258 Oxygen and Carbon Dioxide 



further discussion of the types of metaboHsm displayed by inverte- 

 brate animals capable of anaerobiosis is given by von Brand (1946). 



When the supply of oxygen in a confined aquatic habitat becomes 

 reduced below the level of toleration, the lethal effects sometimes 

 appear quite suddenly. We occasionally see reports that all the fish 

 in a pond have been found dead on one morning whereas the day be- 

 fore they had appeared to be in a perfectly healthy condition. Such a 

 situation occurs most frequently after a hot calm spell in summer. 

 Because of the high temperature the amount of oxygen that the water 

 can hold is low and the rates of respiration by the pond organisms and 

 of the decomposition of organic materials in the water are high. In 

 calm weather no effective stirring takes place to replenish the sub- 

 surface water levels. As a result of this combination of circumstances 

 the oxygen minimum reached by the diurnal pulse at the end of each 

 night becomes progressively lower. Early one morning the oxygen 

 concenti-ation drops below the toleration point, and the whole popula- 

 tion of fish in the pond is suffocated. 



In aquatic habitats permanently and completely devoid of oxygen 

 no aerobic organisms can live. Included in this category are: the 

 deep waters of certain lakes, the Black Sea, and certain arms of the 

 ocean with insufficient circulation; the subsurface layers of mud 

 bottoms; and many heavily polluted waters. Such places can be per- 

 manently inhabited only by forms that are able to function indefinitely 

 on an anaerobic basis. The population of these habitats consists 

 chiefly of anaerobic Protozoa and bacteria. Many of these obligate 

 anaerobes are killed by the presence of oxygen. This fact is put to 

 practical use in the employment of thorough aeration as one step in 

 the purification of water supplies. Certain higher animals, such as 

 slime worms (Tubificidae), pea clams (Pisidium), and insect larvae 

 (Corethra and Chironomus), are found in the bottoms of very deep 

 lakes where oxygen is very low or absent. 



The organisms that inhabit the oxygenless water layers carry out a 

 partial decomposition of the organic matter filtering down from above, 

 and frequently release large quantities of hydrogen sulphide in the 

 process. In the Black Sea the permanent absence of oxygen in the 

 deeper water layer brings the vertical distribution of aerobic benthic 

 animals to an end at depths varying between 115 m and 165 m accord- 

 ing to the locality. The plankton is similarly limited to the upper 125 

 m in the central part of the sea but may extend to 175 m in marginal 

 areas where the surface water is stirred more deeply. Lack of oxygen 

 and abundance of poisonous hydrogen sulphide exclude all life except 

 anaerobic microorganisms from the lower 2000 m of the Black Sea. 



