PHYSICOCHEMICAL AND CHEMICAL PHASES 



175 



or lack of it, in lakes subject to thermal 

 stratification. Diurnal changes in hydrogen 

 ion concentration sometimes come with dia- 

 matic suddenness (see p. 343); seasonal 

 changes take place more slowly. The limits 

 in certain well-studied Wisconsin lakes (Ju- 

 day et al., 1935) seldom pass tliree whole 

 units. Stated thus, the variation seems 

 small, but this is misleading on the pH or 

 any other logarithmic scale. Actually, as 

 shown in Table 12, a change of three such 

 units indicates a thousandfold variation. 



The conditions just outUned indicate that 

 the hydiogen ion concentration of the water 

 is often correlated with the carbon dioxide 

 content. The relationship is imperfect, and 

 pH alone may reveal Uttle or nothing about 

 the amount of carbon dioxide present, since 

 carbon dioxide and carbonates in general 

 are but one of a number of factors known 

 to afifect environmental pH. Even so, under 

 many conditions it is the carbon dioxide 

 content of water, together with the alkaU 

 reserve, rather than pH, that is of primary 

 importance. This conclusion was reached 

 both on the basis of field and of laboratory 

 studies (Powers, 1939). In the laboratory, 

 the most convincing evidence comes from 

 removing all carbonates by bubbling air 

 through water acidified to pH 4 and then 

 establishing the needed pH by treatment 

 with alkali (Clowes and Smith, 1923; Hy- 

 man, 1925). For the planarian, Dugesia, 

 Hyman found that the acidification of natu- 

 ral waters was followed by a decrease in 

 oxygen consumption in all cases where the 

 pH fell below 7.0 and usually when it was 

 reduced from pH 7.8; this effect disap- 

 peared when pH changes were made in 

 water free from carbonates, except when 

 the acidity was produced by adding carbon 

 dioxide. 



Photosynthesis, with its active removal of 

 carbon dioxide and the giving off of oxygen, 

 helps explain the inverse correlation that 

 often exists between hydrogen ion concen- 

 tration and oxygen content of water, wheth- 

 er salt or fresh. Other conditions work to 

 the same end; thus bottom deposits rich in 

 organic matter contribute acid to the water 

 above them and absorb oxygen. With fishes, 

 and many other animals, the reciprocal dis- 

 tribution of hydrogen ions and oxygen pro- 

 duces two important ecological effects. In 

 the first place, many aquatic organisms are 

 less able to tolerate low oxygen tensions in 

 acid waters, and, in the second place, many 



fishes turn back from water of low pH more 

 readily than they do from water that merely 

 has a low content of dissolved oxygen. They 

 turn away from the combination of low 

 oxygen and high hydrogen ion concentra- 

 tion still more readily. Such reactions have 

 survival value. 



Another type of interaction between pH 

 and other environmental factors is shown by 

 the relation of heat resistance to the hy- 

 drogen ion concentration of the medium. 

 Often this is not a straight-fine relationship. 

 When paramecia are transferred from room 

 temperature to 40° C, death occurs within 

 a few minutes before there has been time 

 for accfimatization. Greater resistance to 

 heat is found at about pH 6.8 and 7.8, and 

 definitely lowered heat resistance occurs at 

 about pH 5.7, 7.2, and 8.3 (Chalkley, 1930; 

 Garner, 1934). Chalkley suggests that the 

 results arise from changes in permeability 

 of the cell membrane induced by the differ- 

 ent hydrogen ion concentrations. 



Environmental pH has neither the real 

 importance it frequently has within the or- 

 ganism, the dupfication of which was ex- 

 pected by enthusiastic ecological students in 

 the 1920's, nor the lack of importance some- 

 times expressed and often impfied later. The 

 large amount of evidence available amply 

 supports the following relatively simple 

 generalizations : 



1. Some organisms tolerate a wide range 

 of pH, others only a narrow range, and 

 still others are intermediate. 



2. Some organisms flourish best in acid, 

 others in alkahne environments, and others 

 find their optimum at or near neutrafity. 



3. The reactions may be to the hydrogen 

 ion concentration directly or to changes pro- 

 duced by or related to changes in pH, as is 

 the carbon dioxide concentration in a car- 

 bonate-rich water or soil. 



4. When effective, the relationship be- 

 tween ecological processes and pH is not 

 necessarily represented by a straight-line 

 graph, but may be much more complex. 



5. Hydrogen ion concentration is no 

 more important than many other environ- 

 mental factors and is usually less significant 

 than water (in its various forms), sunlight, 

 heat, the character of the substratum, and 

 some of the other conditions of existence. 



The conclusion is that the hydrogen ion 

 concentration has real, though limited, value 

 as a factor in the environmental complex of 

 animals. 



