120 
BULLETIN OF THE BUREAU OF FISHERIES 
5. The Entomostracan population of the lake itself passed its maximum by 
late August or very early September, though still showing about 5,000 per cubic 
yard at the latter time. 
6. The river and the lake are enriched by the run-off from tributaries, and the 
river itself below the dam is enriched by the run-off from the lake. The enrichment 
from the tributaries holds both as to numbers and species, but the species distinctive 
of a tributary do not survive long in the main channel. Such local enrichment may 
be 100 per cent. The run-off from Lake Keokuk enriches the river below, so that, 
at a point 5 miles below, the plankton Entomostraca were found to be from thirty 
to fifty times as abundant as in the river above the influence of the dam. This was 
in early August, the season of maximum production of Entomostraca in the lake. 
OTHER PLANKTON ANIMALS 
Zooplankton other than Entomostraca came under observation as the collections 
were studied. Chief among these plankton elements were rotifers, which belonged 
mostly to two genera, Asplanchna and Anuraea. The latter genus was represented 
by several species and varieties, chiefly A. aculeata. There were occasionally present 
A. cochlearis, A. tecta, Polyarthra platyptera, Conochilus volvox, Triarthra longiseta, 
and Pedalion miron. Rotifers were seldom absent from the plankton. Until the 
latter part of the summer they were practically a negligible quantity in the main 
body of water. In the tributaries they were frequently abundant, and in places 
they constituted the whole of the plankton. In July and in most of August 
Asplanchna might be regarded as the more important on account of its size and 
because it was at times the dominant factor of the plankton; later in the season it 
became rare. Anursea aculeata was more consistently present than Asplanchna and, 
while usually occurring in greater numbers, its smaller size prevented its contrib- 
uting much to the bulk of the plankton. As the season progressed other species 
of Anursea were encountered, A. cochlearis and A. tecta more than others, the latter 
occasionally being the dominant rotifer. Still other species or varieties of Anursea 
and other genera, some of them mentioned above, were found in the plankton after 
the dominance of Asplanchna had passed. In the latter part of the season — that is, 
in late August and early September — the lower part of the lake reached a stage of 
maturity as to succession of plankton forms such as was attained much earlier in the 
tributaries; then Asplanchna was found and Anursea aculeata became abundant. 
Few Protozoa were recognized, yet a species of Euglena formed a very dense 
bloom upon Skunk River, Flint Creek, and Prairie Slough. Associated sometimes 
with this Euglena were two species of Ceratium. 
Insect larvae were not found in abundance in the plankton of the lake or of its 
communicating waters. Corethra sp. did enter into the catches once in Lake Keokuk, 
sparsely in Skunk River, Spring Slough, Green Bay, and more abundantly in Sulli- 
vans Slough and Devil Creek. 
PHYTOPLANKTON 
Algae rarely occurred in the plankton in measurable quantities until August. 
Traces of green algae were, possibly, present from the beginning. Blue-green algae 
though more easily detected, were not seen until July 28. The algae, once having 
made their appearance, increased as to frequency and amount till near the end of the 
season of observation in early September. The following general considerations as 
to the occurrence of algae stand out clearly from the detailed record. 
