78 



ECOLOGY 



Part II 



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BLUE-GREEN ALGAE 



DESMIDS 



DIATOMS 



Arcella 



Ceratium 



PROTOZOANS 



Cyclops 



ROTIFER 



young stage 

 (Nauplius) 



Bosmina 



CRUSTACEANS 



Fig. 5.12. Important groups in fresh-water plankton. Blue-green algae, common 

 in lakes especially in hot weather, sometimes turn color and create "red water"; 

 green algae (desmids) and diatoms, present the year round with spring and other 

 upswings of abundance; protozoans, few; rotifers, many; crustaceans, abundant, 

 creating the basic fish food. 



respiration. They return it to the atmosphere in combination with carbon as 

 carbon dioxide and with hydrogen as water. In addition green plants release 

 oxygen in photosynthesis. In an aquarium properly arranged for plants and 

 animals, the output of carbon dioxide from respiration and of oxygen from 

 photosynthesis is balanced. 



Nitrogen Cycle. The great reservoir of nitrogen in the atmosphere (78.03 

 per cent of volume) is an inactive associate of oxygen and carbon dioxide. 

 The nitrogen dissolved in bodies of water comes mainly from the atmosphere. 

 Its cycle is more complex than that of carbon because living organisms do not 

 release nitrogen in a form that green plants can use. It is released from animals 

 as nitrogenous waste such as urea (CO(NH2)2) and from decaying tissues 

 after death (Fig. 5.15). Saprophytic bacteria attack these and produce 

 ammonia. Other bacteria feed upon the ammonia, combine oxygen with it, 

 derive energy from the oxidation, and produce nitrites (NOo) — upon which 

 they feed. Still other bacteria (Nitrobacter) attack the nitrites and, through 

 anaerobic (without free oxygen) respiration, derive energy from them and 



