TOTAL PLANKTON OF LAKE MEN DOT A 97 



Mean Quantity and Chemical Composition of Total Plankton 



Table 25 gives a general summary of the results obtained for all of 

 the samples of net plankton and of nannoplankton from Lake Mendota, 

 as well as those from Lakes Monona, Waubesa, and Kegonsa, which 

 are discussed in subsequent chapters. The results obtained by com- 

 bining the net plankton and the nannoplankton into what has been 

 called the total plankton are also indicated in this table. 



The average amount of organic matter in the 184 samples of net 

 plankton from Lake Mendota secured between 1911 and 1917 was 332.5 

 milligrams per cubic meter of water ; in the various samples the quan- 

 tity ranged from a minimum of 42.0 milligrams to a maximum of 

 1,135.0 milligrams per cubic meter of water. The average of 332.5 

 milligrams of organic matter contained 28.7 milligrams of nitrogen 

 which was equivalent to about 180.0 milligrams of crude protein, 39.8 

 milligrams of ether extract, 11.4 milligrams of pentosans, and 22.8 

 milligrams of crude fiber. These four items account for 76.2 per cent 

 of the organic matter. Since some of the pentosans may be derived 

 from carbohydrates in the crude fiber and thus duplicate a certain 

 amount of the latter, the pentosans may be omitted from this computa- 

 tion. The crude protein, ether extract, and crude fiber together give 

 an average of 242.6 milligrams per cubic meter of water which is ap- 

 proximately 73.0 per cent of the organic matter; this leaves 27.0 per 

 cent of the average quantity of the organic matter in the net plankton 

 as nitrogen free extract. 



For direct comparison with the nannoplankton samples, a summary 

 of the corresponding samples of net plankton is included in the table. 

 In making the computations for the total plankton, only the 84 samples 

 of net plankton which cover the same period of time as the nannoplank- 

 ton observations have been used, and not those of the entire series of 

 net catches, namely, 184 samples. The average amount of organic 

 matter in the 84 samples of net plankton collected between 1915 and 

 1917 is somewhat larger than that of the entire series of net catches, 

 or 343.5 milligrams per cubic meter of water as compared with 332.5 

 milligrams; this represents a difference of only 11.0 milligrams, or 

 slightly more than 3 per cent. The nitrogen, ether extract, pentosans, 

 and crude fiber were slightly larger in the average of the 84 samples 

 than in the whole series, the difference ranging from a fraction of a 

 milligram to 2 milligrams per cubic meter of water. The crude protein, 

 ether extract, and crude fiber constituted 70.1 per cent of the organic 

 matter of these samples of net plankton ; this is about 3 per cent below 

 the average for the entire series of net catches. Approximately 30.0 

 per cent of the organic matter in these samples, therefore, consisted of 

 nitrogen free extract. 



