KEOKUK DAM 
123 
Among the small floating animals and plants (the plankton) the minute Crusta- 
cea of many species, grouped together as Entomostraca, are known to constitute an 
important item and are, perhaps, the principal item in the diet of very young striped 
bass, sunfishes, black basses, crappies, gizzard shad, minnows, suckers, buffalo fishes, 
and catfishes; among full-grown fishes they are especially important to the paddlefish, 
crappies, minnows, quillbacks, and buffalo fishes. (Forbes, 1888, pp. 487, 496.) As the 
minnows and gizzard shad constitute, in their turn, the chief food of game fishes and 
others, it is apparent that these tiny animals are very important to the fishery. 
Doolittle found, in July, 1914, that the entomostracan plankton in the river and lake 
proper increased in numbers with approach to the dam in notable degree, the number 
of these small food elements rising from about 50 per cubic yard at the upper end of 
the lake to 150 at Nauvoo and 1,500 at the extreme lower end of the lake. Later in 
the season he observed a similar and even more striking contrast between the food 
supplies under river and lake conditions, respectively, the number of Entomostraca 
per cubic yard rising from 35 at Oquawka (in the river above the lake) and 100 at 
Burlington (in the extreme upper end of the lake) to 1,500 at Nauvoo and 25,000 at 
Keokuk. The increase was attributed to more favorable conditions accompanying 
the slackening of the current, such as the partial loss of sediment in suspension and 
the greater time allowed for multiplication of the Entomostraca. A similar differ- 
ence between the amount of life in rivers and quiet waters occurs in and near the 
Illinois River. (Kofoid, 1903.) The investigation of this river was very thorough and 
extended over several years; wide variations were found in different localities and in 
different months, but the general result showed the quiet waters to be about three and 
one-half t im es as rich as the channel; and Kofoid concluded (p. 545) that the small 
organisms of the river have their source, to a large degree, in impounded backwaters 
and are maintained to a considerable extent by the run-off of the latter. The bearing 
of this on the situation in Keokuk Lake is obvious and is indicated further by 
Doolittle’s observations in the waters tributary to this lake (p. 119). 
The observations and conclusions of Galtsoff (1924) are most pertinent: 
When the water becomes stagnant, or at least flows slowly, the plankton crustaceans grow more 
numerous. This has been observed in both Lake Pepin and Lake Keokuk. The increase of Cope- 
poda and Cladocera is especially noticeable in the backwaters of Lake Keokuk, where the crusta- 
cean population progressively increases from the upper part of the lake to the dam (p. 414). 
The productive capacity of such river-lakes as Lake Keokuk is lessened by the instability of the 
hydrographic conditions. Nevertheless, the increase of plankton in Lake Keokuk during low stages 
of water indicates the increase of its general productive capacity (p. 419). 
Until the point has been determined for Keokuk Lake by a sufficiently extended 
investigation, it is perhaps worth while to consider some of the reasons for expecting 
the lake to have a richer food content that the ordinary channel of the Mississippi. 
In the first place, it should be kept in mind that the mere motion of the water, by 
itself, is almost certainly of no importance to small floating animals. Terrestrial man 
thinks of moving water with reference to fixed bottom or shore. However, the move- 
ment of the medium in which they five is of great importance to aquatic creatures as 
the movement of the earth in space is to man, not because of the motion itself but be- 
cause it changes the relation to other things. In other words, the motion of the water 
is significant to plankton organisms only because it is relative — it tends to change the 
position of the animal with reference to bottom, shores, atmosphere, sunlight, sources 
of supply, injurious objects, the sea, etc. A suggestive paper on this point has been 
published by Lyon (1904). 
