PROGRESS IN BIOLOGICAL INQUIRIES, 1922. 15 



silt. In Lake Keokuk at the rise of the river stage the plankton is 

 washed away and samples taken in this period consist almost ex- 

 clusively of detritus. 



The plankton of the river with regard to its composition is mono- 

 tonic. The plankton of Lake Pepin and Lake Keokuk, as compared 

 with that of the river, may be characterized as richer in Crustacea 

 and Rotifera. No organism was found in the river plankton that 

 would not be present in the lakes. 



The increase of the production of fisheries in Lake Keokuk since 

 the erection of the Keokuk Dam, as reported by the official statistics, 

 can be correlated apparently with increased production of plankton 

 in this lake. 



GREEN LAKE, WIS. 



The bureau has continued to cooperate in a small way with the 

 Wisconsin Geological and Natural History Survey in the investiga- 

 tions of fish-food resources of small lakes. During the past fiscal 

 year a quantitative survey of the fish-food resources of Green Lake, 

 Wis., was conducted under the general direction of President Ed- 

 ward A. Birge, with the cooperation of Chancey Juday and the 

 technical assistance of L. E. Noland and H. W. Rickett. The follow- 

 ing summary of results has been furnished by Mr. Juday. 



PLANKTON. 



The standing crop of net plankton yielded an average of approxi- 

 mately 40 milligrams of dry organic matter per cubic meter of water 

 from April 30 to November 3, 1921; a maximum of 58 milligrams 

 was noted in June, and minima of 21 milligrams were found in 

 October and November, In 1922 a maximum of 30 milligrams of dry 

 organic matter per cubic meter of water was obtained on May 5 and 

 a minimum of 26 milligrams on May 21. Taking the area and volume 

 of the whole lake into account, the average crop of 40 milligrams was 

 equivalent to 13 kilograms of dry organic matter per hectare of sur- 

 face, or 11.8 pounds per acre. 



The standing crop of nannoplankton yielded an average of 742 

 milligrams of dry organic matter per cubic meter of water in 1921; 

 there was a maximum of 1.200 milligrams on April 30 and a mini- 

 mum of 360 milligrams on April 21 and of 1,590 milligrams on May 

 5. For the entire lake the average crop of 742 milligrams per cubic 

 meter in 1921 amounted to 245 kilograms of dry organic material per 

 hectare, or 219 pounds per acre. The average standing crop of net 

 plankton and nannoplankton combined was 258 kilograms per hec- 

 tare, or about 231 pounds per acre, in 1921. The live weight of this 

 material would be fully 10 times as large as the dry weight. 



BOTTOM FAUNA. 



In the quantitative study of the bottom fauna 209 samples were 

 obtained, and the macroscopic organisms therein were enumerated; 

 also the average weight of the various forms was ascertained, so that 

 the results can be expressed in gravimetric as well as numerical units. 



