LIMNETIC COMMUNITY 



75 



ML 





x> 



..^•' 



■«,->: 



mm? 



'•'i.-M-sff" 



12 



city water in a day in August, all together they would be consuming 

 about 10 quarts of solid plant and animal substance — enough to make 

 a meal for about forty people. 



One does not think of the lake as an area of luxuriant vegetation, 

 teeming with animal life, but rather as a barren waste of water. How- 

 ever, if one's vision for small objects were only better, he would see as 

 he passes over the water in a boat, thousands of small animals and plants 

 such as are shown in Figs. 12-18 together with about fifty other forms of 

 protozoa, wheel animal- 

 cules, crustaceans, insects, 

 and small fish. Most of 

 these spend their entire 

 existence freely floating or 

 freely swimming. With 

 the exception of the fish 

 and insects they consti- 

 tute the plankton which is 

 the basis of the food of the 

 millions of pounds of fish 

 taken from Lake Michigan 

 every year. 



From the standpoint 

 of our economic interests, 

 the limnetic formation 

 is of great importance. 

 It deserves comment also 

 because of its scientific 

 interest, and the aes- 

 thetic value of the vari- 

 ous forms of which it is 

 composed. 



a) Its composition (85, 86, 87, 88, 89). — The minutest animals of 

 this formation are the protozoa. About thirteen species have been found 

 to inhabit the open waters of the lake. Of these the sun animalcule 

 {Actinophrys sol) (Fig. 12) and the shelled protozoan {Difflugia globu- 

 losa) (Fig. 14) are easiest to recognize. Nine of the thirteen common 

 species are mixotrophic in their nutrition (i.e., contain chlorophyll and 

 manufacture their own food) (Fig. 13) and share with the algae and 

 diatoms the important function of furnishing food for the rotifers (wheel 

 animalcules) and the crustaceans. 



Fig. 12. — A sun animalcule (Actinophrys sol 

 Ehrbg.); 330 times natural size (after Leidy). 



Fig. 13. — Protozoan (Peridinium tabulatum 

 Ehrbg.); 400 times natural size (after Kent). 



Fig. 14. — A shelled protozoan (Difflugia glohu- 

 losa Duj.); 130 times natural size (after Leidy). 



