3 o8 



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. 



I 



11 



Fig. 184. Diagram illustrating midsummer distribution of 



seven important synthetic organisms in the first one hundred 



feet of depth of Cayuga Lake. &, Ceratium; g, Dinobryon; 



C-Mallomonas; D, Anabaena; ^ Microcystis (Clathrocystis); 



^JB- Asterionella; ^, 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 



