461 
6. THe PLANKTON oF THOMPSON Lake, 1909—1910 
As shown by volumes per cubic meter of silk plankton, based on 
vertical samples, the middle of Thompson Lake and the river channel 
at Havana averaged nearly the same in plankton content during the four 
months March—June, 1910 (Thompson, 12.77 c.c.; river, 13.98 c.c.) ; 
while in the July—November period of five months Thompson Lake 
(5.21 c.c.) averaged about six times as rich as the river (0.87 c.c.). 
Expressed in terms of pounds per acre to a depth of one meter, these 
volumes of plankton amount to around 114 Ibs. for the lake and 124 lbs. 
for the river in the March—June period, and to 46 Ibs. for the lake 
and 8 lbs. for the river in July—November. If these quantities of silk 
plankton are multiplied, both in the case of the river and the lake, by the 
Kofoid silk-filter-paper ratios for river samples, 1897—1899, the pound- 
ages for March—June would be about twice those just given, and 
for July—November about four times those figures. On the other 
hand, if depth is taken into account, and total amounts of plankton 
standing over an acre in the river and the lake to the full average depth 
at the collecting station are figured, the river acre in March—June will be 
found to have more plankton standing over it at a given time than an equal 
area in the central portion of Thompson Lake; and in July—November, 
about half as much, instead of only one sixth as much, as an acre in the 
lake. 
PLANKTON THOMPSON LAKE AND ILLINOIS River, HAVANA, 9 MONTHS, 
MarcH—Novemser, 1909—1910 
_ Silk 
Gage, Depth Depth Silk plankton 
Havana |coll.sta.| coll. sta. | plankton | lbs. per acre 
average | feet, av. | meters, av. | c.c. per m* to depth 
1 meter 
March—June incl. 
Thompson L. 12.04 12.23 BY 12.79 114 
Illinois River 12.04 22.23 6.7 13.98 124 
July—November incl. 
Thompson L. 7.81 8.00- 2.4 5.21 | 46 
Illinois River 7.81 | 18.00 5.4 0.87 »| 8 
