Aquatic Environment 257 



chops below the critical tension for the species concerned. The 

 critical oxygen tensions for a large number of animals, a further dis- 

 cussion of adaptations to low oxygen supply, and a consideration of 

 physiological relations to the oxygen factor in general is to be found 

 in Prosser (1950, Ch. 8). 



It is difficult to determine the minimum concentrations of oxygen 

 required by various aquatic animals. The value is influenced by 

 temperature, pH, and other modifying factors, as well as by the 

 degree and rate of acclimatization to low oxygen tensions. In the 

 hypolimnion of lakes, where oxygen is likely to be depleted, the tem- 

 perature is low, with consequent reduction in the animal's metabolism 

 and in its need for oxygen. Organisms require more oxygen when 

 in an active condition than when quiescent; some species enter a 

 dormant condition in which they can withstand varying degrees of 

 oxygen deprivation for varying lengths of time. The ecologist thus 

 recognizes that the animals caught in a water layer containing little 

 or no oxygen are not necessarily able to live permanently under the 

 observed conditions: they may be in a dormant condition for a 

 limited period, or they may have made a temporary excursion into the 

 poorly aerated zone. Nevertheless we have obtained a knowledge of 

 the approximate oxygen requirements of many common aquatic 

 animals, and even among completely aerobic forms great differences 

 in the minimum values tolerated have been revealed. At one extreme 

 are those like the toadfish that seems to be able to survive if any de- 

 tectible amount of oxygen exists in the water. Carp and other fish 

 adapted to live in muddy ponds often show no harmful effects until 

 oxygen has been reduced to 0.3 cc per liter or even to 0.1 cc per liter. 

 At the other extreme are active fish like the mackerel that requires 

 3.6 cc per liter (70 mm Hg) at 24°C (Hall, 1930). 



In those ponds, lakes, and fjords, in which the oxygen concentration 

 of the lower levels is reduced to zero during the summer ( and in some 

 instances also during the winter) the inhabitants must either migrate 

 out of the oxygenless zones or enter upon some sort of anaerobic 

 existence, if they are to survive. Copepods and other types of zoo- 

 plankton are known to leave the hypolimnion of lakes as summer stag- 

 nation begins. Fish and the motile bottom fauna undertake similar 

 seasonal vertical migrations. Many animals and plants with no 

 means of escape die off in large numbers after the exhaustion of the 

 oxygen supply. However, certain species of worms, crustaceans, 

 insect larvae, mollusks, and other invertebrates are known to remain 

 in the bottoms of lakes and to survive for periods of many weeks each 

 year with no detectible oxygen present in the bottom water. A 



