at the end of Sep- 
tember during the rise of the water the data showed such an extensive decrease 
in the amount of plankton that they could not be used for estimating the productive 
capacity of the river. 
The composition of the plankton is discussed on page 396. We will now 
analyze the results of the volumetric determinations of plankton only, regardless 
of its components. The remark, however, must be made here that the plankton 
samples of the Mississippi, and especially of the part between Rock Island Rapids 
and Burlington, Iowa, contain much detritus. From July to September the amount 
of plankton in the Mississippi River, excluding lakes, averaged 14.5 cm. 3 per cubic 
meter of water. This figure represents the average of the 142 samples collected at 
51 stations. With regard to the amount of plankton a striking difference exists 
between the upper and lower parts of the river. In the lower part, between Rock 
Island Rapids and Burlington, the volume of plankton varied from 3.3 to 6.75 
cm. 3 per cubic meter; the fluctuations in volumes of plankton in the upper part, 
between Hastings and Le Claire, at the head of Rock Island Rapids, ranged from 
12.3 to 33 cm. 3 The data of the volume of plankton measured in different parts 
of the river are given in Table 14. The averages of volumes of plankton in each 
part of the river, expressed in cubic centimeters of plankton per cubic meter of 
water, were as follows: Upper part from Hastings to Le Claire (excluding Lake 
Pepin), July to August 21.3, September 16.2; lower part from Le Claire to Alexandria 
(excluding Lake Keokuk), July to August 5.16, and September 4.8. The average 
was calculated for the upper part from the data obtained at 18 stations in July 
and August and 17 stations in September; the corresponding number of stations 
for the lower part are 10 and 6, respectively. 
The amount of plankton in the upper part of the river gradually increases 
from Hastings (station 116), where its volume is 12.3 cm. 3 per cubic meter, to 
Diamond Bluff, where the volume reaches 22.7 cm. 3 and RedWing (station 110), 
21.5 cm. 3 ; but 5 miles below, just above the head of Lake Pepin (stations 96-98), 
it decreases again, and the average plankton content of water flowing into the lake 
is only 16.6 cm. 3 per cubic meter. 
The water flowing out of Lake Pepin is richer in plankton. The average 
volume of plankton measured in samples taken opposite Reads Landing (stations 
101-103) in the outlet of the lake reached 21.8 cm. 3 
Below Lake Pepin a considerable increase of plankton has been observed near 
Prairie du Chien. This part of the river apparently presents more favorable 
conditions for the development of plankton than any other, on August 15 the 
average amount of plankton observed here reaching 32 cm. 3 (station 54). One 
month later the volume of plankton here was only 17.3 cm. 3 per cubic meter (station 
138), but 60 miles above, opposite the mouth of Root River (station 135), it was 
about twice that much (30.3 cm. 3 ) . It is evident, then, that even during the time 
when hydrographic conditions are stable there are considerable fluctuations in the 
production of plankton in different parts of the river. 
61999°— 24 3 
