14 U. S. BUREAU OF FISHERIES. 



formed by the construction of a dam across the river between Keokuk, 

 Iowa, and Hamilton, Mo. The investigation consisted in the quanti- 

 tative study of plankton and in the measurement of temperature and 

 transparency of water and velocity of current. Plankton collections 

 were made with a pump, and the volume of plankton was determined 

 by the centrifuge method. The field investigation covered a period 

 of three months, July to September, and observations were made at 

 171 different stations, where 673 plankton samples were collected. 

 The results are as follows: 



Assuming that the observations made at various places and on 

 various dates were representative, the mean content of plankton 

 in the Mississippi River, excluding Lake Pepin and Lake Keokuk, 

 during the period in question was 14.5 cm.^ per cubic meter of water. 



With regard to productivity in plankton, there is a marked differ- 

 ence between the upper part of the river, above Rock Island Rapids, 

 and its lower part below the rapids. Planlrton in the upper part, 

 between Hastings and the head of the rapids, excluding Lake 

 Pepin, averaged in August 21.3 and in September 16.2 cm.^ per 

 cubic meter of water. The corresponding figures for the lower part, 

 between the Rock Island Rapids and the head of Lake Keokuk at 

 Burlington, were 5.16 cm.^ in July and 4.8 cm.^ in September. 



Comparative determinations of the amount of plankton made below 

 and just above the rapids showed that the plankton content above 

 the rapids was at least 2.5 times greater than below the rapids. 

 This was possibly due to the destruction of plankton organisms 

 when passing the rapids. 



The mean plankton content in Lake Pepin was 16.6 cm.^ per cubic 

 meter of water. The lower half of the lake was richer in plankton 

 than its upper half; the mean plankton content in the lower half 

 being 22.1, in the upper 13.3 cm.^ per cubic meter of water. 



The mean plankton content in Lake Keolruk in July was 7.25 cm.' 

 per cubic meter of water. The lower part of the lake here also was 

 richer in plankton than the upper part, the plankton content in the 

 upper part, between Burlington and Nauvoo, averaging 5.28, and 

 that in the lower part, from Nauvoo to the dam, 7.7 cm.^ per cubic 

 meter of water. 



The production of plankton in both lakes. Lake Pepin and Lake 

 Keokuk, is greater than in the adjacent parts of the river. 



The plankton Crustacea are very scarce in the lower part of the 

 river, not exceeding 60 individuals per cubic meter of water; they 

 are more abundant in the upper part, varying there from 1,000 to 

 46,000 individuals per cubic meter of water. 



The mean number of copepods in Lake Pepin was 25,800 and in 

 Lake Keokuk 5,400 individuals per cubic meter of water. The mean 

 number of Cladocera in Lake Pepin was 1,020 and in Lake Keokuk 

 2,720 individuals per cubic meter of water. In Lake Pepin the 

 copepods were more numerous in the lower part of the lake, while 

 Cladocera were more numerous in the upper part. In portions of 

 the river adjacent to these lakes the numbers of individuals of crus- 

 tacean species are insignificant. 



The plankton of the river is subject to great fluctuations, depend- 

 ing upon hydrographical conditions. During the rise of the water 

 the plankton of the river is replaced almost entirely by detritus and 



