N— Continued 



neuston— Those organisms living in associa- 

 tion with the surface film layer at the air 

 surface interface. 



neutralism— A symbiotic relationship with 

 no effect on either organism. 



niche— An organism's habitat and func- 

 tional role in the community. 



nitrogen cycle— The numerous biogeochem- 

 ical processes which result in the cycling 

 of nitrogen in the many chemical forms 

 required by various organisms. 



nitrogen fixation— An energy requiring pro- 

 cess whereby nitrogen gas is converted to 

 amino or ammonia nitrogen. 



0-- 



obligate anaerobe — An organism that re- 

 quires the absence of molecular oxygen 

 to survive. See also anaerobe and faculta- 

 tive anaerobe. 



obgo— Prefix referring to or of few. 



oligomictic lake— A lake whose waters are 

 rarely or very slowly mixed. It is ther- 

 mally stable and usually located in the 

 tropics. 



oligotrophic lake— A lake which is low in 

 nutrients and therefore has low produc- 

 tivity. These lakes are generally geologi- 

 cally young. 



omnivore— An animal that eats both animal 

 and plant material. 



organism— Any living individual whether 

 plant or animal. 



osmoregulation— The ability to regulate 

 ionic concentrations in body fluids. 



osmosis— The diffusion of substances, 

 solute or solvent, through a semiperme- 

 able membrance from a region of high 

 concentration to a region of low concen- 

 tration resulting in the tendancy for the 

 concentrations to equilibrate. 



overturn— The mixing of hypolimnion and 

 epilimnion zones in lakes. 



ovigerous— Bearing 

 lobster or crab. 



as in a berried 



oviparous— A form of reproduction where 

 the eggs hatch outside the body. Com- 

 pare with ovoviviparous and viviparous. 



ovoviviparous— A form of reproduction 

 where the eggs are hatched in the 

 mother's body, but derive no nourish- 

 ment directly from her. The young are 

 born alive. Compare with oviparous and 

 viviparous. 



oxygen debt — A temporary phenomenon 

 that occurs in an organism when avail- 

 able oxygen is inadequate for respiratory 

 demand. Partially metabolized products 

 are stored and oxidized when oxygen is 

 available. 



oxygen deficit — The difference between 

 oxygen, concentration actually present 

 and the amount that would theoretically 

 be present at 100 percent saturation for 

 the existing conditions of temperature 

 and pressure. 



oxygen sag — The temporary decrease in 

 oxygen concentration due to reduced 

 photosynthesis or increased oxygen 

 demand. 



13 



