^4 PLANKTON OF WISCONSIN LAKES 



sidered here. Through computations based on data obtained by him- 

 self and by others, Piitter^ became convinced that there is not enough 

 organized food present in most bodies of water to support the animal 

 population ; he concluded, therefore, that the animals are able to make 

 use of the organic compounds that are held in solution by the water, 

 as well as of the organized food. Piitter thought he obtained con- 

 firmatory evidence for his theorj^ in a series of experiments on several 

 kinds of aquatic animals, including plankton Crustacea and fishes; in 

 spite of these results, however, his theory is not generally accepted by 

 other investigators. 



The quantitative results obtained on Lake Mendota show that there 

 was always an abundance of food for the rotifers and Crustacea and 

 these two groups of animals are the chief consumers of the other plank- 

 ton forms. Computations based upon numerical data and upon the aver- 

 age weight of the different kinds of rotifers and Crustacea indicate that 

 from twelve to eighteen times their own weight of available food was 

 present in the plankton of Lake Mendota at the time of this investiga- 

 tion ; it must be concluded, therefore, that these two groups of animals 

 always had an ample supply of organized food. Piitter 's conclusion 

 regarding the lack of food for plankton Crustacea was based upon data 

 obtained from net catches only ; all of the nannoplankton, however, may 

 be used for food by the Crustacea and the quantity of nannoplankton 

 in Lake Mendota was several times as large as the available crustacean 

 food in the net plankton. 



The results for crude protein (N X 6.25) are summarized in table 

 19. The largest percentage of crude protein was found in a sample col- 

 lected in 1917, while the maximum, minimum, and mean percentages 

 were lower in 1916 than in the other two years. On the other hand, the 

 largest quantity of crude protein per cubic meter of water was found in 

 1916, namely, 1,560.0 milligrams; this was more than 43.0 per cent 

 higher than the maximum amount of 1915. The largest minimum 

 amount was found in 1915 and the smallest in 1917, the latter being just 

 a little more than 75.0 per cent of the former. The maximum quantity 

 of crude protein in 1915 was less than three times as large as the mini- 

 mum of that year, but in 1916 there was a fivefold difference, with more 

 than a fourfold difference in 1917. The mean quantity was smallest in 

 1916 and largest in 1917, the former amounting to about 77.0 per cent 

 of the latter. The average amount of crude protein in the nannoplank- 

 ton was three times as large as that in the net plankton in 1916, three 

 and a half times as large in 1915, and a little more than five times as 

 large in the 1917 samples. (Compare tables 8 and 19.) 



''Die Ernahrung der Wassertiere, p. 168. Jena, 1909. 



