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BULLETIN OF THE BUREAU OF FISHERIES 
the biological economy of the lake the fertilizing elements of the bottom soil. (Pond, 
1905.) The virtual absence of rooted aquatic plants in the lake proper (noted also 
by Doolittle) was doubtless characteristic of the immature phase of the lake. As the 
bottom becomes stabler it is probable that rooted forms will appear. The horn- 
weed is considered a good oxygen producer, but the duckweed is not (Titcomb, 1909, 
pp. 11, 16; Barney and Anson, 1921); and oxygen may be a matter of importance on 
the lake, for there were times in 1916 when portions of the open lake near Montrose 
appeared stagnant. 
The author made observations of plants in the vicinity of Dallas City, 111., and 
Fort Madison, Iowa, in September, 1917. In the lake principally great masses of 
duckweed were observed, Spriodela predominating about Fort Madison, and Lemna 
occurring in great rafts about Dallas City and in Peales Lake, so called. The masses 
of Lemna seemed to originate over submerged woodlands, whence they floated out 
into the open lake. No other true aquatic plant (excluding algae) than Lemna was 
observed in Peales Lake, but in Green Bay, on the Iowa side (fig. 16), there were in 
different places acres of smartweed, Polygonum emersum, cat-tails, Typha latifolia, 
price cut-grass, Homalocenchrus (oryzoides?) , and tall tick-seed sunflower, Bidens 
trichosperma, all characteristic of swampy rather than lake conditions. Of plants 
not growing high above the water, there were observed only floating duckweed, 
Lemna and Spirodela; some pondweed, Potamogeton ( natans ?); and the lotus, 
Nelumho lutea. Within two years after this visit the Green Bay region and adjacent 
territory of shallow water from a point about 5 miles above Fort Madison to the 
Sk unk Kiver had been inclosed with levees and drained. 
The Potamogetons are generally very desirable plants in lakes, but it was 
observed in Green Bay that the duckweed accumulated in close drift among the 
floating leaves of the pondweed, making large floating islands, forming a dense shade, 
preventing the growth of submerged leaves, and, no doubt, when the masses are too 
large, checking the growth of algse and other aquatics in the waters beneath. The 
submerged leaves of the pondweed in such cases were either very small or rotted 
off. The excessively abundant duckweed in the middle portion of Lake Keokuk 
at that time was undoubtedly a pest from the point of view of the welfare of fish. 
The various species of emergent vegetation mentioned above seemed to serve 
useful, purposes, both in that their submerged stems gave attachment for algse (not 
present to excess) and protection for small aquatic animals, and in that their emergent 
stems and leaves attracted quantities of grasshoppers and crickets, many of which 
must have found their way into the stomachs of fish while flying from plant to plant. 
The floating leaves of the lotus served as temporary lodging places for such insects, 
while the stems were bored by the larvae of a moth, probably Bellura gortynides. 
May flies were enormously abundant on the lake in 1916. (Needham, 1920.) 
Scattered observations made during the present investigation showed that these 
are of great importance as fish food, and Forbes (1888, p. 488) found that nearly a 
fifth of all the food consumed by all adult fishes examined by him consisted of “neurop- 
terous” insect larvae, the greater part of them being May flies. Their abundance 
in the lake may be related to a great quantity of decaying land vegetation, for at 
least some species of May flies eat such food. (Needham, 1905, p. 40; Needham, 1908, 
p. 262.) Possibly this particular kind of insect will be less abundant when the old 
terrestrial vegetation becomes exhausted and there develops a more normal aquatic 
environment with living water plants. 
