404 
BULLETIN" OF THE BUREAU OF FISHERIES. 
Table 25. — Plankton of the Mississippi River at Fairport, Iowa, in July and August. 
[•, very abundant; O, abundant; ©, frequent; ©, scarce; O, very scarce; . . , absent.] 
JULY. 
Main 
channel. 
Anda- 
lusia 
Slough. 
Main 
channel. 
6 
4 
Species — Continued . 
Brachionus backeri v. cluniorbicu- 
8 
o 
© 
laris S 
Brachionus pala v. amphiceros E 
Anuraea cochlearis typ. G 
cochlearis forma tecta G 
;.<>0 - 
8 
8 
8 
8 
Glochidium sp 
o 
Mayfly larvae 
Mosquito larvae 
Detritus 
8 
o 
• 
6 
Sand 
Anda- 
lusia 
Slough. 
Volume, cubic centimeters per cubic 
meter of water 
Species; 
Melosira crenulata K. 
Cyclotella menenghiniana B . 
Stephanodiscus niagarae E... 
Pediastrum duplex M 
Actinastrum hantzchii L. 
Difflugia pyriformis P 
Arcella sp 
Main 
chan- 
nel. 
Main 
channel 
just 
below 
dike. 
Anda- 
lusia 
Slough. 
Main 
chan- 
nel. 
Main 
channel 
just 
below 
dike. 
Anda- 
lusia 
Slough 
Volume, cubic centimeters per cubic 
meter of water 
Species: 
Clathrocystis aeruginosa H 
Microcystis sp 
Oscillaria sp 
Anabaena spiroides K 
Melosira crenulata K 
Synedra delicatissima W. S 
Eragilaria crotenensis K 
Stephanodiscus niagarae E 
Cyclotella menenghiniana B 
Pediastrum duplex M 
simplex R 
Scenedesmus quadricauda B 
Actinastrum hantzchii L 
Staurastrum gracile R 
Peridinium sp 
4.3 
O 
3.3 
o 
6.6 
o 
Species— Continued . 
Phacus longicaudus E 
Platydorina caudata K. . . 
Plaedorina illinoisensis K. 
Difflugia pyriformis P 
corona W 
Brachionus angularis C 
Polyarthra platyptera W. . . 
Conochilus unicornis R 
Rattulus sp 
Anuraea cochlearis typ. G... 
cochlearis forma tecta G 
Moina rectirostris L 
Nauplii 
Detritus 
Sand 
It can be seen from the table that the plankton in Andalusia Slough in August 
was richer qualitatively as well as quantitatively than that in the main channel. 
Besides that, the samples taken in the almost stagnant water just below the dikes 
in the main channel were the poorest, containing even less plankton than in mid 
river. The composition and the average amount of plankton at the other points 
in the lower section of the Mississippi is almost the same as at Fairport. The 
samples taken in the main stream consist mainly of detritus and contain very few 
organisms. 
A considerable increase in plankton population was observed only in Sturgeon 
Bay near New Boston, 111. Sturgeon Bay is a narrow and shallow bayou extending 
about 7 miles northward from the river. The observations were made only about 
half a mile above the mouth of the bay, because the water was only 18 inches deep 
and it was impossible to go farther. The water was stagnant, but a very slight 
drift of the plankton, caused by the wind, was observed. The surface of the water 
was covered with large groups of water beetles and with empty skins of mayflies, 
the larvae of which were very numerous near the bottom. The amount of plankton 
in Sturgeon Bay was only 5 cm. 3 per cubic meter, which is 1.75 cm. 3 less than in the 
adjacent part of the river, but the analysis of plankton (see Table 26) shows that it 
was composed of Flagellata, Rotifera, and other plankton organisms, whereas in 
the river it consisted mainly of detritus. 
