THE ATMOSPHERIC GASES 



193 



inorganic material. The amount of oxygen 

 present in a lake may be significantly di- 

 luted by the inflow of ground water, which 

 is typically deficient in dissolved oxygen 

 and usually carries a heavy load of nitrogen 

 and of carbon dioxide. Much of the hypo- 

 Umnial deficit in lakes is a result of the 

 oxidation of bottom deposits (Alsterberg, 

 1927), though bacterial oxidation of me- 

 thane and hydrogen may help account for 

 the oxygen deficit in some lakes (RossoHmo 

 and Kusnezowa, 1934). 



have large amounts of hydiogen sulfide 

 and are devoid of animal life. 



STRATIFICATION OF DISSOLVED GASES 

 IN LAKEo 



The presence of gradients of concentra- 

 tion of dissolved gases, especially of oxygen, 

 has been indicated in the pages just pre- 

 ceding. The emphasis to be placed upon 

 stratification in the analysis of com- 

 munities (Chapter 26) requires a brief 

 summary of this phase of environmental 



Fig. 42. Depth profiles of dissolved oxygen in sea water, ( Redrawn from Schmidt. ) 



Water above mats of algae or dense 

 growths of such plants as Cham or Elodea 

 is frequently supersaturated with oxygen 

 during the daylight hours and yet supports 

 a reduced population of animals. Experi- 

 ments show that, as compared with appro- 

 priate control groups, mosquito larvae have 

 a high rate of mortality when reared in 

 aquaria containing dense growth of these 

 water plants (Matheson, 1930). The toxic- 

 ity may be a result of the presence of 

 nascent oxygen; at least nascent oxygen 

 when it is released from ozone, is toxic to 

 fishes in concentrations of only one part in 

 six to twelve and a half, and even to 33 

 million parts of water (Hubbs, 1930). 



The lack of sufiicient dissolved oxygen 

 frequently becomes a limiting factor in the 

 distribution of aquatic organisms. The mass 

 of water in the Black Sea below 200 meters 

 or at the bottom of deep, narrow fjords, 

 contains no dissolved oxygen; such waters 



stratification. Dissolved gases, inorganic 

 compounds of many kinds, innumerable or- 

 ganic substances, and suspended particles 

 are all directly and indirectly involved in 

 the vertical environmental gradients of 

 lakes. All residents must adjust to or tolerate 

 these gradients that, in turn, are affected 

 by the processes and by-products of metab- 

 olism as well as by decomposing dead bod- 

 ies. It is not surprising that, despite much 

 limnological and oceanographic research, a 

 great deal remains unaccomplished in this 

 broad, complex field. 



Dissolved oxygen and carbon dioxide are 

 important in lake biology. As we have al- 

 ready seen (p. 94), in lakes with a sum- 

 mer thermocline the oxygen and carbon 

 dioxide content is largely equalized during 

 the vernal overturn. With the appearance 

 of the thermocUne, the supply of oxygen in 

 the hypolimnion begins to diminish and 

 may disappear from this stratum, although 



