382 
BULLETIN OF THE BUREAU OF FISHERIES. 
these places, and therefore the question of whether the overflowed lands possess a 
greater productive capacity than the adjacent channels with running water remains 
open. 
Fluctuations in hydrographic conditions immediately affect the plankton 
in Lake Keokuk. A rising level causes a sharp decline in plankton content, because 
storm waters mingle with and replace plankton rich backwaters. In September, 
when the river was rising, there was no difference in plankton content of the lower 
and the upper parts of the lake; moreover, no difference at all could be found 
between the river and the lake. All plankton that had been developed during 
the period of stability of hydrographic conditions was washed away with the rise 
of the river. The volume of material suspended in the water at this time varied 
from 2 to 4 cm. 3 and consisted almost exclusively of detritus. (See stations 156-171, 
Table 29, p. 433.) 
LAKE PEPIN. 
The mean volume of plankton in Lake Pepin, computed from the results 
of observations made during the 18-day period (August 18 to September 5), is 16.6 
cm. 3 per cubic meter of water. The figure is the average of 140 samples collected 
at 36 different stations. 
The lower part of the lake, below Lake City, is richer in plankton than the 
upper part. The mean volume of plankton in the lower part averaged 22.1cm. 3 ; 
in the upper part 13.3 cm. 3 If we exclude from the upper part the shallow area 
near Bay City, its mean content of plankton would be 15.7 cm. 3 The increase of 
plankton in the lower part as compared with that in the upper is 8.8 cm. 3 per cubic 
meter of water. Compared with the mean content of plankton in the whole lake 
the amount of plankton is 33.1 per cent greater in the lower part and 19.9 per cent 
less in the upper part. The distribution of plankton in the lake is shown graphically 
on Figure 12, which is, of course, only schematic, because the observations were 
not made simultaneously. 
The possible error due to nonsimultaneousness of observations seems, however, 
not to be great because there were no considerable changes in weather or hydro- 
graphic conditions during the period in question. The results obtained at stations 
72, 75, and 76 on August 24 are very close to those obtained on September 5 at 
station 117, located in the same region. Unfortunately the samples collected 
on September 5 at stations 118 and 119 with the intention of comparing the results 
with those of previous observations made in August were lost before the volume 
of plankton was measured. Only one sample obtained with the plankton net 
at station 118 near Lake City has been saved. The amount of plankton in this 
sample agrees closely with that obtained with the net at station 58 (9.4 cm. 3 of 
plankton per cubic meter observed at station 118, September 5, and 9.5 cm. 3 at 
station 58 August 18). The amount of plankton obtained with a net differs greatly 
from that obtained with a pump. Therefore it is absolutely impossible to compare 
the results obtained by these two different methods, but the results of the net 
method, especially when referred to the same depth, may be compared. The ques- 
tion of the adequacy of the plankton net for quantitative investigation and a 
comparison of the pump and net methods is discussed on page 385. 
