266 Oxygen and Carbon Dioxide 



hard fresh water contain a large reserve supply of carbon dioxide as 

 bicarbonates and carbonates. Plants living in water of high pH must 

 be able to get along by absorbing the tiny amount of free carbon 

 dioxide present (which is then immediately replaced by dissociation) 

 or else they must be able to use the carbon dioxide existing in com- 

 bined form. Many pond plants have been shown to absorb the 

 HCO3 ion and to use it in their photosynthesis (Steemann Nielsen, 

 1952). Although we do not know whether most marine plants rely 

 primarily on free or on combined carbon dioxide we find no evidence 

 that the lack of carbon dioxide ever acts as a limiting factor for plant 

 growth in the sea. 



In soft-water lakes, in which the total supply of carbon dioxide is 

 small, extensive photosynthesis may reduce the concentration of this 

 material to such an extent that further growth of the plants is pre- 

 vented. Water is able to absorb carbon dioxide from the atmosphere 

 only at a slow rate; the supply from ground water and from respira- 

 tion and decomposition may be inadequate to meet the demand of 

 the green plants. The withdrawal of carbon dioxide from the water 

 also causes a concomitant rise in pH. If the change in hydrogen ion 

 concentration is considerable, difficulty is frequently encountered in 

 ascertaining to what extent the limitation of photosynthesis is due to 

 lack of carbon dioxide and to what extent to unfavorable pH values. 

 The growth of populations of planktonic algae in chemically fertilized 

 fish ponds is believed sometimes to be cintailed by the exhaustion of 

 the available carbon dioxide. Lime is commonly added to correct 

 this situation. Improved growth of rooted vegetation in other soft- 

 water ponds has been produced as a result of liming. 



The abundance of carbon dioxide also exerts certain specific efi^ects 

 upon the animals of the aquatic environment. The rates of some 

 developmental and metabolic processes are increased at higher con- 

 centrations of carbon dioxide and are decreased at lower values, but 

 other animal reactions are inhibited by high carbon dioxide concen- 

 trations, so that no general statement of its effect can be made. In 

 vertebrates, and in some arthropods and mollusks the rate of respira- 

 tory movements is definitely raised by increase in carbon dioxide 

 tension, but in others this factor has little or no effect (Sheer, 1948). 



The concentration of carbon dioxide in the water influences its 

 equilibrium in the blood of aquatic animals. An increase of the 

 carbon dioxide in the blood causes a decrease in the oxygen affinity 

 of vertebrate hemoglobins and of some invertebrate pigments (e.g., 

 hemocyanin). In aquatic habitats with high carbon dioxide tensions 

 animals with the type of blood strongly affected by carbon dioxide 



