148 PLANKTON OF WISCONSIN LAEJBS 



per hectare (73 pounds to 558 pounds per acre). For the entire lake 

 the amount varies from about 39 kilograms to 300 kilograms per hectare 

 (35 pounds to 268 pounds per acre), with an average of 161 kilograms 

 per hectare of surface, or about 144 pounds per acre. 



The mean quantity of dry organic matter in the nannoplankton is 

 largest for the deep water area in Lake Monona, namely 494 kilograms 

 per hectare (441 pounds per acre) ; Lake Mendota is second with 359 

 kilograms per hectare (320 pounds per acre), while Lake Waubesa 

 comes last with 336 kilograms per hectare (300 pounds per acre). Com- 

 puted on the basis of the entire body of water, the mean quantities are 

 the same for Mendota and Monona, namely, 198 kilograms per hectare 

 (177 pounds per acre), while the mean quantity for Waubesa is 161 

 kilograms per hectare (144 pounds per acre). Thus the mean quantity 

 per unit of surface in Lake Waubesa is less than 20.0 per cent below 

 that of the other two lakes in spite of the fact that it is a much shal- 

 lower body of water. 



The nannoplankton samples taken on Lake Mendota during the same 

 intervals of time as those obtained from Lake Waubesa give a smaller 

 mean quantity of organic matter than the complete series of catches 

 from the former lake. That is 38 nannoplankton samples secured on 

 Lake Mendota between August 10 and October 31, 1915, and between 

 May 22 and November 1, 1916, give an average of 1,543 milligrams of 

 dry organic matter per cubic meter of water as compared with 1,630 

 milligrams in the complete series of 87 samples. For the deep part of 

 Lake Mendota this means a decrease to 340 kilograms per hectare (303 

 pounds per acre), and for the entire lake a decrease to 87 kilograms per 

 hectare (167 pounds per acre). On the seasonal basis, then, the area 

 within the 10 meter contour line in Lake Waubesa maintains almost 

 as large a standing crop of nannoplankton as the area bounded by the 

 20 meter contour in Lake Mendota. When the entire lake is taken into 

 account on this seasonal basis, the average quantity of dry organic mat- 

 ter in the standing crop of nannoplankton in Lake Mendota is consider- 

 ably smaller during the two periods of time indicated above than in 

 the complete series, but this amount is still about 16.0 per cent larger 

 than the average for Lake Waubesa. 



Total Plankton 



Lake Mendota. The average yield of dry organic matter in the 

 total plankton (net plankton plus nannoplankton) in the series of sam- 

 ples from Lake Mendota is 1,974 milligrams per cubic meter of w^ater. 

 (Table 25.) The various chemical analyses show that nitrogen consti- 

 tutes an average of 7.11 per cent of this organic matter (equivalent 

 to 44.49 per cent of crude protein), the ether extract 7.53 per cent, the 



