420 
BULLETIN OF THE BUREAU OF FISHERIES. 
life in the new lake. The increase in the amount of plankton in the lake as com- 
pared with that in the adjacent part of the river indicates an increase in productive 
capacity, and a confirmation of this statement is found in the fisheries statistics. 
The peculiar conditions existing in Lake Keokuk require more detailed and long- 
continued observations. From the biological point of view the formation of the 
lake is not yet complete and the natural process of the formation of various ecological 
communities can easily be controlled by introducing into the lake such fishes, 
mussels, and water plants as are of greatest practical value. 
SUMMARY. 
The present limnological investigation of the upper Mississippi between Hast- 
ings, Minn., and Alexandria, Mo., covered a period of three months (July to Sep- 
tember) in 1921. The following conclusions are made on the basis of the examina- 
tion of 673 plankton samples collected at 171 stations: 
1. The mean content of plankton in the river, excluding Lake Pepin and Lake 
Keokuk, averaged 14.5 cm. 3 per cubic meter of water (the plankton was collected 
with pump and its volume determined by the centrifuge method) . The production 
of plankton in the upper part of the river between Hastings and Rock Island Rapids, 
excluding Lake Pepin, averaged in August 21.3 cm. 3 and in September 16.2 cm. 3 
per cubic meter. The production of plankton in the lower part of the river, be- 
tween Rock Island Rapids and Burlington (head of Lake Keokuk), averaged in 
July 5.16 cm. 3 and in September 4.8 cm. 3 per cubic meter. 
2. The river below Rock Island Rapids carried less than ^0 per cent of the 
amount of plankton found above the rapids. This is possibly due to the destruc- 
tion of the plankton organisms when passing the rapids. 
3. The mean plankton content in Lake Pepin averaged 16.6 cm. 3 per cubic 
meter of water (August 18 to September 10). In the upper half of the lake the 
average was 13.3 cm. 3 per cubic meter. Excluding the shallow northern part, the 
mean content of plankton in the upper half of the lake reached 15.7 cm. 3 per cubic 
meter. The mean plankton content in the lower half of the lake averaged 22.1 
cm. 3 per cubic meter. The plankton resource of the lake is greater than that of 
the river just above the lake. The water running into the lake contained 16.6 cm. 3 
of plankton (August 29) and leaving the lake it contained 21.8 cm. 3 of plankton 
(August 30) per cubic meter. 
4. The mean plankton content in Lake Keokuk averaged 7.25 cm. 3 per cubic 
meter (July). The mean plankton content in the upper part between Burlington 
and Nauvoo averaged 5.28 cm. 3 and that in the lower part, between Nauvoo and the 
dam, 7.7 cm. 3 per cubic meter. 
5. The crustacean population was very scarce in the lower part of the river, 
where the number did not exceed 60 per cubic meter, and was richer in the upper 
part, varying there from 1,000 to 46,000 per cubic meter. 
6. The mean number of Copepoda in Lake Pepin averaged 25,800 and in 
Lake Keokuk 5,400 per cubic meter. The mean number of Cladocera in Lake 
Pepin averaged 1,020, and in Lake Keokuk 2,720 per cubic meter. 
7. In Lake Pepin the Copepoda were more numerous in the lower part and 
the Cladocera in the upper part. 
