406 
BULLETIN OF THE BUREAU OF FISHERIES. 
cubic meter. This increase in the total amount of plankton was due to the great 
abundance of diatoms ( Melosira crenulata K. ; Fragilaria crotonensis K.) , blue-green 
algae ( Anabsena spiroides Kl.), and copepods. The Rotifera were scarce and were 
represented by Polyarthra euryptera W., Anursea coddearis G., and Brachionus 
angularis G. The Cladocera were represented by Daphnia retrocurva F., which 
formed almost 99 per cent of the Cladocera population; other forms, such as Moina 
rectirostris L. and Dunhevedia setigera B., were also occasionally noticed in some of 
the samples. 
The plankton collected among the water lilies, which grew here in profusion 
along the banks of the river, had almost the same composition as in the main 
channel except that the Cladocera were represented by Moina rectirostris L., which 
was found here in hundreds of individuals per cubic meter, while Daphnia retrocurva 
L. was very scarce. As to the Rotifera, some specimens of Asplanchna brightwelli G 
have been found, but this organism was not noted in the stream. 
In the section from Prairie du Chien to Reads Landing, a distance of 130 miles, 
the river plankton is very uniform and has a great resemblance to that in Lake 
Pepin. The observations made here in August and September showed a considera- 
ble increase in blue-green algae during the latter month, when Aphanizomenon flos- 
aquse R., Anabsena spiroides KL, A. Jlos-aqux Breb., A. circinalis Rab., Clathrocystis 
aeruginosa K., and Microcystis made up a greater part of the plankton. Leptodora 
kindtii L., which is very common in Lake Pepin, was also found in this part of the 
river. The last point below Lake Pepin where this big cladoceran was observed 
is a little below Prairie du Chien opposite the mouth of the Wisconsin River. It 
did not occur farther downstream. Other Cladocera were represented mainly by 
Daphnia retrocurva F., found in nearly all the samples, and by occasional D. arcuata 
F., D. pulex F., Diaphanosoma leuchtenbergianum F., and Sida crystallina O. F. M. 
There was a gradual decline in the number of Daphnia retrocurva from the foot of 
Lake Pepin to Prairie du Chien, and above the mouth of the Wisconsin River it 
disappeared entirely. 
The composition of the plankton of the Mississippi River above Lake Pepin is, 
in general, the same as below the lake except that Ceratium hirundinella Sch. was 
more abundant above the lake and that Asterionella gracillima H. was occasionally 
found at Hastings and Prescott. The Cladocera are less numerous than below the 
lake and are represented by the same species as in Lake Pepin: Diaphanosoma 
leuchtenbergianum F., Sida crystallina O. F. M., Daphnia retrocurva F., D. arcuata 
F., D. pulex v. pulicaria F., Simocephalus vetulus O. F. M., and Leptodora kindtii L. 
Daphnia retrocurva is the most abundant form among the water fleas collected in 
this part of the river. Leptodora kindtii, found in some samples, was represented 
by both the young and the adult organisms, while in the part below Lake Pepin 
only adult Leptodora occurred. 
LAKES. 
The distribution and the composition of the plankton of Lake Pepin has been 
discussed above. Only a few facts can be added now concerning the distribution 
of different plankton forms in the lake. A slight difference can be noticed in the 
composition of plankton taken at different points of the pelagic part of the lake. 
It was observed that Fragilaria crotonensis was most abundant in the upper part 
