142 PLANKTON OF WISCONSIN LAKES 



Multiplying the average quantity 'of organic matter in the 184 samples 

 of net plankton, namely 332.5 milligrams per cubic meter of water, by 

 the above percentage gives an average of 27.8 milligrams of nitrogen 

 per cubic meter of water for the entire series of net catches in Lake 

 Mendota. The other mean percentages and mean quantities were ob- 

 tained in a similar manner. For the crude protein the results for nitro- 

 gen were multiplied by the factor 6.25. 



In the following summary of the results, the fractions of milligrams 

 have been omitted ; they have been used, however, in computing the data 

 into larger units. These fractions of milligrams are shown in table 25 

 and in tables 43 and 48. 



Net Plankton 



Lake Mendota. Table 25 shows that 184 samples of net plankton 

 from Lake Mendota yielded an average of 332 milligrams of dry organic 

 matter per cubic meter of water, while the 84 samples corresponding to 

 the series of nannoplankton samples gave an average of 343 milligrams. 

 In the former series the nitrogen made up 8.37 per cent of the organic 

 matter (equivalent to 52.31 per cent of crude protein), the ether extract 

 11.78 per cent, the pentosans 2.88 per cent, and the crude fiber 6.54 per 

 cent. In other words the crude protein, the ether extract, the pento- 

 sans, and the crude fiber constituted about 73.51 per cent of the 

 average quantity of organic matter. The remaining portion consisted 

 of nitrogen free extract or carbohydrates that are not included in the 

 pentosans and the crude fiber. The percentage of nitrogen in the com- 

 plete series of net catches is substantially the same as in the 84 samples 

 corresponding to the nannoplankton samples, but the percentages of 

 ether extract and of the pentosans are slightly higher and that for 

 crude fiber is somewhat lower. 



In the whole series of net catches from Lake Mendota, the organic 

 matter varied from a minimum of 42 milligrams to a maximum of 1,135 

 milligrams per cubic meter of water. The regular station was located 

 within the area bounded by the 20 meter contour line; when stated in 

 terms of larger units for this area (see p. 98), the amount ranges 

 from about 9 kilograms to 250 kilograms per hectare (8 pounds to 223 

 pounds per acre) for this portion of the lake, with an average of a little 

 more than 72 kilograms per hectare (65 pounds per acre). These 

 figures represent only the dry weight of the organic matter ; the living 

 organic material would weigh approximately ten times as much. For 

 the 84 catches corresponding to the nannoplankton samples the average 

 is somewhat higher, or approximately 76 kilograms per hectare of sur- 

 face (68 pounds per acre). 



Assuming that the net plankton is uniformly distributed over the 

 whole lake, the mean quantity of organic matter (332 milligrams per 



