268 Oxygen and Carbon Dioxide 



by Hasler and Wisby (1951) and Hasler (1954) that olfactory re- 

 sponses involving odor memory may play a part in the orientation of 

 fish. 



Hydrogen Ion Concentration 



To turn to a more specific consideration of the ecological influence 

 of pH in the free natural waters and in the interstitial water of muds 

 and soils, the hydrogen ion concentration is affected not only by the 

 reactions of carbon dioxide discussed above but also by other solutes 

 present, both organic and inorganic. Since any alteration of the pH 

 of natural waters is accompanied by changes in other physicochem- 

 ical aspects of the medium, the ecologist must constantly be on guard 

 against assuming that the easily measured pH exerts the controlling 

 influence before determining what effect related changes in the 

 equilibria of the water may have. Edmondson (1945), for example, 

 showed that certain sessile Rotatoria are very likely excluded from 

 lakes by high bicarbonate concentrations, but not necessarily by high 

 pH. 



Many early investigators believed that pH would prove to be an 

 ecological factor of major importance in controlling the activities and 

 distribution of aquatic plants and animals. Some later workers, go- 

 ing to the opposite extreme, suggested that the pH of the environment 

 has little or no importance. Present information indicates that the 

 effect of pH as a factor is real, but limited and highly variable in its 

 influence from group to group. As would be expected, pH is gen- 

 erally of minor significance in the ocean because of its relatively con- 

 stant value in the highly buffered sea water. 



In bodies of fresh water the usual hydrogen ion concentrations en- 

 countered toward the middle of the pH scale seem to have little dif- 

 ferential effect on the majority of the inhabitants. The distribution 

 of many aquatic organisms is unrelated to pH over a wide range, and, 

 when tested in the laboratory, these species often exhibit a tolerance 

 to pH values well above and below those occurring in their natural 

 habitats. Speckled trout (Salvelinus fontinalis) are found naturally 

 in waters ranging from pH 4.1 to 8.5 and were subjected to values 

 as low as pH 3.3 and as high as 10.7 without apparent harm. The 

 rotifer, Monostyla btdla, is reported to tolerate a range from pH 3.7 

 to 9.1; the water moss Fontinalis dalecarlica can withstand pH values 

 from 3.0 to 10.5. On the other hand, some plants and animals thrive 

 best imder acid, others only under alkaline conditions, and others 

 seem to require a nearly neutral medium. A species of Euglcna is 

 found in water of pH 1.8 draining from mines, and certain bacteria 



