308 



Aquatic Societies 



Each species has its own level ; its own preferred habi- 

 tat, where it finds optimum conditions of pressure, air, 

 temperature and light. Fig. 184 is a diagram of the 

 midsummer distribution in depth of seven important 

 synthetic planctonts of Cayuga Lake. 



B 



D 



E 



Ft. 

 o 



10 

 20 



30 

 40 

 SO 



60 



:o 

 80 

 90 



TOO 



Ft. 

 o 



10 



20 



30 



40 



SO 

 60 



70 



80 



90 



100 



Fig. 184. Diagram illustrating midsummer distribution of 

 seven important synthetic organisms in the first one hundred 

 feet of depth of Cayuga Lake. A, Ceratium; B, Dinobryon; 

 C, Mallomonas; D, Anabaena; E, Microcystis (Clathrocystis) ; 

 7^, Asterionella; G, Fragillaria, 



(Based in part on Juday— 15) 



Light is the principal factor determining distribution 

 in depth. This we have touched upon in Chapter II, 

 under the subject of ''Transparency." It is only in the 

 upper strata of lakes, within the reach of effective light, 

 that green plants can grow. Animals must likewise 

 remain where they can find their food ; whence ^ it 

 results that the bulk of the plancton in a lake lives in its 



