174 



ANALYSIS OF THE ENVIRONMENT 



pH 7,9. They grow in concentrations from 

 pH 2.1 to 7.7, inclusive (von Dach, 1943). 

 Tapewoiins adjust to changes in hydrogen 

 ion concentration from pH 4 to 11, the 

 range to which they are normally exposed; 

 their optimum is at pH 10. Other animals 

 are much more limited; the ciliate, Stentor 

 coeriileus, is reported to be adjusted to 

 only pH 7.7 to 8.0. (Hetherington, 1932), 

 and Spirostomum amhiguum is apparently 

 limited in nature by pH above 7.8 (Hutch- 

 inson, 1941a). 



Some striking correlations between pH 

 and community distribution have been re- 

 ported. In one instance, in southern Canada, 

 a pool located in a granitic outcrop was sep- 

 arated from one in limestone by only a few 

 hundred yards. The pH of the former 

 ranged between 6.2 and 6.8; that of the 

 latter between 7.6 and 9.2. Each pool had 

 its characteristic biota; none of the species 

 of algae, protozoans, and entomostracans 

 recorded from either pool were found in 

 the other (Reed and Klugh, 1924). Since 

 this is approximately all we know about the 

 case, one can only wonder about the role 

 played by pH in controlling the distribution. 

 No analysis was given of the water of the 

 two pools, and it is almost a certainty that 

 the lime content was decidedly diflFerent. 

 The determining factors could be settled by 

 simple toleration experiments. 



Shelford (1925) suggested that pH may 

 be a guiding factor in the return of spawni- 

 ing salmon of northwestern North America 

 to their natal streams. Some mosquito lar- 

 vae are killed by acid water; a pH of 5 is 

 the threshold for development of the first 

 instar of Anopheles maculipennis (Seben- 

 zow and Adova, 1929); tree-hole mosquito 

 larvae can live in much more acid water. In 

 their extensive studies, Jewell and Brown 

 (1929) found snails limited to water wdth 

 a pH of 6. 1 or more and the fingernail clam 

 Pisidiiim to those of 5.8 or more. This last 

 value approximates the acidity at which the 

 deposition of lime becomes theoretically 

 impossible. 



From experience with ecological factors 

 associated with the distribution of com- 

 munities of marine invertebrates near 

 Woods Hole, Massachusetts, Alice (1923) 

 concluded that while a combination of the 

 average pH, the extent of range, and the 

 relative positions of extremes of pH does 

 allow one to place the communities studied 

 in their natural order with considerable 



exactness, such data do not classify the 

 communities with the precision necessary 

 for a successful single-factor index. Salinity 

 changes were more accurately associated 

 with the observed distribution of these com- 

 munities, and the character of the sub- 

 stratum was still more closely correlated. 



Present evidence bearing on the relation 

 between pH and species distribution indi- 

 cates that in general a close correlation can 

 no longer be expected. The limits of pH 

 toleration differ more or less for different 

 species, and yet Jewell and Brown (1929) 

 summarized a considerable amount of 

 first-hand experience with distribution of 

 fresh- water fishes and pH as follows: 

 "While most of the species are found in 

 waters having a pH value between 7.2 and 

 8.6, yet inasmuch as most fresh waters have 

 pH values within these limits, the only in- 

 dication is that the fish are where the water 

 is." The longer these authors worked on this 

 subject, the more convinced they became 

 that pH, as such, is rarely a limiting factor 

 in the distribution of fresh-water fishes in 

 natural waters. Behre (1928) came to a 

 similar conclusion from her studies of fish 

 distribution and the pH of fresh-watei 

 habitats in Panama. 



Observations such as these show the need 

 for caution concerning conclusions about 

 the degree to which pH determinations can 

 be correlated with distribution in nature. 

 Perhaps the correlation is as close as can 

 well be expected, considering the complex- 

 ity of the interactions between pH and 

 other phases of the physical environment. 

 In the main, discussion of the interrelations 

 of different environmental factors is re- 

 served for separate treatment; but with pH, 

 the interplay of various factors has a large 

 importance in comparison with the effec- 

 tiveness of hydrogen ion concentration con- 

 sidered alone and some joint effects need 

 to be considered at once. 



The pH of lake water is low under the 

 ice in late ^vinter. It rises with the spring 

 overturn and then becomes progressively 

 higher in the epilimnion and progressively 

 lower in the hypolimnion as summer strati- 

 fication develops. These changes are asso- 

 ciated with the consumption of carbon 

 dioxide in photosynthesis; that in deeper 

 water, with the accumulation of carbon 

 dioxide and the leaching out of acids from 

 the substratum. There are valid reasons for 

 the use of pH as an indicator of stagnation, 



